Why You Need a Firewall Introduction 2

Why You Need a Firewall Introduction 2
Why You Need a Firewall
With the rapid growth of interest in the Internet and the Windows NT operating system,
network security has become a major concern to companies throughout the world. The fact
that the information and tools needed to penetrate the security of corporate networks are
widely available has only increased that concern.
Because of this increased focus on network security, network administrators often spend
more effort protecting their networks than on actual network setup and administration. New
tools that probe for system vulnerabilities, such as the Security Administrator Tool for
Analyzing Networks (SATAN), assist in these efforts, but these tools only point out areas
of weakness instead of providing a means to protect networks. Thus, as a network
administrator, you are constantly trying to keep abreast of the wide number of security
issues confronting you in today’s world. The next section describes many of the security
issues that arise when connecting a private network to the Internet.
Note Although this discussion focuses on those connections made between a private
internal network and the Internet, the security issues described and the ways in which Cisco
Centri Firewall addresses these issues also applies to private intranetwork connections. The
solutions provided by Cisco Centri Firewall are discussed in Chapter 5, “Inside the Cisco
Centri Firewall.”
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Security Issues When Connecting to the Internet
When you connect your private network to the Internet, you are physically connecting your
network to well over 50,000 unknown networks and all of their users. While such
connections open the door to many useful applications and provide great opportunities for
information sharing, most private networks contain some information that should not be
shared with outside users on the Internet. In addition, not all Internet users are involved in
lawful activities. These two statements foreshadow the key questions behind most security
issues on the Internet:
How do you protect confidential information from those who do not explicitly need to
access it?
How do you protect your network and its resources from malicious users and accidents
that originate outside of your network?
The following sections describe the security issues and types of attacks focused around
these two questions.
Note When people access information that they should not be accessing, or when they
attempt to do something undesirable to a network or its resources, we refer to such attempts
as attacks. An attack is some action, or attempted action, that you do not want to happen on
your network. The person who performs such an action is called an attacker.
Protecting Confidential Information
Confidential information can reside in two states on a network. It can reside on physical
storage media, such as a hard drive or memory, or it can reside in transit across the physical
network wire in the form of packets. These two information states present multiple
opportunities for attacks from users on your internal network, as well as those users on the
Internet. We are primarily concerned with the second state, which involves network
security issues. The following list introduces five common methods of attack that present
opportunities to compromise the information on your network:
network packet sniffers
IP spoofing
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password attacks
distribution of sensitive internal information to external sources
man-in-the-middle attacks
When protecting your information from these attacks, your concern is preventing the theft,
destruction, corruption, and introduction of information. These results can cause
irreparable damage to sensitive and confidential information. Below, we describe these
common methods of attack and provide examples of how your information can be
Network Packet Sniffers
Because networked computers communicate serially (one information piece is sent after
another), large information pieces are broken into smaller pieces. (The information stream
would be broken into smaller pieces even if networks communicated in parallel. The
overriding reason for breaking streams into network packets is that computers have limited
intermediate buffers.) These smaller pieces are called network packets. Currently, Windows
NT distributes network packets in “clear text;” the information sent across the network is
not encrypted. (Encryption is the transformation, or “scrambling,” of a message into an
unreadable format using a mathematical algorithm.) Because the network packets are not
encrypted, they can be processed and understood by any application that can pick them up
off of the network and process them.
Note The Windows NT Remote Access Service (RAS) does provide encryption methods
for protecting the packets that are sent across modem connections. The Point-to-Point
Tunneling Protocol (PPTP) provides encryption between Windows NT clients and
Windows NT servers over RAS, but this solution is limited. It does not include
client-to-client encryption or support for non-Windows NT-based computers. Neither of
these encryption techniques are standard for all TCP/IP-based communications.
Third-party products are available that provide encryption for all TCP/IP-based
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A network protocol specifies how packets are identified and labeled, which enables a
computer to determine whether a packet is intended for it. Because the specifications for
network protocols, such as TCP/IP, are widely published, a third party can easily interpret
the network packets and develop a packet sniffer. (The real threat today results from the
numerous freeware and shareware packet sniffers that are available, which do not require
the user to understand anything about the underlying protocols.) A packet sniffer is a
software application that uses a network adapter card in promiscuous mode (a mode in
which the network adapter card sends all packets received on the physical network wire to
an application for processing) to capture all network packets that are sent across a local area
Because Windows NT distributes network packets in clear text, a packet sniffer can provide
its user with meaningful and often sensitive information, such as user account names and
passwords. If you use networked databases, a packet sniffer can provide an attacker with
information that is queried from the database, as well as the user account names and
passwords used to access the database. The more serious problem with acquiring user
account names and passwords is that users often reuse their login names and passwords
across multiple applications.
In addition, many network administrators use packet sniffers to diagnose and fix
network-related problems. Because in the course of their usual and necessary duties these
network administrators work during regular employee hours (such as those in the Payroll
Department), they can potentially examine sensitive information distributed across the
Many users employ a single password for access to all accounts and applications. If an
application is run in client-server mode and authentication information is sent across the
network in clear text, then it is likely that this same authentication information can be used
to gain access to other corporate resources. Because attackers know and use human
characteristics (attack methods known collectively as social engineering attacks), such as
using a single password for multiple accounts, they are often successful in gaining access
to sensitive information.
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IP Spoofing
An IP spoofing attack occurs when an attacker outside your network pretends to be a trusted
computer either by using an IP address that is within the range of IP addresses for your
network or by using an authorized external IP address that you trust and to which you wish
to provide access to specified resources on your network.
Note A trusted computer is a computer that you have administrative control over or one
that you consciously make a decision to “trust” to allow access to your network.
Normally, an IP spoofing attack is limited to the injection of data or commands into an
existing stream of data passed between a client and server application or a peer-to-peer
network connection. To enable bi-directional communication, the attacker must change all
routing tables to point to the spoofed IP address. Another approach the attacker could take
is to simply not worry about receiving any response from the applications. If an attacker is
attempting to get a system to mail him a sensitive file, application responses are
However, if an attacker manages to change the routing tables to point to the spoofed IP
address, he can receive all of the network packets that are addressed to the spoofed address
and reply just as any trusted user can. Like packet sniffers, IP spoofing is not restricted to
people who are external to your network.
Password Attacks
Password attacks can be implemented using several different methods, including brute
force attacks, Trojan horse programs (discussed later in the Application Layer Attacks
section), IP spoofing, and packet sniffers. Although packet sniffers and IP spoofing can
yield user accounts and passwords, password attacks usually refer to repeated attempts to
identify a user account and/or password. These repeated attempts are called brute force
Often a brute force attack is performed using a program that runs across the network and
attempts to log into a shared resource, such as a server. When an attacker successfully gains
access to a resource, he has the same rights as the user whose account has been
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compromised to gain access to that resource. If this account has sufficient privileges, the
attacker can create a “back door” for future access, without concern for any status and
password changes to the compromised user account.
Distribution of Sensitive Information
Controlling the distribution of sensitive information is at the core of your network security
policy. While such an attack may not seem obvious to you, the majority of computer
break-ins that organizations suffer are at the hands of a disgruntled present or former
employee (Miller, Stewart S., Secure Your Data: Web Site Attacks On The Rise!,
[email protected] Week, January 29, 1996.). At the core of these security breaches is the
distribution of sensitive information to competitors or others who will use it to your
disadvantage. While an outside intruder can use password and IP spoofing attacks to copy
information, an internal user can easily place sensitive information on an external computer
or share a drive on the network with other users.
As an example, an internal user could place a file on an external FTP server without ever
leaving his desk. He could also e-mail an attachment that contains sensitive information to
an external user.
Man-in-the-Middle Attacks
A “man-in-the-middle” attack requires that the attacker have access to network packets that
come across the networks. An example configuration could be someone who is working for
your Internet service provider (ISP), who can gain access to all network packets transferred
between your network and any other network. Such attacks are often implemented using
network packet sniffers and routing and transport protocols. The possible uses of such
attacks are theft of information, hijacking an ongoing session to gain access to your internal
network resources, traffic analysis to derive information about your network and its users,
denial of service, corruption of transmitted data, and introduction of new information into
network sessions.
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Protecting Your Network: Maintaining Internal Network System Integrity
While protecting your information may be your highest priority, protecting the integrity of
your network is critical in your ability to protect the information that it contains. A breach
in the integrity of your network can be extremely costly in time and effort, and it can open
multiple avenues for continued attacks. In this section, we describe five methods of attack
that are commonly used to compromise the integrity of your network:
network packet sniffers
IP spoofing
password attacks
denial of service
application layer attacks
When considering what to protect within your network, you are concerned with
maintaining the integrity of the physical network, your network software, any other
network resources, and your reputation. This integrity involves the verifiable identity of
computers and users, proper operation of the services that your network provides, and
optimal network performance—all of these concerns are important in maintaining a
productive network environment. Below, we describe the previously mentioned attacks and
provide examples of how they can be used to compromise your network’s integrity.
Network Packet Sniffers
As we mentioned earlier, network packet sniffers can yield critical system information,
such as user accounts and passwords. Once an attacker obtains the correct account
information, that attacker has the run of your network. In a worst-case scenario, an attacker
gains access to a system-level user account, which the attacker uses to create a new account
that can be used at anytime as a “back door” to get into your network and its resources. The
attacker can modify system critical files, such as the password for the system administrator
account, the list of services and permissions on file servers, and the login information for
other computers that contain confidential information.
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Packet sniffers provide information about the topology of your network that many attackers
find useful. This information, such as what computers run which services, how many
computers are on your network, which computers have access to others, etc., can be
deduced from the information contained within the network packets that are distributed
across your network as part of necessary daily operations.
In addition, a network packet sniffer can be modified to interject new information or change
existing information in a network packet. By doing so, the attacker can cause network
connections to shut down prematurely, as well as change critical information within the
packet. Imagine what could happen if an attacker modified the information being
transmitted to your accounting system. The effects of such attacks can be hard to detect and
can be very costly to correct.
IP Spoofing
While IP spoofing can yield access to user accounts and passwords, these attacks also can
be used in other ways. One way is where an attacker emulates one of your internal users in
ways that prove embarrassing for your organization. For example, the attacker could send
e-mail messages to business partners that appear to have originated from someone within
your organization. Such attacks are easier when an attacker has a user account and
password, but they are possible by combining simple spoofing attacks with knowledge of
messaging protocols.
Password Attacks
Just as with packet sniffers and IP spoofing attacks, a brute force password attack can
provide access to accounts that can be used to modify critical network files and services.
An example that compromises your network’s integrity is where an attacker modifies the
routing tables for your network. By doing so, an attacker ensures that all network packets
are routed to him before they are transmitted to their final destination. In such a case, an
attacker can monitor all of your network traffic, effectively becoming a “man in the
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Denial-of-Service Attacks
Denial-of-service attacks are different from most other attacks because they are not targeted
at gaining access to your network or the information on your network. These attacks focus
on making a service unavailable for normal use, which is typically accomplished by
exhausting some resource limitation on the network or within an operating system or
When involving specific network server applications, such as a HTTP or FTP server, these
attacks can focus on acquiring and keeping open all of the available connections supported
by that server, effectively locking out those valid users of the server or service.
Denial-of-service attacks can also be implemented using common Internet protocols, such
as TCP and ICMP. Most denial-of-service attacks exploit a weakness in the overall
architecture of the system being attacked rather than a software bug or security hole.
However, some attacks compromise the performance of your networks by flooding the
network with undesired, and often useless, network packets and by providing false
information about the status of network resources.
The New York-based Internet service provider (ISP), Panix Public Access Network
Corporation, recently brought to light just how vulnerable a network can be to
denial-of-service attacks. Panix was subjected to an extended attack that crippled access to
all of its TCP-based services, including the web sites and e-mail services that it hosted for
its corporate clients. In this example, the attacker sent up to 150 connection requests per
second to Panix’s hosts—a number of requests that quickly filled up the hosts’ crucial
memory buffers (TCP pending connection buffers) with pending connection attempts,
making the hosts unreachable by legitimate clients.
Application Layer Attacks
Application layer attacks can be implemented using several different methods. One of the
most common methods is exploiting well-known weaknesses in software commonly found
on servers, such as sendmail, PostScript, and FTP. By exploiting these weaknesses,
attackers can gain access to a computer with the permissions of the account running the
application, which is usually a privileged system-level account.
Trojan horse program attacks are implemented using programs that an attacker substitutes
for common programs. These programs may provide all of the functionality that the normal
program provides, but they also include other features that are known to the attacker, such
as monitoring login attempts to capture user account and password information. These
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programs can capture sensitive information and distribute it back to the attacker. They can
also modify application functionality, such as applying a blind carbon copy to all e-mail
messages so that the attacker can read all of your organization’s e-mail.
One of the oldest forms of application layer attacks is a Trojan horse program that displays
a screen, banner, or prompt that the user believes is the valid login sequence. The program
then captures the information that the user types in and stores or e-mails it to the attacker.
Next, the program either forwards the information on to the normal login process (normally
impossible on modern systems) or simply sends an expected error to the user (for example,
Bad Username/Password Combination), exits, and starts the normal login sequence. The
user, believing that he has incorrectly entered his password (a common mistake
experienced by everyone), retypes the information and is allowed access.
One of the newest forms of application layer attacks exploits the openness of several new
technologies: the HTML specification, web browser functionality, and the HTTP protocol.
These attacks, which include Java applets and ActiveX controls, involve passing harmful
programs across the network and loading them through a user’s browser.
Users of ActiveX controls may be lulled into a false sense of security by the Authenticode
technology promoted by Microsoft. However, attackers have already discovered how to
utilize properly signed and bug-free ActiveX controls to make them act as Trojan horses.
This technique uses VBScript to direct the controls to perform their dirty work, such as
overwriting files and executing other programs.
These new forms of attack are different in two respects:
They are initiated not by the attacker but by the user who selects the HTML page that
contains the harmful applet or script stored using the <OBJECT>, <APPLET>, or <SCRIPT>
Their attacks are no longer restricted to hardware platform and operating systems
because of the portability of the programming languages involved.
The next chapter describes the concepts and terminology used to describe today’s common
firewall architectures, such as security perimeters, packet filters, and proxy services. These
terms and concepts are important to understanding the unique architecture and network
security advancements provided by Cisco Centri Firewall.
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