HTTP/HTTPS protocol constraints. Fortinet FortiWeb 5.1 Patch 3
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Setting name
Description
URL Pattern Depending on your selection in
Type
, enter either:
• the literal URL, such as
/index.php, that is an exception to the generally allowed HTTP request methods. The URL must begin with a slash ( / ).
• a regular expression, such as
^/*.php, matching all and only the URLs which are exceptions to the generally allowed HTTP request methods.
The pattern does not require a slash ( / ); however, it must at match URLs that begin with a slash, such as
/index.cfm.
For example, if multiple URLs on a host have identical HTTP request method requirements, you would type a regular expression matching all of and only those URLs.
Do not include the domain name, such as www.example.com, which is configured separately in the
Host
drop-down list.
To create and test a regular expression, click the >> (test) icon. This opens the Regular Expression Validator window where you can fine-tune the expression (see
“Regular expression syntax” on page 706
).
Allow
Method
Exception
Mark the check boxes of all HTTP request methods that you want to allow.
Methods that you do not select will be denied.
The OTHERS option includes methods not specifically named in the other options. It often may be required by WebDAV ( RFC 4918 ) applications such as Microsoft Exchange Server 2003 and Subversion , which may require
HTTP methods not commonly used by web browsers, such as
PROPFIND and
BCOPY.
Note:
If a WAF Auto Learning Profile will be selected in the policy with an offline protection profile that uses this allowed method exception, you must enable the HTTP request methods that will be used by sessions that you want the FortiWeb appliance to learn about. If a method is disabled, the
FortiWeb appliance will reset the connection, and therefore cannot learn about the session.
8. Click OK.
9. Repeat the previous steps for each exception that you want to add to the allowed method exceptions.
10.To apply the allowed method exception, select it in an allowed method policy. For details,
see “Specifying allowed HTTP methods” on page 462 .
See also
•
Configuring a protection profile for inline topologies
•
Configuring a protection profile for an out-of-band topology or asynchronous mode of operation
HTTP/HTTPS protocol constraints
Protocol constraints govern features such as the HTTP header fields in the protocol itself, as well as the length of the HTML, XML, or other documents or encapsulated protocols carried in the HTTP body payload.
Fortinet 466 FortiWeb 5.1 Patch 3 Administration Guide
Use protocol constraints to prevent attacks such as buffer overflows. Buffer overflows can occur in web servers and applications that do not restrict elements of the HTTP protocol to acceptable lengths, or that mishandle malformed requests. Such errors can lead to security vulnerabilities.
Default HTTP protocol constraint values reflect the buffer size of your FortiWeb model’s HTTP parser. Use protocol constraints to block requests that are too large for the memory size
of FortiWeb’s scan buffers. Failure to block items that are too large to be buffered could compromise your network’s security, and allow requests without scanning or rewriting. See
“Buffer hardening” on page 647 .
For example, if your web applications require HTTP
POST requests with unusually large parameters, you would adjust the HTTP body buffer size (see http-cachesize in the
FortiWeb CLI Reference
). Then, you would configure
Malformed Request
and other HTTP protocol constraints to harden your configuration.
This scan is bypassed if the client’s source IP is a known search engine and you have enabled
To configure an HTTP protocol constraint
1. If you plan to add constraint exceptions to your HTTP protocol constraints, configure the
exceptions first. See “Configuring HTTP protocol constraint exceptions” on page 474
. If you want to use a trigger when the rule is violated, configure it also. See
“Configuring triggers” on page 590
.
2. Go to Web Protection > Protocol > HTTP Protocol Constraints.
To access this part of the web UI, your administrator’s account access profile must have
Read and Write permission to items in the Web Protection Configuration category. For
details, see “Permissions” on page 52
.
3. Click Create New.
A dialog appears.
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4. Configure these settings:
To disable a parameter check, type
0.
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Fortinet
Setting name
Name
Action
Block Period
Description
Type a unique name that can be referenced in other parts of the configuration. Do not use spaces or special characters. The maximum length is 35 characters.
Select which action the FortiWeb appliance will take when it detects a violation of the rule:
• Alert — Accept the connection and generate an alert email and/or log message.
• Alert & Deny — Block the request (reset the connection) and generate an alert and/or log message.
You can customize the web page that will be returned to the client with the HTTP status code. See
“Uploading a custom error page” on page 494
• Period Block — Block subsequent requests from the client for a number of seconds. Also configure
Block Period
.
You can customize the web page that will be returned to the client with the HTTP status code. See
“Uploading a custom error page” on page 494
Note:
If FortiWeb is deployed behind a NAT load balancer, when using this option, you must also define an X-header that indicates the original client’s IP (see
“Defining your proxies, clients, & X-headers” on page 278 ). Failure to do so may cause
FortiWeb to block all connections when it detects a violation of this type.
The default value is Alert.
Caution:
This setting will be ignored if
Note:
Logging and/or alert email will occur only if enabled and
configured. See “Logging” on page 575
.
Note:
If you will use this rule set with auto-learning, you should select Alert. If
Action
is Alert & Deny, or any other option that causes the FortiWeb appliance to terminate or modify the request or reply when it detects an attack attempt, the interruption will cause incomplete session information for auto-learning.
Type the number of seconds that you want to block subsequent requests from the client after the FortiWeb appliance detects that the client has violated the rule.
This setting is available only if
Action
is set to Period Block. The valid range is from 1 to 3,600 (1 hour). The default value is 60. See
also “Monitoring currently blocked IPs” on page 641
.
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Fortinet
Setting name
Severity
Description
When rule violations are recorded in the attack log, each log message contains a Severity Level ( severity_level) field. Select which severity level the FortiWeb appliance will use when it logs a violation of the rule:
• Low
• Medium
• High
The default value is High.
Trigger Action
Illegal Host Name
Illegal HTTP Version Enable to check for invalid HTTP version numbers. Currently, the only valid version strings are
HTTP/1.0 or HTTP/1.1.
Attack log messages contain
Illegal HTTP Version when this feature detects an invalid HTTP version number.
Illegal HTTP
Request Method
Enable to check for invalid HTTP request methods according to RFC
2616 . Any method not defined in that RFC — including misspellings like
GETT as well as HTTP extension methods (e.g. WebDAV and
CalDAV) like
PROPFIND and MKCALENDAR — will be considered invalid.
Attack log messages contain
Illegal HTTP Method when this feature detects an invalid HTTP request method.
HTTP Request
Length
Select which trigger, if any, that the FortiWeb appliance will use when it logs and/or sends an alert email about a violation of the rule.
See
“Configuring triggers” on page 590
.
Enable to check for illegal characters in the
Host: line of the HTTP header, such as null characters or encoded characters.
For example, characters such as
0x0 or %00* will be considered illegal.
Attack log messages contain
Illegal Host Name when this feature detects an invalid host name.
Content Length
Type the maximum acceptable length in bytes of the entire HTTP request, including both headers and body.
Attack log messages contain
HTTP Request Length Exceeded when this feature detects an excessively large HTTP request.
Type the maximum acceptable length in bytes of the request body.
Length is determined by comparing this limit with the value of the
Content-Length: field in the HTTP header.
Attack log messages contain
Content Length Exceeded when this feature detects a content length buffer overflow attempt.
Tip:
RPC requests’ content length often do not match their own
Content-Length: header. Attackers may also intentionally craft mismatching
Content-Length: headers in an attempt to cloak buffer overflows. For those cases, use other limits instead or in addition, such as
Body Length
and “Limiting file uploads” on page 478
.
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Fortinet
Setting name
Body Length
Description
Type the maximum acceptable size in bytes of the HTTP body.
For requests that use the HTTP
POST method, this typically includes parameters from submitted by HTML form inputs. In the case of file uploads, this can normally be many megabytes. For most simple forms, however, the body should be only a few kilobytes in size at maximum.
Attack log messages contain
Body Length Exceeded when this feature detects a body size buffer overflow attempt.
Header Length
Type the maximum acceptable size in bytes of all HTTP header lines.
Attack log messages contain
Total Size of All Headers Too
Large when this feature detects a header size buffer overflow attempt.
Header Line Length
Type the maximum acceptable size in bytes of each line in the HTTP header.
Attack log messages contain
Header Line Too Large when this feature detects an attempted header line length buffer overflow.
Number of Header
Lines In Request
Type the maximum acceptable number of lines in the HTTP header.
Attack log messages contain
Too Many Headers when this feature detects a header line count buffer overflow attempt.
Total URL and Body
Parameters Length
Type the total maximum total acceptable size in bytes of all parameters in the URL and/or, for HTTP
POST requests, the HTTP body.
Question mark ( ? ), ampersand ( & ), and equal ( = ) characters are not included.
Attack log messages contain
Total URL and Body Parameters
Length Exceeded when this feature detects a total parameter size buffer overflow attempt.
Total URL
Parameters Length
Type the total maximum acceptable length in bytes of all parameters, including their names and values, in the URL.
Parameters usually appear after a
?, such as:
/url?parameter1=value1¶meter2=value2
It does not include parameters in the HTTP body, which can occur with HTTP
POST requests. For those, configure
Total URL and Body
Parameters Length
or
Body Length
instead.
Attack log messages contain
Total URL Parameters Length
Exceeded when this feature detects a URL parameter line length buffer overflow attempt.
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Setting name
Number of URL
Parameters
Description
Type the maximum number of parameters in the URL. The maximum number is 104.
It does not include parameters in the HTTP body, which can occur with HTTP
POST requests.
Attack log messages contain
Too Many Parameters in
Request when this feature detects a URL parameter count buffer overflow attempt.
Number of Cookies
In Request
Type the maximum acceptable number of cookies in an HTTP request.
Attack log messages contain
Too Many Cookies in Request when this feature detects a cookie count buffer overflow attempt.
Number of ranges in
Range Header
Type the maximum acceptable number of
Range: lines in each
HTTP header. The default value is 5.
Attack log messages contain
Too Many Range Headers when this feature detects too many
Range: header lines.
Tip:
Some versions of Apache are vulnerable to a denial of service
(DoS) attack on this header, where a malicious client floods the server with many
Range: headers. The default value is appropriate for un-patched versions of Apache 2.0 and Apache 2.1.
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Fortinet
Setting name
Malformed Request Enable to inspect the request for:
• syntax errors
• exceeding the maximum buffer size allowed by FortiWeb’s HTTP parser
Errors and buffer overflows can cause problems in web servers that do not handle them gracefully. Such problems can lead to security vulnerabilities.
Attack log messages contain
Too Many Parameters or Too
Many Flash Parameters or another message that indicates the specific cause when this feature detects a request with parser errors or a FortiWeb buffer overflow attempt.
Caution:
Fortinet strongly recommends to enable this option unless large requests/parameters are required by the web application. If part of a request is too large for its scan buffer, FortiWeb cannot scan it for attacks. It also cannot perform rewrites. Unless you
configure it to block, FortiWeb will allow oversized requests to
pass through without scanning or rewriting. This could allow padded attacks to pass through, and rewriting to be skipped.
If feasible, instead of disabling this option:
• Enlarge the scan buffer for each parameter (see http-cachesize in the
FortiWeb CLI Reference
). Requests larger than the buffer will be flagged as potentially malformed by
FortiWeb’s parser, causing FortiWeb to block normal requests
(i.e. false positives). For more buffer specifications, see
“Buffer hardening” on page 647
.
• Disable this setting only for URLs that require oversized parameters (see
“Configuring HTTP protocol constraint exceptions” on page 474 )
Exception Name
Description
Select the HTTP constraints exception, if any, that you want to apply to this policy (see
“Configuring HTTP protocol constraint exceptions” on page 474
).
If you want to view or change the information associated with a exception, select the Detail link. The HTTP Constraints Exception dialog appears, where you can view and edit the exceptions.
5. Click OK.
6. To apply the HTTP protocol constraint profile, select it in an inline or offline protection profile
(see “Configuring a protection profile for inline topologies” on page 495
or
).
See also
•
•
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Configuring HTTP protocol constraint exceptions
You can configure exceptions for use with HTTP protocol constraints.
Exceptions define HTTP constraints that will not be subject to HTTP protocol constraint.
Exceptions are useful when you know that some HTTP protocol constraints, during normal use, will cause false positives by matching an attack signature.
For example, if no exceptions are defined, FortiWeb executes the HTTP protocol constraint as defined in
“HTTP/HTTPS protocol constraints” on page 466
. But, if you mark the check box for
Header Length
in a HTTP protocol constraint exception for a specific host, FortiWeb will skip the HTTP header length check when executing the web protection profile for that host.
As another example, some web applications require very large HTTP
POST requests. You can use
Malformed Request
to create an exception from the constraint for those requests.
Like any software, FortiWeb’s buffers are not endless. If an HTTP request overall or its individual components such as parameters are too long to fit the scan buffer, they will you do not want to
To configure an HTTP constraint exception
1. Go to Web Protection > Protocol > HTTP Constraints Exception.
To access this part of the web UI, your administrator’s account access profile must have
Read and Write permission to items in the Web Protection Configuration category. For
details, see “Permissions” on page 52
.
2. Click Create New.
A dialog appears.
Fortinet
3. In Name, type a unique name that can be referenced by other parts of the configuration. Do not use spaces or special characters. The maximum length is 35 characters.
4. Click OK.
5. Click Create New to add an entry to the set.
A dialog appears.
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6. Configure these settings:
Fortinet
Setting name
Host Status
Host
Request Type
Description
Enable to apply this HTTP constraint exception only to HTTP requests for specific web hosts. Also configure
Host
.
Disable to apply the exceptions to all web hosts.
Select the IP address or fully qualified domain name (FQDN) of the protected host to which this exception applies.
This setting is available only if
Host Status
is enabled.
Select whether the
URL Pattern
field will contain a literal URL
(Simple String), or a regular expression designed to match multiple
URLs (Regular Expression).
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Fortinet
Setting name
URL Pattern
Header Length
Content Length
Description
Depending on your selection in the Request Type field, enter either:
• the literal URL, such as
/index.php, that the HTTP request must contain in order to match the input rule. The URL must begin with a backslash ( / ).
• a regular expression, such as
^/*.php, matching all and only the
URLs to which the input rule should apply. The pattern does not require a slash ( / ); however, it must at match URLs that begin with a slash, such as
/index.cfm.
Do not include the domain name, such as www.example.com, which is configured separately in the Host drop-down list.
To create and test a regular expression, click the >> (test) icon. This opens the Regular Expression Validator window where you can fine-tune the expression (see
“Regular expression syntax” on page 706
)n.
Enable to omit the constraint on the maximum acceptable size in bytes of the HTTP header.
Enable to omit the constraint on the maximum acceptable size in bytes of the request body.
Body Length
Enable to omit the constraint on the maximum acceptable size in bytes of the HTTP body.
Parameter Length
Enable to omit the constraint on the maximum acceptable size in bytes of parameters in the URL or, for HTTP
POST requests.
Header Line Length
Enable to omit the constraint on the maximum acceptable size in bytes of each line in the HTTP header.
HTTP Request
Length
URL Parameter
Length
Enable to omit the constraint on the maximum acceptable length in bytes of the HTTP request.
Enable to omit the constraint on the maximum acceptable size of an
URL parameter (including the name and value).
Number of Cookies
In Request
Enable to omit the constraint on the maximum acceptable number of cookies in an HTTP request.
Number of Header
Lines In Request
Enable to omit the constraint on the maximum acceptable number of lines in the HTTP header.
Illegal HTTP
Request Method
Number of URL
Parameters
Illegal Host Name
Enable to omit the constraint on to check for invalid HTTP version numbers.
Enable to omit the constraint on the maximum number of parameters in the URL.
Enable to omit the constraint on invalid characters in the
Host: line of the HTTP header, such as null characters or encoded characters.
476 FortiWeb 5.1 Patch 3 Administration Guide
Setting name Description
Number of ranges in
Range Header
Enable to omit the constraint on the maximum acceptable number of
Range: lines in an HTTP header.
Tip:
Some versions of Apache are vulnerable to a denial of service
(DoS) attack on this header, where a malicious client floods the server with many
Range: headers. If your web servers do not run
Apache and are not vulnerable to this attack, mark this check box to omit it from the scan and improve performance.
Malformed Request Enable to omit the constraint on syntax and FortiWeb parsing errors.
Caution:
Some web applications require abnormal or very large
HTTP
POST requests. Since allowing such errors and excesses is generally bad practice and can lead to vulnerabilities, use this option to omit the malformed request scan only if absolutely necessary.
7. Click OK.
8. Repeat the previous steps for each rule you want to add to the exception.
9. Group the HTTP protocol constraint exception in an HTTP protocol constraint profile (see
“HTTP/HTTPS protocol constraints” on page 466
).
See also
•
Configuring a protection profile for inline topologies
•
Configuring a protection profile for an out-of-band topology or asynchronous mode of operation
Fortinet 477 FortiWeb 5.1 Patch 3 Administration Guide
Limiting file uploads
You can restrict file uploads based upon file type and size.
Detection and restriction are performed by scanning
Content-Type: and Content-Length: headers in HTTP
PUT and POST request methods submitted to your web servers.
For example, if you want to allow only specific types of files (MP3 audio files, PDF text files and
GIF and JPG picture files) to be uploaded to: http://www.example.com/upload.php
create a file upload restriction policy that contains rules that define only those specific file types.
When FortiWeb receives an HTTP
PUT or POST request for the /upload.php URL with Host: www.example.com, it scans the HTTP request and allows only the specified file types to be uploaded. FortiWeb blocks file uploads for any HTTP request that contains non-specified file types.
To configure a file upload restriction
1. Go to Web Protection > Input Validation > File Upload Restriction Rule.
To access this part of the web UI, your administrator’s account access profile must have
Read and Write permission to items in the Web Protection Configuration category. For
details, see “Permissions” on page 52
.
2. Click Create New.
A dialog appears.
Fortinet
3. In Name, type a unique name that can be referenced by other parts of the configuration. Do not use spaces or special characters. The maximum length is 35 characters.
478 FortiWeb 5.1 Patch 3 Administration Guide
4. If you want to apply this file upload restriction rule only to requests for specific web hosts:
• Enable Host Status.
• From Host, select the IP address or FQDN of a protected host.
Disable Host Status to match the file upload restriction rule based upon the other criteria, such as the URL, but regardless of the
Host: field
5. In Request URL, type the literal URL, such as
/upload.php, to which the file upload restriction applies. The URL must begin with a slash ( / ).
Do not include the name of the host, such as www.example.com, which is configured separately in the Host drop-down list.
6. In File Upload Limit, type a number to represent the maximum size in kilobytes for any individual file. The upload rule rejects allowed files larger than this number. The valid range is from 0 to 5,120 KB (5 MB).
7. Click OK.
8. To add or remove file types, click Add File Types.
A dialog appears.
Fortinet
9. In the File Types pane, select the file types to allow, then click the right arrow (->) to move them to the Allow Files Types pane.
Microsoft Office Open XML file types such as .docx, xlsx, .pptx, and .vsdx are a type of
ZIP-compressed XML. If you specify restrictions for them, those signatures will take priority.
However, if you do not select a MSOOX restriction but do have an XML or ZIP restriction, the
XML and ZIP restrictions will still apply, and the files will still be restricted.
10.Click OK.
11.Go to Web Protection > Input Validation > File Upload Restriction Policy.
To access this part of the web UI, your administrator’s account access profile must have
Read and Write permission to items in the Web Protection Configuration category. For
details, see “Permissions” on page 52
.
12.Click Create New.
A dialog appears.
479 FortiWeb 5.1 Patch 3 Administration Guide
13.Configure these settings:
Setting name
Name
Description
Type a unique name that can be referenced in other parts of the configuration. Do not use spaces or special characters. The maximum length is 35 characters.
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Fortinet
Action
Block Period
Severity
Select which action the FortiWeb appliance will take when it detects a violation of the rule:
• Alert — Accept the connection and generate an alert email and/or log message.
• Alert & Deny — Block the request (reset the connection) and generate an alert and/or log message.
You can customize the web page that will be returned to the client with the HTTP status code. See
“Uploading a custom error page” on page 494
.
• Period Block — Block subsequent requests from the client for a number of seconds. Also configure
Block Period
.
You can customize the web page that will be returned to the client with the HTTP status code. See
“Uploading a custom error page” on page 494
.
Note:
If FortiWeb is deployed behind a NAT load balancer, when using this option, you must also define an X-header that indicates the original client’s IP (see
“Defining your proxies, clients, &
X-headers” on page 278 ). Failure to do so may cause FortiWeb to
block all connections when it detects a violation of this type.
The default value is Alert.
Caution:
This setting will be ignored if
is enabled.
Note:
Logging and/or alert email will occur only if enabled and configured. See
Note:
If you will use this rule set with auto-learning, you should select
Alert. If
Action
is Alert & Deny, or any other option that causes the
FortiWeb appliance to terminate or modify the request or reply when it detects an attack attempt, the interruption will cause incomplete session information for auto-learning.
Type the number of seconds that you want to block subsequent requests from the client after the FortiWeb appliance detects that the client has violated the rule.
This setting is available only if
Action
is set to Period Block. The valid range is from 1 to 3,600 (1 hour). The default value is 1. See also
“Monitoring currently blocked IPs” on page 641
.
When rule violations are recorded in the attack log, each log message contains a Severity Level ( severity_level) field. Select which severity level the FortiWeb appliance will use when it logs a violation of the rule:
• Low
• Medium
• High
The default value is High.
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Fortinet
Trigger Action
Antivirus Scan
Select which trigger, if any, that the FortiWeb appliance will use when it logs and/or sends an alert email about a violation of the rule. See
“Configuring triggers” on page 590 .
Enable to scan for trojans. Also enable the signatures (
configure the antivirus-specific
,
(see
“Blocking known attacks & data leaks” on page 408 ).
Attackers often modify HTTP header so that the request’s
Content-Type: does not match — it indicates an allowed file type, but the byte code contained in the body is actually a virus. This scan ensures that the request actually contains the file type that it professes, and that it is not infected.
14.Click OK.
15.Click Create New to include a rule in the set.
A dialog appears.
16.From the File Upload Restriction Rule drop-down list, select an existing file upload restriction rule that you want to use in the policy.
To view or change the information associated with the item, select the Detail link. The File
Upload Restriction Rule dialog appears. Use the browser Back button to return.
17.Click OK.
18.Repeat the previous steps for each rule that you want to add to the file upload restriction policy.
19.To apply the file upload restriction policy, select it in an inline or offline protection profile (see
“Configuring a protection profile for inline topologies” on page 495
).
See also
•
•
Connecting to FortiGuard services
•
How often does Fortinet provide FortiGuard updates for FortiWeb?
•
482 FortiWeb 5.1 Patch 3 Administration Guide
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