Preventing zero-day attacks > HTTP/HTTPS protocol constraints

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.

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.

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 Allow Known Search Engines.

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. If you want to use a trigger when the rule is violated, configure it also. See Viewing log messages.

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.

3.  Click Create New.

Settings for the following HTTP protocol constraints are displayed. To display a brief description of a rule, click its name:

Setting name Description
General  
  Illegal Content Type Enable to check whether the Content Type: value uses the format <type>/<subtype>.
  Illegal Response Code Enable to check whether the HTTP response code is a 3-digit number.
  Illegal Host Name

Enable to check for illegal characters in the Host: line of the HTTP header, such as null characters or encoded characters.

For example, 0x0 or %00* are illegal.

Attack log messages contain Illegal Host Name when this feature detects an invalid host name.

  Illegal HTTP Version

Enable to check for invalid HTTP version numbers. Currently, the only valid version strings are HTTP/0.9, HTTP/1.0 or HTTP/1.1.

Attack log messages contain Illegal HTTP Version when this feature detects an invalid HTTP version number.

  Body Length

Specifies the maximum acceptable size in bytes of the HTTP body.

For requests that use the HTTP POST method, this typically includes parameters 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.

  Number of Cookies In Request

Specifies 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

Specifies 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.

  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 allows 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.
  • Disable this setting only for URLs that require oversized parameters (see Configuring HTTP protocol constraint exceptions)
HTTP Header  
  Header Length

Specifies 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 Name Length Specifies the maximum acceptable size in bytes of a single HTTP header name (for example, Host:, Content-Type:, User-Agent:).

The default is 50 bytes.
  Header Value Length Specifies the maximum acceptable size in bytes of a single HTTP header value.

The default is 4096 bytes.
  Illegal Character in Header Name Enable to check whether the HTTP header name contains illegal characters.
  Illegal Character in Header Value Enable to check whether the HTTP header value contains illegal characters.
HTTP Request  
  Illegal HTTP Request Method

Enable to check for invalid HTTP request methods according to RFC 2616 or RFC 4918. Any method not defined in these RFCs — including misspellings like GETT as well as other HTTP extension methods (e.g. CalDAV) like MKCALENDAR — are considered invalid.

Attack log messages contain Illegal HTTP Method when this feature detects an invalid HTTP request method.

  HTTP Request Filename Length Specifies the maximum acceptable length in bytes of the HTTP request filename.
  HTTP Request Length

Specifies 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.

  Number of Header Lines in Request

Specifies 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.

  Missing Content Type Enable to check whether the Content-Type: header is available.
HTTP Parameter  
  Total URL Parameters Length

Specifies 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&parameter2=value2

The count does not include:

  • Question mark ( ? ), ampersand ( & ), and equal ( = ) characters are not included.
  • Parameters in the HTTP body, which can occur with HTTP POST requests. For these parameters, configure Total 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.

  Total Body Parameters Length

Specifies the total maximum acceptable size in bytes of all the parameters in the HTTP body of HTTP POST requests.

Question mark ( ? ), ampersand ( & ), and equal ( = ) characters are not included.

Attack log messages contain Total Body Parameters Length Exceeded when this feature detects a total parameter size buffer overflow attempt.

  Number of URL Parameters

Specifies the maximum number of parameters in the URL. The maximum number is 1024.

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.

The default is 128.

  NULL Character in Parameter Name Enable to check for null characters in parameter names.
  NULL Character in Parameter Value Enable to check for null characters in parameter values.
Content Length  
  Content Length

Specifies 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.

  Illegal Content Length Enable to check whether the Content-Length: header includes numeric characters only.

4.  Configure these settings:

Setting name Description
Name 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 63 characters.
Exception Name

Select the HTTP constraints exception, if any, that you want to apply to this policy (see Configuring HTTP protocol constraint exceptions).

If you want to view or change the exception configuration, click Detail.

Status Specify whether the rule applies when you apply this constraint to a profile.
Length For rules that specify maximums, enter a maximum value.
Action

Select the action the FortiWeb appliance takes when it detects a violation of the rule:

  • Alert — Accept the connection and generate an alert email and/or log message.
  • Alert & DenyBlock the request (reset the connection) and generate an alert and/or log message.

    You can customize the web page that FortiWeb returns to the client with the HTTP status code. See Customizing error and authentication pages (replacement messages).

  • Period Block Block subsequent requests from the client for a number of seconds. Also configure Block Period.

    You can customize the web page that FortiWeb returns to the client with the HTTP status code. See Customizing error and authentication pages (replacement messages).

    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). 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 is ignored when Monitor Mode is enabled.

Note: Logging and/or alert email occur only if you enable and configure it. See Logging and Alert email.

Note: To use this rule set with auto-learning, 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 causes incomplete session information for auto-learning.

Block Period

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.

Severity

When rule violations are recorded in the attack log, each log message contains a Severity Level (severity_level) field. Select which severity level to use when FortiWeb logs a violation of the rule:

  • Low
  • Medium
  • High
Trigger Action Select which trigger, if any, to use when FortiWeb logs and/or sends an alert email about a violation of the rule. See Viewing log messages.

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 or Configuring a protection profile for an out-of-band topology or asynchronous mode of operation).

See also

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. 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.

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.

2.  Click Create New.

A dialog appears.

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 63 characters.

4.  Click OK.

5.  Click Create New to add an entry to the set.

A dialog appears.

6.  Configure these settings:

Setting name Description
 Host Status

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.

 Host

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.

  Request Type Select whether the  URL Pattern field will contain a literal URL (Simple String), or a regular expression designed to match multiple URLs (Regular Expression).
 URL Pattern

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).

General  
  Illegal Content Type Enable to omit the constraint on whether the Content Type: value uses the format <type>/<subtype>.
  Illegal Reponse Code Enable to omit the constraint on whether the HTTP response code is a 3-digit number.
  Illegal Host Name Enable to omit the constraint on invalid characters in the Host: line of the HTTP header, such as null characters or encoded characters.
  Body Length Enable to omit the constraint on the maximum acceptable size in bytes of the HTTP body.
  Number of Cookies In Request Enable to omit the constraint on the maximum acceptable number of cookies in an HTTP request.
  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.

HTTP Header  
  Header Length Enable to omit the constraint on the maximum acceptable size in bytes of the HTTP header.
  Header Name Length Enable to omit the constraint on the maximum acceptable size in bytes of a single HTTP header name.
  Header Value Length Enable to omit the constraint on the maximum acceptable size in bytes of a single HTTP header value.
  Illegal Character in Header Name Enable to omit the constraint on whether the HTTP header name contains illegal characters.
  Illegal Character in Header Value Enable to omit the constraint on whether the HTTP header value contains illegal characters.
HTTP Request  
  Illegal HTTP Request Method Enable to omit the constraint on to check for invalid HTTP version numbers.
  HTTP Request Length Enable to omit the constraint on the maximum acceptable length in bytes of the HTTP request.
  Number of Header Lines In Request Enable to omit the constraint on the maximum acceptable number of lines in the HTTP header.
  Post Request -- Missing Content Type Enable to omit the constraint on whether the Content-Type: header is available.
HTTP Parameter  
  Total URL Parameter Length Enable to omit the constraint on the maximum acceptable size of an URL parameter (including the name and value).
  Total Body Parameters Length Enable to omit the constraint on the maximum acceptable size in bytes of all parameters in the HTTP body of HTTP POST requests.
  Number of URL Parameters Enable to omit the constraint on the maximum number of parameters in the URL.
  NULL Character in Parameter Name Enable to omit the constraint on null characters in parameter names.
  NULL Character in Parameter Value Enable to omit the constraint on null characters in parameter values.
Content Length  
  Content Length Enable to omit the constraint on the maximum acceptable size in bytes of the request body.
  Illegal Content Length Enable to omit the constraint on whether the Content-Length: header includes numeric characters only.

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).

See also