CSRF form tagging check

The Cross Site Request Forgery (CSRF) Form Tagging check tags each web form sent by a protected web site to users with a unique and unpredictable FormID, and then examines the web forms returned by users to ensure that the supplied FormID is correct. This check protects against cross-site request forgery attacks. This check applies only to HTML requests that contain a web form, with or without data. It does not apply to XML requests.

The CSRF Form Tagging check prevents attackers from using their own web forms to send high volume form responses with data to your protected web sites. This check requires relatively little CPU processing capacity compared to certain other security checks that analyze web forms in depth. It is therefore able to handle high volume attacks without seriously degrading the performance of the protected web site or the App Firewall itself.

Before you enable the CSRF Form Tagging check, you should be aware of the following:

  • You need to enable form tagging. The CSRF check depends on form tagging and does not work without it.
  • You should disable the NetScaler Integrated Caching feature for all web pages containing forms that are protected by that profile. The Integrated Caching feature and CSRF form tagging are not compatible.
  • You should consider enabling Referer checking. Referer checking is part of the Start URL check, but it prevents cross-site request forgeries, not Start URL violations. Referer checking also puts less load on the CPU than does the CSRF Form Tagging check. If a request violates Referer checking, it is immediately blocked, so the CSRF Form Tagging check is not invoked.
  • The CSRF Form Tagging check does not work with web forms that use different domains in the form-origin URL and form-action URL. For example, CSRF Form Tagging cannot protect a web form with a form-origin URL of http://www.example.com and a form action URL of http://www.example.org/form.pl, because example.com and example.org are different domains.

If you use the wizard or the GUI, in the Modify CSRF Form Tagging Check dialog box, on the General tab you can enable or disable the Block, Log, Learn and Statistics actions.

If you use the command-line interface, you can enter the following command to configure the CSRF Form Tagging Check:

  • set appfw profile <name> -fieldConsistencyAction [block] [log] [learn] [stats] [none]

To specify relaxations for the CSRF Form Tagging check, you must use the GUI. On the Checks tab of the Modify CSRF Form Tagging Check dialog box, click Add to open the Add CSRF Form Tagging Check Relaxation dialog box, or select an existing relaxation and click Open to open the Modify CSRF Form Tagging Check Relaxation dialog box. Either dialog box provides the same options for configuring a relaxation.

An alert is generated when you set the NetScaler Appfirewall session limit to a value of 0 or lower, because such a setting affects advanced protection check functionality that requires a properly functioning App Firewall session.

Following are examples of CSRF Form Tagging check relaxations:

Note: The following expressions are URL expressions that can be used in both the Form Origin URL and Form Action URL roles.

  • Choose URLs beginning with http://www.example.com/search.pl? and containing any string after the query, except for a new query:

    ^http://www[.]example[.]com/search[.]pl?[^?]*$
    
  • Choose URLs that begin with http://www.example-español.com and have paths and filenames that consist of upper-case and lower-case letters, numbers, non-ASCII special characters, and selected symbols in the path. The ñ character and any other special characters are represented as encoded UTF-8 strings containing the hexadecimal code assigned to each special character in the UTF-8 charset:

    ``` ^http://www[.]example-espa\xC3\xB1ol[.]com/(([0-9A-Za-z]|\x[0-9A-Fa-f][0-9A-Fa-f]) ([0-9A-Za-z_-]|\x[0-9A-Fa-f][0-9A-Fa-f])/)([0-9A-Za-z]|\x[0-9A-Fa-f][0-9A-Fa-f])([0-9A-Za-z_-]|\x[0-9A-Fa-f][0-9A-Fa-f])*.$

  • Choose all URLs that contain the string /search.cgi?:

    ^[^?<>]*/search[.]cgi\?[^?<>]*$
    

    Important

    Regular expressions are powerful. If you are not thoroughly familiar with PCRE-format regular expressions, double-check any regular expressions you write. Make sure that they define exactly the URL that you want to add as an exception, and nothing else. Careless use of wildcards, and especially of the dot-asterisk (.*) metacharacter/wildcard combination, can have results you do not want, such as blocking access to web content that you did not intend to block or allowing an attack that the check would otherwise have blocked. Tip

    When enableValidate referrer header is enabled under the Start URL Action, ensure that the Referrer Header URL is added to StartURL as well. Note

    When NetScaler reaches the appfw_session_limit and CSRF checks are enabled, the web application freezes.

    To prevent web application freeze, decrease the session timeout and increase the session limit by using the following commands:

```

From CLI: > set appfw settings –sessiontimeout 300 From shell: root@ns# nsapimgr_wr.sh -s appfw_session_limit=200000 ```

Logging and generating SNMP alarms when appfw_session_limit is reached assists you in troubleshooting and debugging issues.

CSRF form tagging check

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