Product Documentation

Form field consistency check

The Form Field Consistency check examines the web forms returned by users of your web site, and verifies that web forms were not modified inappropriately by the client. This check applies only to HTML requests that contain a web form, with or without data. It does not apply to XML requests.

The Form Field Consistency check prevents clients from making unauthorized changes to the structure of the web forms on your web site when they fill out and submit a form. It also ensures that the data a user submits meets the HTML restrictions for length and type, and that data in hidden fields is not modified. This prevents an attacker from tampering with a web form and using the modified form to gain unauthorized access to web site, redirect the output of a contact form that uses an insecure script and thereby send unsolicited bulk email, or exploit a vulnerability in your web server software to gain control of the web server or the underlying operating system. Web forms are a weak link on many web sites and attract a wide range of attacks.

The Form Field Consistency check verifies all of the following:

  • If a field is sent to the user, the check ensures that it is returned by the user.
  • The check enforces HTML field lengths and types.

    Note: The Form Field Consistency check enforces HTML restrictions on data type and length but does not otherwise validate the data in web forms. You can use the Field Formats check to set up rules that validate data returned in specific form fields on your web forms.

  • If your web server does not send a field to the user, the check does not allow the user to add that field and return data in it.
  • If a field is a read-only or hidden field, the check verifies that the data has not changed.
  • If a field is a list box or radio button field, the check verifies that the data in the response corresponds to one of the values in that field.

If a web form returned by a user violates one or more of the Form Field consistency checks, and you have not configured the Web App Firewall to allow that web form to violate the Form Field Consistency checks, the request is blocked.

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

You also configure Sessionless Field Consistency in the General tab. If Sessionless Field Consistency is enabled, the Web App Firewall checks only the web form structure, dispensing with those parts of the Form Field Consistency check that depend upon maintaining session information. This can speed the Form Field Consistency check with little security penalty for web sites that use many forms. To use Sessionless Field Consistency on all web forms, select On. To use it only for forms submitted with the HTTP POST method, select postOnly

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

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

To specify relaxations for the Form Field Consistency check, you must use the GUI. On the Checks tab of the Modify Form Field Consistency Check dialog box, click Add to open the Add Form Field Consistency Check Relaxation dialog box, or select an existing relaxation and click Open to open the Modify Form Field Consistency Check Relaxation dialog box. Either dialog box provides the same options for configuring a relaxation, as described in “Manual Configuration By Using the GUI.”

Following are examples of Form Field Consistency check relaxations:

Form Field Names:

  • Choose form fields with the name UserType:

     ^UserType$
    
  • Choose form fields with names that begin with UserType_ and are followed by a string that begins with a letter or number and consists of from one to twenty-one letters, numbers, or the apostrophe or hyphen symbol:

     ^UserType_[0-9A-Za-z][0-9A-Za-z'-]{0,20}$
    
  • Choose form fields with names that begin with Turkish-UserType_ and are otherwise the same as the previous expression, except that they can contain Turkish special characters throughout:

     ^T\xC3\xBCrk\xC3\xA7e-UserType_([0-9A-Za-z]|\\x[0-9A-Fa-f][0-9A-Fa-f])+$
    

    Note: See “PCRE Character Encoding Format” for a complete description of supported special characters and how to encode them properly.

  • Choose form field names that begin with a letter or number, consist of a combination of letters and/or numbers only, and that contain the string Num anywhere in the string:

     ^[0-9A-Za-z]*Num[0-9A-Za-z]*$
    

Form Field Action URLs:

  • 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])*[.](asp|htp|php|s?html?)$
    
  • Choose all URLs that contain the string /search.cgi?:

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

Caution: Regular expressions are powerful. Especially 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 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 or expect, such as blocking access to web content that you did not intend to block or allowing an attack that the Cookie Consistency check would otherwise have blocked.

Form field consistency check

In this article