The CERT_CONTEXT Structure Variable Is Not Available for Web Filters in ISA

When you try to write a Web filter for Internet Security and Acceleration (ISA) Server that does client certificate Certificate Revocation List (CRL) validation, you cannot use the CertVerifyRevocation application programming interface (API) because no CERT_CONTEXT structure server variable is available.

Because ISA Server does not fully expose a pointer to the CERT_CONTEXT structure, it cannot be used from a Web filter to perform actions such as CRL checking.

You must install ISA Server Service Pack 1 (SP1) before you apply the following hotfix.

For additional information about how to obtain the latest ISA Server service pack, click the article number below to view the article in the Microsoft Knowledge Base: A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language. To install the fix, run the self-extracting file. You do not have to restart the ISA Server computer. If the computer is part of an ISA Server array, you do not have to shut the whole array down; you can still install this fix on a one-by-one basis.

The English version of this fix should have the following file attributes or later:
This fix also applies to the French, German, Spanish, and Japanese versions of ISA Server.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.

This hotfix makes the CERT_CONTEXT structure server variable available in the HTTP_FILTER_AUTH_COMPLETE_INFO Web filter notification. The CERT_CONTEXT structure is not available in any other notifications. Note that Web filters receive a const pointer to the CERT_CONTEXT structure. You do not have to free the memory that is used by the structure because the Web proxy automatically cleans it up at the end of each request.

The following code can be used to get CERT_CONTEXT structure: ISA does not do client certificate CRL checking in a Web publishing scenario when Secure Sockets Layer (SSL) is used and when client certificates are used as the authentication method. To use CRL checking, you must both use a Web filter and install this hotfix.

When the certificate is validated against the CRL, it may generate a network request. You can make any networking call from a web filter. However, a static packet filter must be created for the traffic if the target of the request is behind the external network adapter and packet filtering is turned on. This filter can be created exactly to the target you need so this does not introduce a security hole.

For more information about writing Web filters, visit the following Microsoft Web site: