Access Violation in W3proxy.exe Because of HTTP VARY Header Processing

When an outgoing HTTP request is made through Internet Security and Acceleration (ISA) Server, an access violation may occur in W3proxy.exe, causing the Web Proxy service to terminate.

The access violation may occur if the Web browser client's HTTP request contains more than eight HTTP headers in a specific order and the Web server's HTTP response contains a VARY header for some of the HTTP request headers.

Typically, this access violation occurs only if there is a downstream (intermediate) proxy server that is adding HTTP request headers to the Web browser client's request between the Web browser client and ISA Server.

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. The English version of this fix should have the following file attributes or later:

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

The HTTP VARY response header is used by a Web server to signal that it has multiple possible responses and, using server-driven negotiation, is returning its best guess as to the proper response. The VARY response header contains field names that are request headers that may be used to select that specific response. The VARY header may have Accept-Language, Accept-Encoding, Accept-Charset, User-Agent, or other fields that define how the page was selected.