Software Engineering Institute

ISAPI is a programming interface to IIS that can be used to modify or extend the behavior of IIS. Programs written using ISAPI are known as either filters or extension, depending on the way in which they're implemented. One of the primary differences is that extensions only run when called (by a reference in a URL) whereas filters run for every URL submitted to the server. For more information on the difference between filters an extensions, see http://msdn.microsoft.com/library/en-us/vccore/html/_core_What_Is_the_Difference_Between_an_ISAPI_Server_Extension_and_a_Filter.3f.asp.

There is an ISAPI filter that ships as part of Microsoft Fronpage Extensions and ASP.NET that handles overly long URLs by replacing them with a null value. When IIS attempts to process the page (to send it back to the client as an error message) an access violation results. Other ISAPI filters may engage in similar behavior, which would trigger the same vulnerability. However, the only known extension to trigger this error ships with ASP.NET and Front Page Extensions. Quoting from Microsoft Security Bulletin MS02-018,

The sole ISAPI filter known to generate the error that results in the access violation ships only as part of FrontPage Server Extensions and ASP.NET. ASP.NET is not installed by default, and FPSE can be uninstalled if desired. 

If you have custom extensions or filters, you may wish to examine them to see if they would trigger this behavior too.

An intruder could cause the IIS service to fail. IIS 5.0 and 5.1 will automatically restart, though a sustained attack could be mounted relatively easily; IIS 4.0 would have to be manually restarted.

Apply a patch as described in MS02-018.