source: http://www.securityfocus.com/bid/1861/info

A cross-site scripting vulnerability has been reported in Microsoft Indexing Services for Windows 2000/NT4 and its handling of the .htw extension. If a user inadvertantly opened a hostile link through a browser or HTML compliant e-mail client, active content such as JavaScript may be executed. For example, the following link when processed by IIS will yield successful exploitation:

http://target/null.htw?CiWebHitsFile=filename.htm&CiRestriction="<SCRIPT>Active Scripting</SCRIPT>"

It is not necessary to specify a valid .htw file because the virtual file null.htw is stored in memory and the .htw extension is mapped by default to webhits.dll.

Indexing Services is shipped with Windows 2000, however is not started by default. Those who are running a web server and have enabled Indexing Services are recommended to apply the patch.