Software Engineering Institute

HTR is a server-side scripting technology for IIS which has largely been supplanted by ASP.

There is a buffer overflow in the HTR ISAPI extension. By submitting a malicious HTR request, and intruder can exploit this buffer overflow.

An intruder can interrupt the normal operation of the IIS server. In addition, an intruder may be able to use this vulnerability to execute arbitrary code with the privileges of the HTR ISAPI extension. On IIS 4.0, this permits administrative control of the oeprating system; on IIS 5.0 and 5.1, this permits access with the privileges of the IWAM_computername account.

Apply a patch as decribed in MS02-018.

In addition to applying the patch (or if the patch cannot be applied) we strongly recommend disabling the HTR ISAPI extension. It has been the source of a number of vulnerabilities, including

http://www.kb.cert.org/vuls/id/28565
http://www.kb.cert.org/vuls/id/264272
http://www.kb.cert.org/vuls/id/35085

For some time, Microsoft has recommended disabling HTR unless there is a business-critical need to support it. Quoting from Microsoft Security Bulletin MS02-018:

HTR is a first-generation advanced scripting technology delivered as part of IIS 2.0. HTR was never widely adopted, largely because a far superior technology, Active Server Pages (.ASP), was introduced in IIS 4.0 and became popular before customers had invested significant development resources in HTR. However, all versions of IIS through version 5.1 do provide support for HTR, for purposes of backward compatibility.

Microsoft has long advocated that customers disable HTR on their web servers, unless there is a business-critical need for the technology. By default, the IIS Lockdown Tool disable HTR support, by unmapping the HTR ISAPI extension. 

As with any technology, we recommend disabling it if its not required.