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Microsoft Internet Information Server (IIS) vulnerable to heap overflow during processing of crafted ".htr" request by "ISM.DLL" ISAPI filter

Vulnerability Note VU#363715

Original Release Date: 2002-04-10 | Last Revised: 2004-02-23


A buffer overflow in the HTR ISAP extension on IIS servers could permit an intruder to interrupt the normal operation of IIS or possibly execute arbitrary code with the privileges of the HTR extension.


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

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.

Vendor Information


Microsoft Corporation Affected

Updated:  September 13, 2002



Vendor Statement

We have not received a statement from the vendor.

Vendor Information

The vendor has not provided us with any further information regarding this vulnerability.


Microsoft has released Microsoft Security Bulletin MS02-18.

If you have feedback, comments, or additional information about this vulnerability, please send us email.

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Our thanks to Microsoft Corporation upon whose advisory this document is based. Microsoft has publicly thanked Dave Aitel of @Stake and Peter Grundl of KPMG for reporting this vulnerability.

This document was written by Shawn V. Hernan.

Other Information

CVE IDs: CVE-2002-0071
Severity Metric: 9.00
Date Public: 2002-04-10
Date First Published: 2002-04-10
Date Last Updated: 2004-02-23 22:20 UTC
Document Revision: 6

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