Vulnerability Note VU#363715

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

Overview

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.

I. Description

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.

II. Impact

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.

III. Solution

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.

Systems Affected

Vendor	Status	Date Notified	Date Updated
Microsoft Corporation	Vulnerable	13-Sep-2002

References

http://www.atstake.com/research/advisories/2002/a041002-1.txt
http://www.microsoft.com/technet/security/bulletin/MS02-018.asp
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=37931
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=37824
http://www.securityfocus.com/bid/4474

Credit

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

Date Public:	2002-04-10
Date First Published:	2002-04-10
Date Last Updated:	2004-02-23
CERT Advisory:
CVE-ID(s):	CVE-2002-0071
NVD-ID(s):	CVE-2002-0071
US-CERT Technical Alerts:
Metric:	9.00
Document Revision:	6

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