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Vulnerability Note VU#454091Microsoft Internet Information Server (IIS) vulnerable to buffer overflow via inaccurate checking of delimiters in HTTP header fieldsOverviewA buffer overflow in IIS could allow an intruder to execute arbitrary code the the privileges of the ASP ISAPI extension.I. DescriptionLike all web servers, IIS parses HTTP headers and decomposes them into the constituent parts. As part of this processing, IIS checks for delimiters that are supposed to mark boundaries within the headers. By constructing a carefully chosen HTTP request, and intruder to cause IIS to incorrectly parse an HTTP header, and place the incorrect results into a buffer that is too small. For more information, see Microsoft Security Bulletin MS02-018.II. ImpactAn intruder can interrupt the ordinary operation of a vulnerable IIS server or execute arbitrary code with the privileges of ASP ISAPI extension, ASP.DLL. On IIS 4.0, ASP.DLL runs as part of the operating system thus allowing an intruder to take full administrative control. On IIS 5.0 and 5.1, ASP.DLL runs with the privileges of the IWAM_computername account.III. SolutionApply a patch as described in Microsoft Security Bulletin MS02-018.Until a patch can be applied, you may wish to disable the ASP ISAPI extension by using the IIS Lockdown tool, available at http://www.microsoft.com/technet/security/tools/locktool.asp. Additionally, the URLScan tool can help reduce the impact of this vulnerability.
References
Our thanks to Microsoft Corporation, upon whose advisory this document is based. This document was written by Shawn V. Hernan.
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