Patch Available for "Undelimited .HTR Request" and "File Fragment Reading via .HTR" Vulnerabilities

Originally Posted: May 10, 2000

Updated: June 16, 2000

Summary
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Microsoft has released a patch that eliminates two security vulnerabilities in Microsoft® Internet Information Server. The vulnerabilities could, respectively, be used to slow an affected web server´s response or to obtain the source code of certain types of files under very restricted conditions.

Frequently asked questions regarding this vulnerability and the patch can be found at http://www.microsoft.com/technet/security/bulletin/fq00-031.asp

Issue
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This patch eliminates two security vulnerabilities that are unrelated except by virtue of the fact that both exist in the ISAPI extension that provides web-based password administration via .HTR scripts.

- The "Undelimited .HTR Request" vulnerability is a denial of service vulnerability. If a malicious user provided a password change request that was missing an expected delimiter, the algorithm would conduct an unbounded search. This would prevent it from servicing additional .HTR requests, and could also slow the overall response of the server.

- The ".HTR File Fragment Reading" vulnerability could allow fragments of certain types of files to be read by providing a malformed request that would cause the .HTR processing to be applied to them. However, the vulnerability could only be exploited under extremely restrictive conditions, and the most valuable data in the files would be the least likely to actually appear in the fragments sent to the user.

Neither of these vulnerabilities would allow data to be added, deleted or changed on the server, nor would they allow any administrative control on the server to be usurped. Although .HTR files are used to allow web-based password administration, neither of these vulnerabilities involve any weakness in password handling. Also, if security best practices have been followed, and unneeded script mappings have been removed, many customers will have removed the .HTR script mapping and thus be unaffected by either vulnerability.

Download for IIS 4
Download for IIS 5