Microsoft Security Bulletin MS99-053

Patch Available for "Windows Multithreaded SSL ISAPI Filter" Vulnerability

Originally Posted: December 02, 1999

Summary

Microsoft has released a patch that eliminates a vulnerability in the SSL ISAPI filter that ships with Microsoft® Internet Information Server and is used by other Microsoft products. If called by a multi-threaded application under very specific, and fairly rare, circumstances, a synchronization error in the filter could allow a single buffer of plaintext to be transmitted back to the data's owner.

Frequently asked questions regarding this vulnerability can be found at http://www.microsoft.com/technet/security/bulletin/fq99-053.mspx.

Issue

The SSL ISAPI filter provided as part of IIS supports concurrent use. When used in this mode, a synchronization problem could induce a race condition and cause a single buffer of plaintext to be leaked. The conditions under which this could happen are very rare, and could only occur when a single user's session was multi-threaded and traffic volumes were extremely high.

The scope of this vulnerability is very limited. The leaked plaintext would always be sent to its owner, never another user. Also, because the leaked data would fail its integrity check, the effect of the leak would be to cause the SSL session to immediately collapse. The condition could not be induced by a hostile user, and would offer at best a target of opportunity. Finally, it is worth noting that this vulnerability only affects the SSL ISAPI filter, not the secure communications capability provided by Windows NT via Schannel.

Affected Software Versions

Vulnerability Identifier: CVE-1999-0861

Patch Availability

More Information

Please see the following references for more information related to this issue.

Obtaining Support on this Issue

This is a fully supported patch. Information on contacting Microsoft Technical Support is available at http://support.microsoft.com/contactussupport/?ws=support.

Acknowledgments

Microsoft acknowledges Wall Data for bringing this issue to our attention.

Revisions

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