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(r) 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/security/bulletins/MS99-053faq.asp.



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

==========================

 - Microsoft IIS 4.0

 - Microsoft Site Server 3.0

 - Microsoft Site Server Commerce Edition 3.0



Patch Availability

==================

 - x86:

   http://www.microsoft.com/downloads/release.asp?ReleaseID=16186

 - Alpha:

   http://www.microsoft.com/downloads/release.asp?ReleaseID=16187



   NOTE: This and other patches are available from the Microsoft

   Download Center (http://www.microsoft.com/downloads/search.asp?

   Search=Keyword&Value='security_patch'&OpSysID=1)



More Information

================

Please see the following references for more information related to this

issue.

 - Microsoft Security Bulletin MS99-053: Frequently Asked Questions,

   http://www.microsoft.com/security/bulletins/MS99-053faq.asp.

 - Microsoft Knowledge Base (KB) article Q244613,

   IIS 4.0 SSL ISAPI Filter Can Leak Single Buffer of Plaintext,

   http://support.microsoft.com/support/kb/articles/q244/6/13.asp.

   (Note: It may take 24 hours from the original posting of this

   bulletin for this KB article to be visible.)

 - Microsoft Security Advisor web site,

   http://www.microsoft.com/security/default.asp.



Obtaining Support on this Issue

===============================

This is a fully supported patch. Information on contacting Microsoft

Technical Support is available at

http://support.microsoft.com/support/contact/default.asp.



Acknowledgments

===============

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



Revisions

=========

 - December 02, 1999: Bulletin Created.



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