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Microsoft Security Bulletin (MS99-021)
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Patch Available for "CSRSS Worker Thread Exhaustion" Vulnerability
Originally Posted: June 23, 1999
Summary
=======
Microsoft has released a patch that eliminates a vulnerability in the
Microsoft(r) Windows NT(r) CSRSS process that could be used to create a
denial of service condition against a machine that allows interactive
logons.
Frequently asked questions regarding this vulnerability, its background,
and the patch can be found at
http://www.microsoft.com/security/bulletins/ms99-021faq.asp
Issue
=====
If all worker threads in CSRSS.EXE are occupied awaiting user input, no
other requests can be serviced, effectively causing the server to hang.
When user input is provided, processing returns to normal. The patch
eliminates the vulnerability by ensuring that the last CSRSS worker thread
services only requests that do not require user input.
Affected Software Versions
==========================
- Microsoft Windows NT 4.0 Workstation
- Microsoft Windows NT 4.0 Server
- Microsoft Windows NT 4.0 Server, Enterprise Edition
Patch Availability
==================
The patch is available at ftp://ftp.microsoft.com/bussys/winnt/
winnt-public/fixes/usa/nt40/Hotfixes-PostSP5/CSRSS-fix/
NOTE: A line break has been inserted into the above URL for readability.
More Information
================
Please see the following references for more information related to this
issue.
- Microsoft Security Bulletin MS99-021: Frequently Asked Questions,
http://support.microsoft.com/support/bulletins/ms99-021faq.asp.
- Microsoft Knowledge Base (KB) article Q2133323,
"Exceeding MaxRequestThreads may Cause Windows NT to Hang",
http://support.microsoft.com/support/kb/articles/q231/3/23.asp.
(Note: It may take 24 hours from the original posting of this bulletin
for the 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 Jeff Sumner for discovering this vulnerability and
reporting it.
Revisions
=========
- June 23, 1999: Bulletin Created.
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