Microsoft Security Bulletin (MS00-027)

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Patch Available for "Malformed Environment Variable" Vulnerability

Originally Posted: April 20, 2000

Summary

Microsoft has released a patch that eliminates a security
vulnerability in Microsoft(r) Windows NT(r) 4.0 and Windows(r) 2000.
The vulnerability could allow a malicious user to make some or all of
the memory on an affected server unavailable, potentially slowing or
stopping an affected server's response time.

Frequently asked questions regarding this vulnerability
and the patch can be found at
http://www.microsoft.com/technet/security/bulletin/fq00-027.asp

Issue

CMD.EXE, the command processor for Windows NT 4.0 and Windows 2000,
has an unchecked buffer in part of the code that handles environment
strings. Although we believe that it could not be exploited to run
arbitrary code, it could be used to mount denial of service attacks
in certain cases. If a server provides batch or other script files, a
malicious user could potentially provide arguments that would create
an extremely large environment string and overflow the buffer. This
would cause the process to fail, and the memory allocated to the
process would not be made available again until a dialogue had been
cleared on the operator's console. By repeatedly running the batch
file, the malicious user could potentially make some or all of the
memory on the server temporarily unavailable.

As noted above, Microsoft believes that arbitrary code cannot be made
to execute via this vulnerability. The machines most likely to be
affected would be web servers, as they are the most likely types of
machines to offer batch files for use by remote users. However, even
an otherwise-affected web server would not be vulnerable to this
problem if an operator were present at the console to clear the error
dialogue promptly.

Affected Software Versions

• Microsoft Windows NT 4.0 Workstation
• Microsoft Windows NT 4.0 Server
• Microsoft Windows NT 4.0 Server, Enterprise Edition
• Microsoft Windows NT 4.0 Server, Terminal Server Edition
• Microsoft Windows 2000 Professional
• Microsoft Windows 2000 Server
• Microsoft Windows 2000 Advanced Server

Patch Availability

• Windows NT 4.0:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=20494
• Windows 2000:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=20503

Note Additional security patches are available at the Microsoft
Download Center

More Information

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

• Frequently Asked Questions: Microsoft Security Bulletin MS00-027,
  http://www.microsoft.com/technet/security/bulletin/fq00-027.asp
• Microsoft Knowledge Base article Q259622 discusses this issue
  and will be available soon.
• Microsoft TechNet Security web site,
  http://www.microsoft.com/technet/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 thanks David Litchfield of Cerberus Information Security,
Ltd (http://www.cerberus-infosec.co.uk/) for reporting this issue to
us and working with us to protect customers.

Revisions

• April 20, 2000: Bulletin Created.

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Last updated April 20, 2000

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