K-033: Microsoft "Myriad Escaped Characters" Vulnerability

PROBLEM:       A vulnerability has been identified in Microsoft Internet
               Information Server (IIS), versions 4 & 5. Malicious users could
               embed a large number of escaped character sequences within a
               URL causing a denial of service attack.
PLATFORM:      Any Microsoft Internet Information Server system.
DAMAGE:        A Internet Information Server reading the malformed URL will
               suffer a denial of service attack. Attempting to interpret the
               URL increases the CPU load on a server. The increase in CPU
               load lasts only until the URL is processed by the CPU. A string
               of malformed URL's could make a system unusable for other
               processes, causing a denial of service for users. This
               vulnerability does not have the capability of deleting/adding
               files to the server or to crash the server.
SOLUTION:      Install the patch as provided by Microsoft.
VULNERABILITY  The risk is medium. No system compromise is provided by this
ASSESSMENT:    vulnerability, but the ability to perform a denial of service
               attack does exist.


[  Start Microsoft Advisory  ]

Microsoft Security Bulletin (MS00-023)
- --------------------------------------

Patch Available for "Myriad Escaped Characters" Vulnerability

Originally Posted: April 12, 2000

Summary
=======
Microsoft has released a patch that eliminates a security
vulnerability in Microsoft(r) Internet Information Server. The
vulnerability could allow a malicious user to slow a web server's
response or prevent it from providing service altogether for a period
of time.

Frequently asked questions regarding this vulnerability and the
patch can be found at

http://www.microsoft.com/technet/security/bulletin/fq00-023.asp

Issue
=====
Special characters can be embedded in URLs by use of so-called escaped
character sequences. By providing a specially-malformed URL with an
extremely large number of escaped characters, a malicious user could
arbitrarily increase the work factor associated with parsing the
escaped characters, thereby consuming much or all of the CPU
availability on the server and preventing useful work from being
done.

The vulnerability does not provide any capability to cause the server
to fail, or to add, change or delete data on it. The slowdown would
only last until the URL had been processed, at which point service
would return to normal.

Affected Software Versions
==========================
 - Microsoft Internet Information Server 4.0
 - Microsoft Internet Information Server 5.0

Patch Availability
==================
 - Internet Information Server 4.0
   http://www.microsoft.com/Downloads/Release.asp?ReleaseID=20292
 - Internet Information Server 5.0
   http://www.microsoft.com/Downloads/Release.asp?ReleaseID=20286

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 MS00-023,
   http://www.microsoft.com/technet/security/bulletin/fq00-023.asp
 - Microsoft Knowledge Base article Q254142 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 Vanja Hrustic of the Relay Group for reporting the
"Myriad Escaped Character" vulnerability to us and working with us to
protect customers.






Revisions
=========
 - April 12, 2000: Bulletin Created.

- -------------------------------------------------

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Last updated April 12, 2000

(c) 2000 Microsoft Corporation. All rights reserved. Terms of use.

[  End Microsoft Advisory  ]

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