Microsoft Security Bulletin (MS99-061)

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



Patch Available for "Escape Character Parsing" Vulnerability

Originally Posted: December 21, 1999



Summary

=======

Microsoft has released a patch that eliminates a vulnerability in

Microsoft(r) Internet Information Server and products that run atop it. The

vulnerability could allow files on a web server to be specified using an

alternate representation, in order to bypass access controls of some

third-party applications.



Frequently asked questions regarding this vulnerability can be found at

http://www.microsoft.com/security/bulletins/MS99-061faq.asp.



Issue

=====

RFC 1738 specifies that web servers must allow hexadecimal digits to be

input in URLs by preceding them with the so-called "escape" character, a

percent sign. IIS complies with this specification, but also accepts

characters after the percent sign that are not hexadecimal digits. Some of

these translate to printable ASCII characters, and this could provide an

alternate means of specifying files in URLs.



The vulnerability does not affect IIS; even specifying a file name via this

alternate method does not bypass IIS' access controls. However, third-party

software that runs atop IIS but does not perform canonicalization is

affected by it.



Affected Software Versions

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

 - Microsoft Internet Information Server 4.0

 - Microsoft Site Server 3.0

 - Microsoft Site Server Commerce Edition 3.0



Patch Availability

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

 - Intel:

   http://www.microsoft.com/Downloads/Release.asp?ReleaseID=16357

 - Alpha:

   http://www.microsoft.com/Downloads/Release.asp?ReleaseID=16358



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 MS99-061,

   http://www.microsoft.com/security/bulletins/MS99-061faq.asp.

 - Microsoft Knowledge Base (KB) article Q246401,

   IIS may improperly parses specific escape characters,

   http://support.microsoft.com/support/kb/articles/q246/4/01.asp.

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

   for this KB article to be visible.)

 - RFC 1738,

   Uniform Resource Locators,

   http://www.ietf.org/rfc/rfc1738.txt.

 - 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 the ACROS Security Team, Slovenia, for bringing this

issue to our attention.



Revisions

=========

 - December 21, 1999: Bulletin Created.



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