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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