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Security Bulletin

Microsoft Security Bulletin MS99-061 - Critical

Patch Available for "Escape Character Parsing" Vulnerability

Published: December 21, 1999

Version: 1.0

Originally Posted: December 21, 1999

Summary

Microsoft has released a patch that eliminates a vulnerability in Microsoft® 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 https://www.microsoft.com/technet/security/bulletin/fq99-061.mspx.

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

Vulnerability Identifier: CVE-2000-0024

Patch Availability

More Information

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

  • Frequently Asked Questions: Microsoft Security Bulletin MS99-061, https://www.microsoft.com/technet/security/bulletins/fq99-061.mspx..

  • Microsoft Knowledge Base (KB) article 246401, IIS may improperly parses specific escape characters, https:.
    (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, </https:>https:.

  • Microsoft Security web site, </https:>https:.

Obtaining Support on this Issue

This is a fully supported patch. Information on contacting Microsoft Technical Support is available at </https:>https:.

Acknowledgments

Microsoft acknowledges ACROS Security Team, Slovenia, for bringing this issue to our attention.

Revisions

  • December 21, 1999: Bulletin Created.

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