Title: Heap Overrun in HTR Chunked Encoding Could Enable Web
Server Compromise (Q321599)
Date: 12 June 2002
Software: Internet Information Server
Impact: Run Code of Attacker's Choice
Max Risk: Moderate
Bulletin: MS02-028

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS02-028.asp.

Issue:

This patch eliminates a newly discovered vulnerability affecting
Internet Information Services. Although Microsoft typically delivers
cumulative patches for IIS, in this case we have delivered a patch
that eliminates only this new vulnerability, while completing a
cumulative patch. When the cumulative patch is customer-ready, we
will update this bulletin with information on its availability. The
FAQ provides information on the circumstances surrounding the
vulnerability, and why we believe releasing a singleton patch
immediately is in customers' best interests. To ensure that servers
are fully protected against past as well as current vulnerabilities,
we strongly recommend installing the previous cumulative patch
(discussed in Microsoft Security Bulletin MS02-018) before
installing this patch.

The vulnerability is similar to the first vulnerability discussed
in Microsoft Security Bulletin MS02-018. Like that vulnerability,
this one involves a buffer overrun in the Chunked Encoding data
transfer mechanism in IIS 4.0 and 5.0, and could likewise be used
to overrun heap memory on the system, with the result of either
causing the IIS service to fail or allowing code to be run on the
server. The chief difference between the vulnerabilities is that
the newly discovered one lies in the ISAPI extension that
implements HTR - an older, largely obsolete scripting technology -
where the previous one lay in the ISAPI extension that implements
ASP.

Mitigating Factors:

• Microsoft has long recommended disabling HTR functionality
  unless there is a business-critical reason for retaining it.
  Systems on which HTR is disabled would not be at risk from this
  vulnerability.

• The IIS Lockdown Tool disables HTR by default in all server
  configurations.

• The current version of the URLScan tool provides a means of
  blocking chunked encoding transfer requests by default.

• On default installations of IIS 5.0, exploiting the
  vulnerability to run code would grant the attacker the privileges
  of the IWAM_computername account, which has only the privileges
  commensurate with those of an interactively logged-on
  unprivileged user.

Risk Rating:

• Internet systems: Moderate
• Intranet systems: Moderate
• Client systems: Moderate

Patch Availability:

• A patch is available to fix this vulnerability. Please read the
  Security Bulletin at
  http://www.microsoft.com/technet/security/bulletin/ms02-028.asp
  for information on obtaining this patch.

Acknowledgment:

• eEye Digital Security (http://www.eeye.com/)

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