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Thanks to ahodes for the heads up :)

UPDATED: MS02-028 Heap Overrun in HTR Chunked Encoding Could Enable Web Server Compromise

Title: Heap Overrun in HTR Chunked Encoding Could Enable Web Server Compromise (Q321599)
Released: 12 June 2002
Revised: 01 July 2002 (version 2.0)
Software: Internet Information Server
Impact: Run Code of Attacker's Choice
Max Risk: Critical
Bulletin: MS02-028

Reason for Revision:
====================
On June 12, 2002, Microsoft released the original version of this bulletin. On July 1, 2002, the bulletin was updated to revise the severity rating. Specifically, Microsoft has increased the severity rating of this issue to "critical ." The revision is in response to a significant change in the threat environment due to an increased focus on chunked encoding vulnerabilities in general, and the discovery of hostile code attempting to exploit similar vulnerabilities on other platforms. Customers who have already disabled HTR or applied this patch need not take any action. Customers who have not disabled HTR should do so as soon as possible. Alternately, customers who cannot disable HTR should apply the patch immediately.

View: MS02-028

Download locations for this patch:

Download: Microsoft IIS 4.0
Download: Microsoft IIS 5.0

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: Critical
- Intranet systems: Critical
- Client systems: Critical

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/)