Sophos Antivirus CHM File Heap Overflow Vulnerability

iDefense Security Advisory 12.08.06
http://labs.idefense.com/intelligence/vulnerabilities/
Dec 08, 2006

I. BACKGROUND

Sophos AntiVirus offers protection from the latest Trojans, worms and
Viruses. More information is available on the vendors site at:

http://sophos.com/products/

II. DESCRIPTION

Sophos AntiVirus Engine is vulnerable to a Heap Overflow attack when
scanning malformed CHM archives.

Specifically, if the CHM file has a Window_size of 0 set in a LZX
decompression header then memory corruption will occur.

III. ANALYSIS

Malformed CHM files can lead to a heap overflow condition on the local
machine. Files received as email attachments, uploaded via web forms or
otherwise saved to disk may trigger this condition if auto scanning is
enabled on the target machine. Archive scanning is disabled by default and
must be specified in order for this vulnerability to trigger.

IV. DETECTION

iDefense has confirmed this vulnerability in the following Sophos Antivirus
products:

 * Sophos Antivirus for Linux
   Product version: 4.03
   Engine version : 4.05

Previous versions of Sophos Antivirus are suspected vulnerable.  Other
Sophos Antivirus products may also be vulnerable.

Windows versions of the Sophos Antivirus product do not appear to be
vulnerable.

V. WORKAROUND

Disabling the option to scan within archives will prevent exploitation of
this vulnerability.

VI. VENDOR RESPONSE

Sophos has addressed this problem with new versions of their products. See
http://www.sophos.com/support/knowledgebase/article/7609.html for more
information.

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2006-5646 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

09/28/2006  Initial vendor notification
10/02/2006  Initial vendor response
12/08/2006  Coordinated public disclosure

IX. CREDIT

This vulnerability was reported to iDefense by Damian Put
<pucik@overflow.pl>.

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X. LEGAL NOTICES

Copyright © 2006 iDefense, Inc.

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Disclaimer: The information in the advisory is believed to be accurate at
the time of publishing based on currently available information. Use of
the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct, indirect,
or consequential loss or damage arising from use of, or reliance on, this
information.