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Date: Fri, 08 Dec 2006 17:56:23 -0500
From: iDefense Labs <>
Subject: iDefense Security Advisory 12.08.06: Multiple
 Vendor Antivirus RAR File Denial of Service Vulnerability

Multiple Vendor Antivirus RAR File Denial of Service Vulnerability

iDefense Security Advisory 12.08.06
Dec 08, 2006


AntiVirus products typically handle searching files for known viruses
within their scan engines.  Most scan engines support searching inside of
known archive types for viruses as well.  For more information refer to
any of the popular AntiVirus vendors' web sites.


Remote exploitation of a denial of service vulnerability in Multiple
Vendors' Antivirus engines allows an attacker to cause the engines to
consume excessive resources.

The affected vendors' scan engines are vulnerable to a DoS attack when
scanning specially malformed RAR archives. Specifically, the malformed
archives will have the head_size and pack_size fields set to zero in
Archive Header section. When such a file is encountered, the affected scan
engines will enter an infinite loop.


Successful exploitation will allow an attacker to cause the affected scan
engine to consume excessive CPU, and in some cases memory, resources. The
malicious RAR file would need to be uploaded to a server to initiate the
attack. Several common ways this could be achived are e-mail attachments,
available network shares, FTP accounts, or Web form uploads.

The impact of the vulnerability varies slightly from vendor to vendor as
described below.

Scanning of archives is not enabled by default and must be specified by
the user. This denial of service attack will prevent the scanner from
scanning other files on disk while it is stuck on the exploit file. The
hung process can be stopped by the user.

Trend Micro:
Once attacked, the scan engine will consume 99 percent of CPU resources and
the affected computer will require a reboot to recover from the condition.
The scan engine process cannot be forced to quit, although its thread
priority can be lowered to regain some use of the system before reboot.


iDefense has confirmed this vulnerability exists in the following vendors'
products. This should not be considered an exhaustive list as these vendors
tend to include the scan engine in many of their products. Previous
versions are likely to be affected as well.

 * Sophos Small business edition (Windows/Linux) 4.06.1 with
   engine version 2.34.3.
 * Trend Micro PC Cillin - Internet Security 2006
 * Trend Micro Office Scan 7.3
 * Trend Micro Server Protect 5.58


For Sophos' scan engine, this exploit will not have any effect if the
"Enabled scanning of archives" option is not set. iDefense is currently
unaware of a workaround for this issue for the remaining vendor's engines.


Sophos has addressed this problem with new versions of their products. See for more

Trend Micro stated that this vulnerability does not affect version 8.320 of
their Windows scan engine. Additionally, they have released version 8.150
of the HPUX and AIX builds of their scan engine to address this problem in
those environments.


The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2006-5645 to this issue. This is a candidate for inclusion in the
CVE list (, which standardizes names for security


09/27/2006  Initial vendor notifications
09/27/2006  Initial vendor response - Trend Micro
09/28/2006  Initial vendor response - Sophos
12/08/2006  Coordinated public disclosure


The vulnerability was reported by Titon of BastardLabs, Damian Put
<>, and an anonymous researcher.

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Copyright © 2006 iDefense, Inc.

Permission is granted for the redistribution of this alert electronically.
It may not be edited in any way without the express written consent of
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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

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