Multiple Vendor ClamAV CAB File Denial of Service Vulnerability

iDefense Security Advisory 02.15.07
http://labs.idefense.com/intelligence/vulnerabilities/
Feb 15, 2007

I. BACKGROUND

Clam AntiVirus is a multi-platform GPL anti-virus toolkit. The main purpose
of which is integration into electronic mail servers. More information
about ClamAV can be found at http://clamav.net/. Microsoft CAB files are
the native compressed file format for Windows.

II. DESCRIPTION

Remote exploitation of a resource consumption vulnerability in Clam
AntiVirus' ClamAV allows attackers to degrade the service of the clamd
scanner.

The vulnerability specifically exists due to a file descriptor leak.  When
clam encounters a cabinet header with a record length of zero it will
return from a function without closing a local file descriptor. This can
be triggered multiple times, eventually using up all but three of its
available file descriptors. This prevents clam from scanning most
archives, including zip and tar files.

III. ANALYSIS

Exploitation allows attackers to degrade the functionality of the ClamAV
virus scanning service. Exploitation requires that attackers send a
specially constructed CAB file through an e-mail gateway or personal
anti-virus client using the ClamAV scanning engine.

When ClamAV is unable to scan an archive successfully because it has run
out of descriptors, it will return an error status. Several mail servers
that use clam were tested to see how they handled this status. Exim, as of
version 4.50, features an option to build clamd support into it. It will
reject a mail if clamd fails to scan it properly. Amavisd will also deny a
mail that clamd cannot properly scan. This vulnerability can be used to
deny service to users trying to send legitimate archives through the mail
gateway.

IV. DETECTION

iDefense has confirmed this vulnerability affects Clam AntiVirus ClamAV
v0.90RC1.1. All versions prior to the 0.90 stable release are suspected
to be
vulnerable.

V. WORKAROUND

iDefense is unaware of any effective workarounds for this issue.

VI. VENDOR RESPONSE

Clam AntiVirus has addressed this vulnerability in the version 0.90 stable
release.

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-0897 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

02/07/2007  Initial vendor notification
02/13/2007  Initial vendor response
02/15/2007  Coordinated public disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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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.