Public Vulnerability Reports

Clam AntiVirus tnef_attachment() DoS Vulnerability

11.04.05

BACKGROUND

Clam AntiVirus is a GPL anti-virus toolkit for Unix.

DESCRIPTION

Remote exploitation of a design error in Clam AntiVirus ClamAV allows
attackers to cause a denial of service (DoS) condition.

The vulnerability specifically exists in the tnef_attachment function
within tnef.c. A user controlled value is used to fseek into the file
that is being processed; this allows a user to specify the same block
for scanning repeatedly, thus leading to an infinite loop. The following
one line code snippet from the tnef processor demonstrates the flaw:

fseek(fp, (long)(offset + length), SEEK_SET);  /* shouldn't be needed */ 

When this code is reached, length is supplied by the file being
processed. This makes it possible to seek back to the previous header,
causing the infinite loop to begin. Furthermore, if the block is marked
as an attachment, the data that is repeatedly read will be added to a
dynamically allocated memory buffer, making memory exhaustion trivial.

ANALYSIS

Successful exploitation requires an attacker to send a specially
constructed CAB file through a mail gateway or personal anti-virus
client utilizing the ClamAV scanning engine.

The infinite loop will cause the ClamAV software to use all available
processor resources, resulting in a DoS or severe degradation of system
performance. Ultimately, the Out of Memory handler will terminate a task
to alleviate the stress on the system.

DETECTION

iDEFENSE has confirmed this vulnerability on ClamAV 0.86.1. All previous
versions are suspected vulnerable to this issue.

WORKAROUND

Archive file analysis can be disabled (--no-archive) but this can have
severe impacts on the virus detection functionality.

VENDOR RESPONSE

The vendor has released clamav 0.87.1 to address this vulnerability. It
is available for download at:

  http://prdownloads.sourceforge.net/clamav/clamav-0.87.1.tar.gz?download

CVE INFORMATION

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.

DISCLOSURE TIMELINE

10/07/2005   Initial vendor notification
10/12/2005   Initial vendor response
11/04/2005   Coordinated public disclosure

CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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LEGAL NOTICES

Copyright © 2005 Verisign, 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
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