Public Vulnerability Reports

Clam AntiVirus tnef_attachment() DoS Vulnerability



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


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.


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.


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


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


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


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


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


The discoverer of this vulnerability wishes to remain anonymous.

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Copyright © 2005 Verisign, Inc.

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