Clam AntiVirus tnef_attachment() DoS Vulnerability
iDEFENSE Security Advisory 11.04.05
November 4, 2005
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 OOM 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.
VI. VENDOR RESPONSE
The vendor has released clamav 0.87.1 to address this vulnerability. It
is available for download at:
VII. CVE INFORMATION
A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.
VIII. DISCLOSURE TIMELINE
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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X. LEGAL NOTICES
Copyright (c) 2005 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
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There are no warranties with regard to this information. Neither the
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