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Multiple Vendor ClamAV CAB File Denial of Service Vulnerability

02.15.07

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.

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.

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.

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.

WORKAROUND

iDefense is unaware of any effective workarounds for this issue.

VENDOR RESPONSE

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

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.

DISCLOSURE TIMELINE

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

CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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LEGAL NOTICES

Copyright © 2006 Verisign, Inc.

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