Clam AntiVirus tnef_attachment() DoS Vulnerability iDEFENSE Security Advisory 11.04.05 http://www.idefense.com/application/poi/display?type=vulnerabilities November 4, 2005 I. BACKGROUND Clam AntiVirus is a GPL anti-virus toolkit for Unix. II. 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. III. 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 OOM handler will terminate a task to alleviate the stress on the system. IV. DETECTION iDEFENSE has confirmed this vulnerability on ClamAV 0.86.1. All previous versions are suspected vulnerable to this issue. V. WORKAROUND 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: http://prdownloads.sourceforge.net/clamav/clamav-0.87.1.tar.gz?download 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 IX. CREDIT The discoverer of this vulnerability wishes to remain anonymous. Get paid for vulnerability research http://www.idefense.com/poi/teams/vcp.jsp Free tools, research and upcoming events http://labs.idefense.com X. LEGAL NOTICES Copyright (c) 2005 iDEFENSE, Inc. Permission is granted for the redistribution of this alert electronically. It may not be edited in any way without the express written consent of iDEFENSE. If you wish to reprint the whole or any part of this alert in any other medium other than electronically, please email customerservice (at) idefense (dot) com [email concealed] for permission. 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.