Clam AntiVirus Cabinet-file handling Denial of Service 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 with the libmspack library, which is included in ClamAV. The vulnerability can be found in the cabd_find function within mspack/cabd.c: for (offset = 0; offset < flen; offset += length) { length = flen - offset; [... read length from file ...] for (p = &buf[0], pend = &buf[length]; p < pend; ) { switch (state) { [ ... ] case 19: => header has been completely read, cablen has been read from bytes 8-11 [...] /* likely cabinet found -- try reading it */ cab = sys->alloc(sys, sizeof(struct mscabd_cabinet_p); => an mscabd_cabinet_p entry will be allocated each time /* cabinet read correctly! */ offset = caboff + cablen; => if cablen (and caboff) == 0, offset will be equal to 0 [...] /* restart search */ [...] length = 0; p = pend; state = 0; => if offset == 0, we will restart at the beginning of the file => (because length == 0, so offset will still be zero in the next iteration) break; } } } If this function is called with a length value of zero, an infinite loop occurs. The comments above explain the scenario. 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 denial of service or severe degradation of system performance. Ultimately, the OOM handler will terminate a task in order 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.