Clam AntiVirus ClamAV Cabinet File Handling DoS Vulnerability iDEFENSE Security Advisory 06.29.05 www.idefense.com/application/poi/display?id=275&type=vulnerabilities June 29, 2005 I. BACKGROUND Clam AntiVirus is a GPL anti-virus toolkit for Unix. II. DESCRIPTION Remote exploitation of an input validation error in Clam AntiVirus ClamAV allows attackers to cause a denial of service condition. The cabinet file format is a Microsoft archive format used for distributing Microsoft software. The vulnerability specifically exists due to insufficient validation on cabinet file header data. The ENSURE_BITS() macro fails to check for zero length reads, allowing a carefully constructed cabinet file to cause an infinite loop. The ENSURE_BITS() macro is executed indirectly through the READ_BITS macro and is called from the mszipd_compress function. The vulnerable code from libclamav/mspack/mszipd.c is detailed below: #define ENSURE_BITS(nbits) do { \ while (bits_left < (nbits)) { \ if (i_ptr >= i_end) { \ if (zipd_read_input(zip)) return zip->error; \ i_ptr = zip->i_ptr; \ i_end = zip->i_end; \ } \ bit_buffer |= *i_ptr++ << bits_left; bits_left += 8; \ } \ } while (0) #define READ_BITS(val, nbits) do { \ ENSURE_BITS(nbits); (val) = PEEK_BITS(nbits); REMOVE_BITS(nbits); \ } while (0) static int zipd_read_input(struct mszipd_stream *zip) { int read = zip->sys->read(zip->input, &zip->inbuf[0], (int)zip->inbuf_size); if (read < 0) return zip->error = MSPACK_ERR_READ; zip->i_ptr = &zip->inbuf[0]; zip->i_end = &zip->inbuf[read]; return MSPACK_ERR_OK; } int mszipd_decompress(...) { [...] do { READ_BITS(i, 8); if (i == 'C') state = 1; else if ((state == 1) && (i == 'K')) state = 2; else state = 0; } while (state != 2); [...] } Modification of the cffile_FolderOffset to 0xff in a CAB file can cause a read of zero, resulting in the infinite loop. III. ANALYSIS ClamAV is used in a number of mail gateway products. 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 infinate loop will cause the ClamAV software to use all available processor resources, resulting in a denial of service or severe degradation to system performance. Remote exploitation can be achieved by sending a malicious file in an e-mail message or during an HTTP session. IV. DETECTION iDEFENSE has confirmed the existence of this vulnerability in ClamAV 0.83. It is suspected that all versions of ClamAV prior to 0.86 are vulnerable. The following vendors distribute susceptible ClamAV packages within their respective operating system distributions: FreeBSD Project: FreeBSD 5.4 and prior Gentoo Foundation Inc.: Gentoo Linux 1.1a, 1.2, 1.4, 2004.0, 2004.1 and 2004.2 Mandrakesoft SA: Mandrakelinux 10.1 and Corporate Server 3.0 Novell Inc.: SuSE Linux 9.1, 9.2 V. WORKAROUND As a temporary workaround solution, scanning of archive files can be disabled by supplying the --no-archive flag; however, this will significantly impact the effectiveness of the AV engine. VI. VENDOR RESPONSE This vulnerability has been patched in clamav 0.86 which is available for download at: http://www.clamav.net/stable.php#pagestart VII. CVE INFORMATION The Common Vulnerabilities and Exposures (CVE) project has assigned the name CAN-2005-1923 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org), which standardizes names for security problems. VIII. DISCLOSURE TIMELINE 06/07/2005 Initial vendor notification 06/08/2005 Initial vendor response 06/29/2005 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@idefense.com 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.