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AUSCERT External Security Bulletin Redistribution
ESB-2005.0485 -- iDEFENSE Security Advisory 06.29.05
Clam AntiVirus ClamAV Cabinet File Handling DoS Vulnerability
30 June 2005
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AusCERT Security Bulletin Summary
---------------------------------
Product: ClamAV 0.86 and prior
Publisher: iDEFENSE
Operating System: Linux variants
UNIX variants
Impact: Denial of Service
Access: Remote/Unauthenticated
CVE Names: CAN-2005-1923
Original Bulletin: http://www.idefense.com/application/poi/display?id=275
Comment: ClamAV 0.86.1 has been released following this iDEFENSE advisory,
which corrects a denial of service vulnerability in another
decompression library.
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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.
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