Clam AntiVirus ClamAV MS-Expand File Handling DoS Vulnerability 

iDEFENSE Security Advisory 06.29.05
www.idefense.com/application/poi/display?id=276&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 vulnerability specifically exists due to improper behavior during 
exceptional conditions. The following piece of code from 
libclamav/scanners.c shows where the vulnerability exists: 


    static int cli_scanszdd(...)
    {
      [...]
      FILE *tmp = NULL, *in;

      cli_dbgmsg("in cli_scanmscomp()\n");

      if((in = fdopen(dup(desc), "rb")) == NULL) {
        cli_dbgmsg("SZDD: Can't open descriptor %d\n", desc);
        return CL_EMSCOMP;
      }

      if((tmp = tmpfile()) == NULL) {
        cli_dbgmsg("SZDD: Can't generate temporary file.\n");
        fclose(in);
        return CL_ETMPFILE;
      }

      if(cli_msexpand(in, tmp) == -1) {
        cli_dbgmsg("SZDD: msexpand failed.\n");
        return CL_EMSCOMP;
      }

      [...]
    }


Each time the cli_msexpand() function fails , two file descriptors ('in'

and 'tmp') are leaked. As they are both opened using fopen(), there is 
also a memory leak (2 * 364 bytes). This allows an attacker to exhaust 
all of the available file descriptors fairly quickly with around 1,000 
malformed files.

III. ANALYSIS

Successful exploitation allows attackers to exhaust file descriptors 
pool and memory. Anti-virus detection functionality will fail if there 
is no file descriptors available with which to open files. 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-2005-1922 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.