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Clam AntiVirus ClamAV Cabinet File Handling DoS Vulnerability

06.29.05

BACKGROUND

Clam AntiVirus is a GPL anti-virus toolkit for Unix.

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.

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.

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

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.

VENDOR RESPONSE

This vulnerability has been patched in clamav 0.86 which is available
for download at:

   http://www.clamav.net/stable.php#pagestart

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.

DISCLOSURE TIMELINE

06/07/2005   Initial vendor notification
06/08/2005   Initial vendor response
06/29/2005   Public disclosure

CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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LEGAL NOTICES

Copyright © 2005 Verisign, Inc.

Permission is granted for the redistribution of this alert
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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.