Public Vulnerability Reports

Clam AntiVirus Cabinet-file handling Denial of Service Vulnerability

11.04.05

BACKGROUND

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

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.

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.

DETECTION

iDEFENSE has confirmed this vulnerability on ClamAV 0.86.1. All previous
versions are suspected vulnerable to this issue.

WORKAROUND

Archive file analysis can be disabled (--no-archive) but this can have
severe impacts on the virus detection functionality.

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

CVE INFORMATION

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.

DISCLOSURE TIMELINE

10/07/2005   Initial vendor notification
10/12/2005   Initial vendor response
11/04/2005   Coordinated public disclosure

CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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LEGAL NOTICES

Copyright © 2005 Verisign, Inc.

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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
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There are no warranties with regard to this information. Neither the
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