[UNIX] Clam AntiVirus Multiple DoS

From: SecuriTeam (support_at_securiteam.com)
Date: 11/06/05

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      Clam AntiVirus Multiple DoS
    ------------------------------------------------------------------------

    SUMMARY

    " <http://www.clamav.net/> Clam AntiVirus is a GPL anti-virus toolkit for
    UNIX."

    Exploitation of a design error in Clam AntiVirus ClamAV allows attackers
    to cause a denial of service (DoS) condition.

    DETAILS

    Vulnerable Systems:
     * ClamAV version 0.86.1

    Immune Systems:
     * ClamAV version 0.87.1

    Cabinet-file DoS:
    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.

    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.

    tnef_attachment() DoS:
    Remote exploitation of a design error in Clam AntiVirus ClamAV allows
    attackers to cause a denial of service (DoS) condition.

    The vulnerability specifically exists in the tnef_attachment function
    within tnef.c. A user controlled value is used to fseek into the file that
    is being processed; this allows a user to specify the same block
    for scanning repeatedly, thus leading to an infinite loop. The following
    one line code snippet from the tnef processor demonstrates the flaw:

    fseek(fp, (long)(offset + length), SEEK_SET); /* shouldn't be needed */

    When this code is reached, length is supplied by the file being processed.
    This makes it possible to seek back to the previous header, causing the
    infinite loop to begin. Furthermore, if the block is marked as an
    attachment, the data that is repeatedly read will be added to a
    dynamically allocated memory buffer, making memory exhaustion trivial.

    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 DoS or severe degradation of system
    performance. Ultimately, the OOM handler will terminate a task to
    alleviate the stress on the system.

    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>
    http://prdownloads.sourceforge.net/clamav/clamav-0.87.1.tar.gz?download

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

    ADDITIONAL INFORMATION

    The information has been provided by
    <mailto:idlabs-advisories@lists.idefense.com> iDEFENSE Labs .
    The original article can be found at:
    <http://www.idefense.com/application/poi/display?id=333&type=vulnerabilities> http://www.idefense.com/application/poi/display?id=333&type=vulnerabilities,
     
    <http://www.idefense.com/application/poi/display?id=334&type=vulnerabilities> http://www.idefense.com/application/poi/display?id=334&type=vulnerabilities

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