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Clam AV Win32-FSG File Handling DoS Vulnerability

09.19.05

BACKGROUND

Clam Antivirus is a GPL anti-virus toolkit for Unix. It is used in
several anti-virus gateways and available for free on multiple platforms
(Unix, Windows, MacOS X, BeOS, etc.).

More information about Clam AntiVirus is available from:

  http://www.clamav.net/

DESCRIPTION

Remote exploitation of a file handling vulnerability in Clam AntiVirus
ClamAV allows attackers to cause a denial of service (DoS) condition.

Fast Small Good (FSG) is a compressor for small Win32 EXE files. Remote
exploitation of an unexpected condition in the decompression of FSG
(v1.33) files allow attackers to cause a denial of service condition.

The vulnerability specifically exists in the unfsg_133() [fsg.c]
function when trying to sort the sections. The sort algorithm does not
expect two sections to have the same Relative Virtual Address (RVA),
leading to an infinite loop (permanently swapping the two sections). The
following code excerpt illustrates this issue:

 int upd = 1;

  [...]

  while (upd) {
    upd = 0;
    for (i = 0; i < sectcount  ; i++) {
      if ( sections[i].rva < sections[i+1].rva )
        continue;

      [...swap 'i' and 'i+1' section...]

      upd = 1;
    }
  }

ANALYSIS

Successful exploitation requires an attacker to send a specially
constructed FSG 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 to system performance.

Remote exploitation can be achieved by sending a malicious file in an
e-mail message or during an HTTP session.

DETECTION

This issue was tested to work with ClamAV 0.86. The vulnerable code
was added in Clam AntiVirus 0.80. All versions between .80 and .86
are believed vulnerable

WORKAROUND

The following patch can be applied to fix the vulnerability:

diff -ruN clamav-0.86.1/libclamav/fsg.c
  clamav-0.86.1-fsg-fix/libclamav/fsg.c
--- clamav-0.86.1/libclamav/fsg.c Thu Jun 23 22:03:12 2005
+++ clamav-0.86.1-fsg-fix/libclamav/fsg.c Tue Jul 12 09:40:18 2005
@@ -235,7 +235,7 @@
     for (i = 0; i < sectcount  ; i++) {
       uint32_t trva,trsz,traw;

-      if ( sections[i].rva < sections[i+1].rva )
+      if ( sections[i].rva <= sections[i+1].rva )
  continue;
       trva = sections[i].rva;
       traw = sections[i].raw;

Windows PE EXE files analysis can be disabled (--no-pe) but this can have
severe impacts on the virus detection functionality.

VENDOR RESPONSE

ClamAV has released version 0.87 to correct this issue.

  http://sourceforge.net/project/showfiles.php?group_id=86638

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2005-2919 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

09/08/2005   Initial vendor notification
09/19/2005   Initial vendor response
09/19/2005   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
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
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