iDefense Security Advisory 04.14.08
http://labs.idefense.com/intelligence/vulnerabilities/
Apr 14, 2008

I. BACKGROUND

Clam AntiVirus is a multi-platform GPL anti-virus toolkit. ClamAV is
often integrated into e-mail gateways and used to scan e-mail traffic
for viruses. It supports virus scanning for a wide variety of packed
Portable Executable (PE) binaries. PeSpin is one of the supported
packer/protectors. For more information visit the vendor's web site at
the following URL.

http://www.clamav.net/

II. DESCRIPTION

Remote exploitation of a heap overflow vulnerability in Clam AntiVirus'
ClamAV, as included in various vendors' operating system distributions,
allows attackers to execute arbitrary code with the privileges of the
affected process.

The vulnerability exists within the code responsible for decompressing
sections within a PE binary packed with the PeSpin executable
protector. See the following excerpt from libclamav/spin.c:

  417    key32 = cli_readint32(ep+0x2fee);
  ...
  427        cli_dbgmsg("spin: Resources (sect%d) appear to be 
compressed\n\tuncompressed offset %x, len %x\n\tcompressed offset %x, 
len %x\n", j, sections[j].rva, key32 - sections[j].rva, key32, 
sections[j].vsz - (key32 - sections[j].rva));
  428
  429        if ( (curr=(char *)cli_malloc(sections[j].vsz)) != NULL ) {
  430          memcpy(curr, src + sections[j].raw, key32 - 
sections[j].rva); /* Uncompressed part */
  431          memset(curr + key32 - sections[j].rva, 0, sections[j].vsz 
- (key32 - sections[j].rva)); /* bzero */

On line 417, a 32-bit value is read from the file into the "key32"
variable. Then a heap buffer is allocated using the "sections[j].vsz"
value on line 429. The "memcpy" call on line 430 then copies data into
the newly allocated buffer.

No validation is performed on the "key32", "sections[j].raw", and
"sections[j].rva" values before they are used in the memory copy
operation. Since these values are under attacker control, this can lead
to an exploitable heap corruption condition.

III. ANALYSIS

Exploitation of this vulnerability results in the execution of arbitrary
code with the privileges of the process using libclamav. In the case of
the clamd program, this will result in code execution with the
privileges of the clamav user. Unsuccessful exploitation results in the
clamd process crashing.

Although it would appear that the following "memset" call will cause a
DoS condition, iDefense Labs confirmed that it is possible to bypass
this call. This is accomplished through manipulating the file such that
the memory layout allows the "sections" structure to be completely
controlled via an overwrite by the "memcpy" call.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in ClamAV
0.92.1. Previous versions may also be affected.

V. WORKAROUND

Disabling the scanning of PE files will prevent exploitation.

  If using clamscan, this can be done by running clamscan with the 
'--no-pe' option.
  If using clamdscan, set the 'ScanPE' option in the clamd.conf file to 
'no'.

VI. VENDOR RESPONSE

The ClamAV team has addressed this vulnerability within version 0.93.
Additionally, the ClamAV team reports, "the vulnerable module was
remotely disabled via virus-db update in March."

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2008-0314 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

03/04/2008  Initial vendor notification
03/06/2008  Initial vendor response
04/14/2008  Coordinated public disclosure

IX. CREDIT

This vulnerability was reported to iDefense by Damian Put.

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X. LEGAL NOTICES

Copyright © 2008 iDefense, 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.

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