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ClamAV libclamav PeSpin Heap Overflow Vulnerability

14.04.08

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/

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 compressedntuncompressed offset %x, len %xntcompressed offset %x, len %xn", 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.

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.

DETECTION

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

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'. 

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."

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.

DISCLOSURE TIMELINE

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

CREDIT

This vulnerability was reported to iDefense by Damian Put.

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LEGAL NOTICES

Copyright © 2008 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 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.