Sophos Anti-Virus Zip File Handling DoS Vulnerability

iDEFENSE Security Advisory 07.14.05
www.idefense.com/application/poi/display?id=283&type=vulnerabilities
July 14, 2005

I. BACKGROUND

Sophos Small Business Suite includes the Sophos PureMessage Small
Business Edition, combining virus and spam protection for the email
gateway, and Sophos Anti-Virus Small Business Edition, which offers
desktop and server defense against the virus threat. More information
is available at:

    http://www.sophos.com

II. DESCRIPTION

Remote exploitation of a denial of service vulnerability in
Sophos Plc.'s Sophos Anti-Virus engine allows attackers to exhaust CPU
resources on the target system and prevent further scans.

The problem specifically exists in the handling of Zip files compressed
using the BZIP2 algorithm. When scanning within a BZIP2 archive the
Sophos engine will not perform any sanity checks on the 'Extra field
length' value. By specifying an abnormally large value for this field
the analysis routine is forced into an infinite loop leading to CPU
exhaustion. The following is a hex dump of an example file that will
trigger the issue:

00  50 4b 03 04 0a 00 00 00  0c 00 5c 84 67 32 5b 26  |PK........\.g2[&|
10  b9 09 01 00 00 00 01 00  00 00 03 00 ff ff 66 6f  |..............fo|
20  6f 55 54 09 00 03 a0 74  2c 42 a1 74 2c 42 55 78  |oUT....t,B.t,BUx|
30  04 00 e8 03 e8 03 2a 50  4b 01 02 17 03 0a 00 00  |......*PK.......|
40  00 00 00 5c 84 67 32 5b  26 b9 09 01 00 00 00 01  |...\.g2[&.......|
50  00 00 00 03 00 0d 00 00  00 00 00 01 00 00 00 a4  |................|
60  81 00 00 00 00 66 6f 6f  55 54 05 00 03 a0 74 2c  |.....fooUT....t,|
70  42 55 78 00 00 50 4b 05  06 00 00 00 00 01 00 01  |BUx..PK.........|
80  00 3e 00 00 00 37 00 00  00 00 00                 |.>...7.....|
      
Bytes 08-09: Compression method - 0x000c == BZIP2 algorithm
Bytes 28-29: Extra field length - 0xffff
      
The function BZ2_decompress(), among others, is called within the
infinite loop.

III. ANALYSIS
      
Successful exploitation allows remote attackers to trigger a denial of
service condition on the target system. Remote exploitation can be
achieved by sending a malicious file in an e-mail message or during an
HTTP session. Exploitation is only possible if the 'Scan inside archive
files' option is enabled. This is not a default option but is a common
setting. Both Unix and Windows systems running a vulnerable version of
Sophos Anti-Virus are affected.

IV. DETECTION

iDEFENSE has confirmed the existence of this vulnerability against
Sophos Anti-Virus version 5.0.1. Exploitation is reliant on the
non-default option "Scan inside archive files" found under the
"On-access scanning" configuration panel. It has been reported that
Sophos Anti-Virus versions 3.91 and 3.90 are also vulnerable. It is
suspected that earlier versions are also susceptible to exploitation.
The vulnerability exists across both the Unix and Microsoft Windows
platforms.

V. WORKAROUND

Disable the "Scan inside archive files" option until a vendor fix is
made available.

VI. VENDOR RESPONSE

"We have now included protection against this vulnerability into the
latest versions of Sophos Anti-Virus on Windows, UNIX, Linux, Netware,
OpenVMS and Mac OSX (version 3.95.0 on all those platforms as well as
version 5.0.4 on Windows 2000/XP/2003 and versions 4.5.3 on Windows NT
and Windows 95/98/Me) as well as into the latest updated versions of
PureMessage for UNIX, PureMessage for Windows/Exchange and MailMonitor
(any version running anti-virus engine 2.30.4 or above)

All of these versions would be updated automatically to customers using
our automatic updating through EM Library or direct from Sophos.

They are also available to customers to download at
http://www.sophos.com/support/updates"

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2005-1530 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

05/12/2005  Initial vendor notification
05/16/2005  Initial vendor response
07/14/2005  Public disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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X. LEGAL NOTICES

Copyright (c) 2005 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.