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Category:   Application (Security)  >   Sophos Anti-Virus Vendors:   Sophos
Sophos Anti-Virus Engine Infinite Loop in Processing BZIP2 Archives Lets Remote Users Deny Service
SecurityTracker Alert ID:  1014488
SecurityTracker URL:
CVE Reference:   CVE-2005-1530   (Links to External Site)
Updated:  Jun 15 2008
Original Entry Date:  Jul 15 2005
Impact:   Denial of service via network
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): prior to versions 3.95.0, 4.5.3, and 5.0.4
Description:   iDEFENSE reported a vulnerability in the Sophos Anti-Virus engine. A remote user can cause denial of service conditions.

The software does not properly validate the 'Extra field length' parameter value in BZIP2 archives. A remote user can create a specially crafted archive that, when processed by the anti-virus engine, will cause the engine to enter an infinite loop. This will CPU utilization on the target system to reach 100%, preventing further scans.

This can be exploited via e-mail or a web page.

The vendor was notified on May 12, 2005.

The original advisory is available at:

Impact:   A remote user can cause CPU utilization on the target system to reach 100%, preventing further scans.
Solution:   The vendor has issued fixed versions (3.95.0 on Windows, UNIX, Linux, Netware, OpenVMS and Mac OSX; version 5.0.4 on Windows 2000/XP/2003; and versions 4.5.3 on Windows NT and Windows 95/98/Me).

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) also include the fix.

Updates are available at:

Vendor URL: (Links to External Site)
Cause:   Input validation error
Underlying OS:  Linux (Any), OpenVMS, UNIX (Any), Windows (Any)

Message History:   None.

 Source Message Contents

Subject:  iDEFENSE Security Advisory 07.14.05: Sophos Anti-Virus Zip File

Sophos Anti-Virus Zip File Handling DoS Vulnerability

iDEFENSE Security Advisory 07.14.05
July 14, 2005


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:


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  ||
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.

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.


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


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


"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"


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 (, which standardizes names for
security problems.


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


The discoverer of this vulnerability wishes to remain anonymous.

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Copyright (c) 2005 iDEFENSE, Inc.

Permission is granted for the redistribution of this alert
electronically. It may not be edited in any way without the express
written consent of iDEFENSE. If you wish to reprint the whole or any
part of this alert in any other medium other than electronically, please
email for permission.

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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