> Advisories > rt-sa-2005-013 vertical divider

Sophos does not recognize keylogger after string alteration

During a Penetrationtest RedTeam found out that Sophos Anti-Virus
(SAV for short) won't recognize a keylogger as malware, after
alteration of a string in the keylogger's binary.


Product: Sophos Anti-Virus
Affected Version: <= 5.0.2
Immune Version: None known
OS affected: tested on Win2k, GNU/Linux, probably all supported by
Security-Risk: medium
Remote-Exploit: no
Vendor-URL: http://www.sophos.com
Vendor-Status: informed
Advisory-URL: http://www.redteam-pentesting.de/advisories/rt-sa-2005-013
Advisory-Status: published


"Sophos Anti-Virus provides integrated virus detection on a wide
range of Windows platforms. Our award-winning technology protects
corporate servers, desktops and laptops from viruses, Trojans, worms
and malicious spyware." (from Vendor's page)

SAV fails to recognize a keylogger binary after altering a few bytes
in a string contained in the program.

More Details

During a Penetrationtest, RedTeam wanted to install a keylogger on a
victim's system. Klogger (written by Arne Vidstrom, see [1]) was chosen
because of its small size, simplicity, and the ability to be executed
from the command prompt. Since we knew that SAV was running on the
target system, we did a test in our lab at RWTH-Aachen University. This
test revealed that SAV would recognize the Klogger binary as malicious
and raise alarm.

In a simplistic attempt to confuse SAV, a few bytes in the Klogger
binary (there is no source code available) which belonged to a string
containing the author's name where changed with a hex editor. To our
astonishment this was enough to foil SAV - no alarms where raised for
the modified binary. Apparently the only detection method deployed by
SAV for this binary was a hash comparison or something to the same

Tests with other antivirus programs showed that all of them
recognized the binary even after the string alteration. As for SAV,
additional tests with more popular malware showed that for these,
proper heuristics were used: it was not enough just to change a few
bytes with other malware binaries we tested.

This example shows impressively, how easy some virusscanners can be
bypassed. An attacker just has to spend less than one minute to
manipulate the keylogger to prevent SAV from detecting the file.

As keyloggers are more and more used by criminals like phishers to
get e.g. online-banking data, it is important that protection
software has robust detection mechanisms for malware. Simple
circumvention of protection mechanisms could lead to a severe
information leakage and compromise of the user. It is not uncommon
for malware code to be hex-edited by the entities deploying them or
even to change itself, thus potentially circumventing SAV if this
practice is used with other malicicous code, too.

[1] http://ntsecurity.nu/toolbox/klogger/

Proof of Concept

Just download klogger and change some bytes.


Never rely only on your antivirus program, regardless how good it is.
Those programs can only detect known malware with 100% certainty.
Unknown but also slightly modified malicious code is only recognized
using heuristics, which fail much too often. Always use common sense
and don't execute or even open files you don't exactly know where they
come from.


None known.

Security Risk

As users should not rely only on their antivirus programs (as stated
above) in the first place, the security risk may be seen as medium.


2005-04-14  discovery of SAV's behaviour
2005-04-21  additional tests with other programs
2005-05-10  advisory is written
2005-06-03  contacted Sophos. Answer: the attachement you sent is clean.
            Eh? Apparently, they sent the attached pgp-signature to their
            virus-lab... Asked for a security contact. Got back the
            offer that if we send a file with a virus, they can scan it.
            Okaaaay, that was not the question, was it? Told them we
            were short of viruses, sorry. Contact promised
            to sent the mail to their headquarter in England. Never
            heard from them again.
2005-06-16  Advisory released
2009-05-08 Updated Advisory URL and date format


RedTeam is a penetration testing group working at the Laboratory for
Dependable Distributed Systems at RWTH-Aachen University. You can
find more Information on the RedTeam Project at