Date: Tue, 25 Oct 2005 22:47:04 +0530
From: "Debasis Mohanty" <mail@...kingspirits.com>
To: "'Andrey Bayora'" <andrey@...urityelf.org>,
	<full-disclosure@...ts.grok.org.uk>
Cc: <bugtraq@...urityfocus.com>
Subject: RE: [Full-disclosure] Multiple Vendor Anti-Virus Software DetectionEvasion Vulnerability through forged magic byte


Hello Andrey, 
Few comments on this - 
Correct me if I am wrong, "forged magic byte" might not always be able to
fool the AV in real scenario (especially EXEs) unless you are talking about
Static Virus scanners. In past few years the AV scanning technology has
improved a lot and has gone even beyond "heuristic scanning techniques".


>> The problem exists in the scanning engine - in the routine that
determines the file type. 
>> If some file types (file types tested are .BAT, .HTML and .EML) changed
to have the MAGIC BYTE 
>> of the EXE files (MZ) at the beginning, then many antivirus programs will
be unable to detect 
>> the malicious file. It will break the normal flow of the antivirus
scanning and many existent 
>> and future viruses will be undetected.

Especially in case of EXEs, AFAIK not all EXEs has the same 'MAGIC BYTE'
(MZ). MZ only appears in the first two bytes of Win32 executable files. Most
older file types such as original .com files, any Linux/Mac files, and
almost all scripting files do not contain MZ in the header. In fact the
EICAR test virus which can be represented as a .txt or a .com file is one
such file. It is a fully executable .com file that does not contain the MZ
bytes and still executes on Win32. This implies that the AV scan engine
doesn't just rely on the 'magic byte'. Changing the magic byte might fool
the static AV scanners and maybe some current Avs but this might not work in
case of real Viruses. As the scan engine do a heuristic scan and doesn't
just rely upon the magic byte. I published a paper on similar topi
"Anti-Virus Evasion Techniques" almost a year back which talks about various
evasion techniques. It can be downloaded from here :
http://hackingspirits.com/eth-hac/papers/whitepapers.asp


As I haven't tested your finding on real viruses so can't say if at all I am
wrong especially incase comments related to EXEs. However, in anycase if
this exploit works for real viruses then this will imply that heuristic scan
is a Joke ;-). Although heuristics can be fooled by many advance techniques
(eg - obfuscation / polymorphism) but if it is fooled by this technique then
I believe there are lot of work waiting for Guys @ AV Schools ;-)


- Tr0y (www.hackingspirits.com) 





 

-----Original Message-----
From: full-disclosure-bounces@...ts.grok.org.uk
[mailto:full-disclosure-bounces@...ts.grok.org.uk] On Behalf Of Andrey
Bayora
Sent: Tuesday, October 25, 2005 8:38 AM
To: full-disclosure@...ts.grok.org.uk
Cc: bugtraq@...urityfocus.com
Subject: [Full-disclosure] Multiple Vendor Anti-Virus Software
DetectionEvasion Vulnerability through forged magic byte

Multiple Vendor Anti-Virus Software Detection Evasion Vulnerability through
forged magic byte.



AUTHOR: Andrey Bayora (www.securityelf.org)



For more details, screenshots and examples please read my article "The Magic
of magic byte" at www.securityelf.org . In addition, you will find a sample
"triple headed" program which has 3 different 'execution entry points',
depending on the extension of the file (exe, html or eml) - just change the
extension and the SAME file will be executed by (at least) THREE DIFFERENT
programs! (thanks to contributing author Wayne Langlois from
www.diamondcs.com.au).

DATE: October 25, 2005



VULNERABLE vendors and software (tested):



1.  ArcaVir 2005 (engine 2005-06-03,vir def 2005-06-27, scanner ver
2005-03-06, package ver 2005-06-21)

2.  AVG 7 (updates 24 June, ver.7.0.323, virus base 267.8.0/27)

3.  eTrust CA (ver 7.0.1.4, engine 11.9.1, vir sig. 9229)

4.  Dr.Web (v.4.32b, update 27.06.2005)

5.  F-Prot (ver. 3.16c, update 6/24/2005)

6.  Ikarus (latest demo version for DOS)

7.  Kaspersky (update 24 June, ver. 5.0.372)

8.  McAfee Internet Security Suite 7.1.5 (updates 25 June, ver 9.1.08,
engine 4.4.00, dat 4.0.4519 6/22/2005)

9.  McAfee Corporate (updates 25 June, ver. 8.0.0 patch 10, vir def 4521,
engine 4400)

10. Norman ( ver 5.81, engine 5.83.02, update 2005/06/23)

11. TrendMicro PC-Cillin 2005 (ver 12.0.1244, engine 7.510.1002, pattern
2.701.00)

12. TrendMicro OfficeScan (ver7.0, engine 7.510.1002, vir pattern 2.701.00
6/23/2005)

13. Panda Titanium 2005 (updates 24 June, ver 4.02.01)

14. UNA - Ukrainian National Antivirus (ver. 1.83.2.16 kernel v.265)

15. Sophos 3.91 (engine 2.28.4, virData 3.91)



IMPORTANT NOTE:

Similar vulnerability may exist in many other antivirus\anti-spyware desktop
and gateway products. In addition, various "file filter" solutions may be
affected as well.



NOT VULNERABLE vendors and software (tested):



1.  F-Secure (updates 24 June, ver 5.56 b.10450)

2.  Avast (ver. 4.6.655, vir databas 0525-5 06/25/2005)

3.  BitDefender (ver. 8.0.200, update 6/24/2005, engine 7.01934)

4.  ClamWin (ver. 0.86.1, upd 24 June 2005)

5.  NOD32 (updates 24 June, ver 2.50.25, vir database 1.1152)

6.  Symantec Corporate (ver 10.0.0.359, engine 103.0.2.7)

7.  Norton Internet Security 2005 (ver 11.5.6.14)

8.  VBA32 (ver 3.10.4, updates 27.06.2005)

9.  HBEDV Antivir Personal (ver 6.31.00.01, engine 6.31.0.7, vir def
6.31.0.109 6/24/2005)

10. Sophos 5 (ver. 5.0.2, vir def 3.93, upd 6/30/2005)

11. Sophos 3.95 (engine 2.30.4)



SEVERITY: critical



DESCRIPTION:



The problem exists in the scanning engine - in the routine that determines
the file type. If some file types (file types tested are .BAT, .HTML and
.EML) changed to have the MAGIC BYTE of the EXE files (MZ) at the beginning,
then many antivirus programs will be unable to detect the malicious file. It
will break the normal flow of the antivirus scanning and many existent and
future viruses will be undetected.



NOTE: In my test, I used the EXE headers (MZ), but it is possible to use
other headers (magic byte) that will lead to the same effect.



ANALYSIS:



Some file types like .bat, .html and .eml can be properly executed even if
they have some "unrelated" beginning. For example, in the case of .BAT files
- it is possible to prepend some "junk" data at the beginning of the file
without altering correct execution of the batch file. In my tests, I used
the calc.exe headers (first 120 bytes - middle of the dosstub section) to
change 5 different files of existing viruses. In addition, the simplest test
of this vulnerability is to prepend only the magic byte (MZ) to the existing
malicious file and check if this file is detected by antivirus program.



NOTE, that this is NOT the case where the change of existing virus file
resulted in the "broken" detection signature (see details and the test logic
in "The Magic of magic byte" article at www.securityelf.org).



WORKAROUND:

I did not found any effective one besides of patching the vulnerable engine.



CREDITS:

The idea for this vulnerability came during discussions from Wayne Langlois
at diamondcs.com.au, who hinted that JPEGs could probably be exploited in
this way.



TIME LINE:



July 13, 2005 - Initial vendor notification

July 16, 2005 - Second vendor notification

.....Waiting.....Waiting....

October 24, 2005 - Public disclosure (uncoordinated)



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