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Date: Fri Oct 28 07:05:42 2005
From: x at (x)
Subject: Re: Multiple Vendor Anti-Virus Software Detection
	Evasion Vulnerability through forged magic byte

Andrey Bayora said:
+ > If your altered virus sample
+ > still executes correctly, you have simply created a new 
+ > virus variant.
+ Not exactly, please look at this log
+ The altered (120 bytes prepended) TXT_* variant is STILL 
+ detected by your product (CA), but when I change the first 
+ byte from "Z" to "M" - your product fails (MZ_* variant). 
+ I believe, that if I PREPEND 120 bytes to known virus and 
+ the virus is still detected with the SAME signature - 
+ then I DID NOT create a new variant. Now one more example: 
+ try to change the first byte "Z" in the TXT_* variant to 
+ any value, but not to "M" - this virus will be detected, 
+ but when you change to "M", thus creating the .EXE magic 
+ byte - the variant is not detected !!!  My conclusion: 
+ the antivirus ?thought? that the file is the executable 
+ type instead of determining the file type by the 
+ extension.
+ That is my point, if you still think that your product is 
+ OK - do not do anything.

Thierry Zoller said:
+ WJK> You are effectively altering existing viruses to the 
+ WJK> point that AV scanners do not detect them.
+ No, he is changing a few bytes only.
+ WJK> If your altered virus sample still executes 
+ WJK> correctly, you have simply created a new virus 
+ WJK> variant.
+ No, there is no variant, the virus executes EXACTLY as 
+ before. A variant acts differenlty then a precedent 
+ version, else it would be no variant. To your AV engine it 
+ is a variant, yes, but only because it is flawed.

Why are you guys having such a difficult time comprehending 
this?  Read both the general and AV-specific definitions of 
the word "variant".

If you take an existing virus and modify it, you have 
created a variant.

The AV vendors aren't going to patch their products if they 
don't detect your PoC; they're just going to write a new 
signature or modify an existing signature to detect your 
new variants.  The fact that it can and will be fixed by 
AV signatures instead of product patches should help you 
figure out if this is a product vulnerability issue or just
a "new virus variant" issue.

BTW, Andrey, did you bother to use the "deep scan", 
"heuristic mode", "reviewer mode", etc to see if any
of those AV scanners picked up your new variants?  I bet
you didn't.

Thierry Zoller said:
+ WJK> Consequently, the issue that you describe is *not* a
+ WJK> vulnerability issue, but rather just an example of a 
+ WJK> new variant that has not yet been added to an AV 
+ WJK> vendor's database of "known viruses".
+ Thank you James, this _to my knowledge_ (perhaps the guy 
+ from vmyths knows better) is the first time the complete 
+ failure of todays AV solutions is shown naked publicaly 
+ directly by a representant of an AV company. This 
+ statement coming from a AV vendor is simply exposing what 
+ is known in the sec. community since many years.

To say that an AV scanner is a "complete failure" because it 
fails to detect a variant you just created is inane.  Each 
of the top 10 AV scanners detects well over 95% of all known 
viruses.  The AV scanners aren't perfect, but they 
definitely make a BIG BIG difference wrt malware risk 

Thierry Zoller said:
+ The solution was to make the engines a bit "smarter", i.e 
+ analyse the header to determine the type and then ONLY 
+ apply the signatures/heuristics which apply to the type of 
+ the file (i am not speaking about the extension of the 
+ file here) thus speeding up the process. Changing the 
+ header just makes the smart engines look...well...  a bit 
+ dumb in my regards.

There are two types of people in the world:  those who 
complain about problems, and those who find solutions to
problems.  Where's your superior AV scanner?

x @ bos

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