[Full-disclosure] Re: Multiple Vendor Anti-Virus Software Detection Evasion Vulnerability through forged magic byte

[Andrey Bayora]

Dear Ken,

> If your altered virus sample
> still executes correctly, you have simply created a new virus
> variant.

Not exactly, please look at this virustotal.com log
http://www.securityelf.org/updmagic.html

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.

Regards,
Andrey Bayora.


Quoting "Williams, James K" <James.Williams at ca.com>:

>
>> Subject: Re: Multiple Vendor Anti-Virus Software Detection
>> Evasion Vulnerability through forged magic byte
>> From: "Andrey Bayora" <andrey () securityelf ! org>
>> Date: 2005-10-25 3:07:51
>>
>> [...]
>>
>> VULNERABLE vendors and software (tested):
>>
>> [...]
>>
>> 3.  eTrust CA (ver 7.0.1.4, engine 11.9.1, vir sig. 9229)
>>
>> [...]
>> 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.
>
> Andrey,
>
> Thank you for the report.
>
> You are effectively altering existing viruses to the point that
> AV scanners do not detect them.  If your altered virus sample
> still executes correctly, you have simply created a new virus
> variant.  If your altered virus sample does not execute properly,
> you have created nothing more than a corrupt virus sample.
>
> Consequently, the issue that you describe is *not* a
> vulnerability issue, but rather just an example of a new variant
> that has not yet been added to an AV vendor's database of "known
> viruses".
>
> Note that CA eTrust Antivirus, when running in Reviewer mode,
> should already detect these new variants.
>
> Regards,
> Ken
>
> Ken Williams ; Dir. Vuln Research
> Computer Associates ; 0xE2941985
>
>