RE: [Full-Disclosure] Exploiting Multiple Flaws in Symantec Antivirus 2004 for Windows Mobile

["Matthew J. Brown" <matthew.brown@xxxxxxxxx>]

Don Cheatham writes:
> Vulnerability #2: The Virus scanner does not appear to work at all!
> 
> [snip]
> An AV scanner should be able to detect a byte stream anywhere in the
> file, but Symantec is easily bypassed with this rudimentary trick.

This is not a valid test.  Virus scanners search for viruses in ways
that vary from virus to virus.  They generally do NOT search for
possible remnants of virus code scattered randomly within data files;
that's a recipe for false positives all over the place.  False positives
are painful.

Some viruses do indeed insert random (but valid) instructions in front
of their unvarying section (instructions whether in machine code, or
macro language, or whatever, depending on the environment in which the
virus runs).  This is a very rudimentary form of disguise.  Others
'encrypt' their viral code (generally by some simple XOR scheme or the
like) with a varying key, so that there is no easily detectable common
code.

Techniques for dealing with all these vary, but the one thing that's
almost universal is that the detection parameters for each virus in the
virus scanner encode how this particular virus is found in files.  Thus,
the search string for the EICAR test file quite probably explicitly
encodes that this is only valid at the START of a file, to reduce the
false positive likelihood.  This does NOT mean that the virus scanner
would be incapable of detecting a virus that did pad in front of itself
with a random instruction stream; in that case, the flags for that virus
string would be set differently.

The only valid test of a virus scanner is its detection of real viruses
-- which the EICAR file is not in the first place.  The EICAR test file
is not a virus but merely a functionality-tester, and all scanners that
support it guarantee to report if they find THE PRECISE EICAR FILE and
not otherwise.  In other words, Symantec is performing exactly as
specified and exactly as it should.

-Matt, who once upon a time worked in the AV field

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