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Re: Bypass Virus Checking
From: nick () VIRUS-L DEMON CO UK (Nick FitzGerald)
Date: Fri, 4 Feb 2000 10:08:11 +1200


Neil wrote about Edward Salm's EICAR.COM comments:

 "The reason I mentioned the other eicar.com is I noticed NAV on my
test machine wouldn't detect your version of eicar.com unless
bloodhound was activated!  When I turned bloodhound heuristics off
(even though autoprotect was sill running), I could put your
eicar.com anywhere on the drive!" Doesn't that give you a good
pointer. Bloodhound seems pretty important. It's also possible that
bloodhound ignores the default exclusions. I have a contact at SARC
whom I'll ask about this and let you all know the response. Oh, and
in case you're wondering, there was only a difference of one byte
between our copies of EICAR.COM. Mine terminated in an <LF>, Ed's in
a <CR><LF>.

As Elias has already commented, the lowdown on the EICAR test string
is at http://www.eicar.org/, although not immediately obvious, as it
once was...  FWIW, here is an extract from the original article/white
paper/whatever that explained the raison d'etre for the existence of
EICAR.COM:

   Any anti-virus product which supports the EICAR test file should
   "detect" it in any file which starts with the following 68 characters:

   X5O!P% () AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*

   [Source: \EICAR\TESTING\testfile.htm from EICAR'99 conference CD]

Note all you who got a virus warning from your Email scanners that
this message is "infected" with the EICAR test string/virus/whatever
that according to the people who made it and its definition, your
product has just false alarmed.  (In fact, it may suggest that your
scanner is doing a lot of brain-dead grunt scanning which means it
will be slow and poor at detecting viruses in general, but that is a
separate discussion we won't start here.)

Note the spec is very clear -- the string above *is* the EICAR test
string.  It does not matter what follows it.  Thus, files consisting
of just those 68 bytes, of them plus CR LF, of them plus LF, of them
plus CR, of them plus <any arbitrary string of characters> should
*all* be detected as the EICAR test string (the last example may be
problematic in some cases, as some scanners may complain about
invalid COM format files based on the size and the fact that the file
is not internally an EXE, if too large a string is appended to the
EICAR string).

Here's an idea. The statement by McAfee that they can't go looking
for XORed files because it's not feasible got me thinking. It seems
to me that it's not feasible because it would take too long. People

Well, it is unfeasible *eventually*.  Note that very similar issues
are faced by scanner developers now with run-time decompressors. How
these are handled by (some) extant scanners is that commonly used
RTDs are detected from the constant-ish stub and/or header structure
the compressor attaches to the compressed EXE.  If such an EXE is
found, it is decompressed and then scanned.  If a common run-time
EXE encryptor evolved (why would it, other than as a lame proof of
concept that "known virus scanners" can only detect known forms of
malware?) then a similar approach would be adopted -- detect the
run-time decryption code, decrypt then scan...

would be annoyed at 2 second waits for their files to open and
whatnot. Now, I'm no AV expert and some even may work like this, but
here's what I came up with. An AV checker could do a real hard look
at a file, doing whatever it needed to be really thorough with the
file (I understand that breaking XOR programmatically is pretty
straight forward). It would then the store an MD5 hash for that file
in an index. Whenever it needed to scan a file, it would just
compare hashes (which is quick), and only re-scan the files if they

In fact, as you suspect, some scanners do implement schemes along
these lines.

had been changed. Special handling would probably be needed for data
files as they get changed all the time, but overall it seems

Where it gets harder is "data" files that *can* also harbour code --
embedded macros in MS Office, MS Project, Viso, AutoCAD, etc files
and embedded scripts in HTML, HLP, I presume CHM, and so on.

reasonable to me. I'd also think that AV scanners could do more
advanced scans in the background with CPU idle cycles. There are a
LOT of spare cycles on the average desktop.

I have been advocating for some time that the scanner developers
should move to a "code integrity management" model, or at least
provide that as an option for their corporate customers.  They
already have tested technology to intercept very low-level file
system processes to inspect file accesses for change vs. straight
reading, complex file format decomposition (to check for and identify
macros in Word documents, etc), and most of the other building blocks
for doing what is needed.  This should be layered this with an access
control system allowing the sys-admin to specify who could run what
*code*.  Because of the failure of OS, browser and productivity app
developers to keep data and code separate, this is *not* the same
thing as file-level permissions and is the necessary improvement over
what MS seems to think is the panacea -- code signing.  Such tools
could actually *prevent* infection of (virtually) all future (i.e.
currently unknown) viruses.  (The weakness is the arrival of new
compound "data and code together" file formats that typically require
intensive reverse engineering before their code resources can be
reliably extarcted and identified, but that is an equal shortcoming
in today's "known virus scanning" technology.  We could partly get
around that as a professional group by simply refusing en masse to
use products whose developers do not provide adequate and timely file
format details to our security product developers -- if no-one is
using a product there cannot be a compelling business reason to do
so, so ill-informed managerial pressure to adopt the product would be
weakened.  Cue the open source chorus...)


--
Nick FitzGerald
Computer Virus Consulting Ltd. (NZ)
Ph/FAX: +64 3 3529854



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