HTTP-Inspect / Stream4 Reassembly question

[Daniel Purcell <dpurcell () nitrosecurity com>]

List users,

For some reason, my snort rule is not detecting a simple signature.  The 
EICAR virus is a simple, sample virus that can be used to detect
whether or not a virus-detection engine is working or not. It is 68
bytes long. It is in fact an executable (an MS-DOS .com file) that is
also entirely printable ASCII characters (all 68 bytes).  You can see it
here:

        http://www.eicar.org/anti_virus_test_file.htm

I have a rule in the database to detect it. It is really simple (almost
overly so). Here it is:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"EICAR test virus";
content:"X5O!P%@AP[4|5c|PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*";
classtype:not-suspicious;
reference:url,www.eicar.org/anti_virus_test_file.htm; rev:1;)

It is just a rule with one content. I can't get it to detect with my 
2.3.x or 2.2.x copy of snort when  I go to the above
web site. I have found a way, however, to force it to detect it, but it
is pretty bad - I have to turn off _BOTH_ the Stream-4-Reassembly, _AND_
turn off the HTTP_INSPECT preprocessors. Then, it will detect it every
time. Otherwise, it will never detect it. And, it is not just that. I
can't even detect, for example, the "X5" from the front of the string,
or the "EICAR" from the middle of the string without turning those off.

Does the content: tag do some mangling of the packet before it inspects 
it?  I thought that's what uricontent did...

-Daniel

- --
Dyslexics have more fnu.



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