[Snort-sigs] Snort rule for a pattern match?

Shields, Joseph (NIH/NIEHS) [C] joseph.shields at ...3788...
Wed Mar 27 10:45:43 EDT 2013

   Thanks for the advice.  However, the problem is the look ahead needs to do a calculation based on a matched value of a dot (.).  so we are looking for a pattern.  The first value can be an ascii (inc extended ascii) character.  Here is the original posting of the requirement:

I am looking for a pattern that identifies a threat I am tracking and need to write a signature to find it.  The problem is that I don’t know what the starting character will be but I will always know what the difference between two given characters will be.
A simple, human readable, example is:
The difference between each character is:
[A] is 1 SMALLER than [B] is 1 SMALLER than [C] is 1 SMALLER than [D] is 16 SMALLER than [T] is 1 BIGGER than [S] is 1 BIGGER than [R] is 1 BIGGER than [Q]
The pattern in this example is -1,-1,-1,-16,+1,+1,+1.
BCDEXWVU would match this pattern and so would HIJKZXYW.
How can I write this rule?

-----Original Message-----
From: lists at ...3397... [mailto:lists at ...3397...] 
Sent: Wednesday, March 27, 2013 10:40 AM
To: Lay, James; Shields, Joseph (NIH/NIEHS) [C]
Cc: snort-sigs at lists.sourceforge.net
Subject: Re: [Snort-sigs] Snort rule for a pattern match?

On 03/27/2013 09:29 AM, Lay, James wrote:
> James,
>    The traffic could be on most any port, though it likely will be 
> web.  I think PCRE would be possible if the PERL look ahead with calc 
> capability is supported.  I’ve not seen anything showing this 
> implementation.  Namely, (?{ code }).

Look-aheads work, check out SID 2016551 in the ET ruleset.  Check the PCRE with the negated look ahead.

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"ET CURRENT_EVENTS Possible Neutrino EK Downloading Jar"; flow:established,to_server; content:"?h"; http_uri; content:" Java/1."; http_header; fast_pattern; pcre:"/\/[a-z]+\?h(?!ash)[a-z]{5,}=[a-f0-9]{24}$/U"; classtype:trojan-activity; sid:2016551; rev:3;)

Cheers, Nathan

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