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

Lay, James james.lay at ...3513...
Wed Mar 27 10:29:18 EDT 2013


Brian,

 

Yea I'm not sure on that, so I'll defer to the much smarter folks in
this group J

 

James

 

From: Shields, Joseph (NIH/NIEHS) [C] [mailto:joseph.shields at ...3788...] 
Sent: Tuesday, March 26, 2013 1:56 PM
To: Lay, James; snort-sigs at lists.sourceforge.net
Subject: RE: [Snort-sigs] Snort rule for a pattern match?

 

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 }).  

 

Brian

 

From: Lay, James [mailto:james.lay at ...3513...] 
Sent: Tuesday, March 26, 2013 3:29 PM
To: snort-sigs at lists.sourceforge.net
Subject: Re: [Snort-sigs] Snort rule for a pattern match?

 

From: Shields, Joseph (NIH/NIEHS) [C] [mailto:joseph.shields at ...3788...] 
Sent: Tuesday, March 26, 2013 12:02 PM
To: snort-sigs at lists.sourceforge.net
Subject: Re: [Snort-sigs] Snort rule for a pattern match?

 

I'm reposting this question as I have not seen any responses yet.
Perhaps this can't be done at this time.

 

Brian

 

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:

 

ABCDTSRQ

 

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?

 

Brian


Brian,

 

What port are we talking here?  If this is port 80 then ick, but if it's
something obscure it could be as simple as a pcre and we could forego
the computations:

 

pcre:"/[A-Z]{7}/";

 

Got a pcap or is there ANYTHING that's a constant that we can also match
on?

 

James

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