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Re: Triggering a complex snort rule (packet forging)
From: Asiri Rathnayake <asiri.rathnayake () gmail com>
Date: Tue, 2 Apr 2013 23:01:04 +0100

Following reply was sent only to wkitty by mistake. Re-sending it to the
list just for reference.

Thanks all!

- Asiri

On Tue, Apr 2, 2013 at 5:21 PM, Asiri Rathnayake <asiri.rathnayake () gmail com
wrote:

Hello,

On Tue, Apr 2, 2013 at 4:36 PM, waldo kitty <wkitty42 () windstream net>wrote:

On 4/2/2013 07:28, Asiri Rathnayake wrote:
May be I should've been more specific, sorry about that. I need to
trigger the
rule from the outside, without depending on the client.

your rule requires an "established" connection so there has to be another
end of
the pipeline... the "server" is one end but where is the data going if
there is
no client involved?

it may be possible, as others have pointed out, to simulate it via
constructed
pcaps, though... not really something i'd want to attempt unless there is
a tool
that can easily generate such a pcap of sufficient size... i'm not aware
of one
but others may be...

my initial gut reaction says the /easiest/ method would be to use a
scripted
client and a remote server...


I agree with you on all the points. However, I have a specific requirement
of being able to trigger the rule from the outside.

This requirement came from a research we're currently working on:

http://www.cs.bham.ac.uk/~hxt/research/rxxr.shtml

It's possible for rule writers to introduce vulnerable regular expressions
into their PCRE rules which could be exploitable. We found several such
rules but all of them seem to be looking at some sort of response traffic.

This is why I started investigating if it's possible to trigger those
rules without involving a client. If I can figure out a way to trigger the
rules that way, then I might be able to send malicious packets to a snort
protected network and see how snort will handle the situation.

I didn't want to go into these details because our research is very
specific. But may be I over-simplified the problem by trying to avoid
talking about it.

It seems what I'm trying to do is extremely uncommon, and the usual
approach is to get some support from the client. Having the support from
the client would work really well for testing this kind of rules, but as
far as I can understand, it wouldn't help much if I'm trying to
(repeatedly) trigger a rule from the outside.

Many thanks for all of your inputs!

- Asiri

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