[Snort-sigs] ASN1 question

Joel Esler jesler at ...435...
Wed Dec 19 16:18:59 EST 2012


On Dec 19, 2012, at 2:51 PM, Eric G <eric at ...3692...> wrote:

> On Dec 18, 2012 3:40 PM, "Patrick Mullen" <pmullen at ...435...> wrote:
> >
> > James,
> >
> > ASN.1 stuff really has to be done using an SO rule.
> 
> I don't mean to thread hijack, but I thought SO rules were used solely for rule obsfucation... your reply to the original question kind of implies more advanced rule logic can be rolled into SO rules, presumably at the expense of some performance in rule processing. Is that correct?
> 
> I'm just trying to strengthen my Snort Kung Fu a bit... didn't know SO rules can be used like that
> 
SO rules are rules written in C.  Their main use is to do things in Snort that you can't do in a regular rule language.  Complex math, loops, advanced comparison stuff.

Someone has went out on the internet and said that SO rules are strictly used for hiding things, spreading FUD around, and it's gotten around so much that you wind up being lied to and having to believe what you are told.

We used to use (and still do for very very few things) SO rules to obfuscate detection that we would receive advanced notice about from certain vendors.  But we haven't put out an obfuscated SO rule for that type of stuff for almost two years. 

We can also put out rules for vulnerabilities that we discover that we've reported that vendors have not fixed yet so that people are protected.  But the primary use of SO rules (and the large majority of SO rules) is for extremely complex detection that can't be done via the Snort rule language, regardless of platform.

We still have been releasing SO rules (with their source code, right in the tarball!) for complex things that no other engine can cover.  We try and avoid SO rules at all costs, but sometimes it's the only correct way to write detection for vulnerabilities, and we have to do it.

--
Joel Esler
Senior Research Engineer, VRT
OpenSource Community Manager
Sourcefire
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