[Snort-sigs] Why not the otherway around??

Edwin Eefting edwin at ...149...
Wed Oct 17 07:15:19 EDT 2001


Chris Green wrote:
> 
> Edwin Eefting <edwin at ...149...> writes:
> 
> > Ok..let me explain the weird subject. The rules that currently generate
> > the most false positives are rules like this:
> >
> > #alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS 80 (msg:"WEB-IIS cmd.exe
> > access"; flags: A+; content:"cmd.exe"; nocase; classtype:attempted-user;
> > sid:1002; rev:1;)
> >
> > (you can even see i commented it out ;-)
> > These rules have two huge disadvantages:
> > 1)They are triggered by worms VERY often. (serveral thousand events/hour
> > at our isp.)
> 
> Now, yes they are.  They have been used for a lot more than just IIS
> worms though in the past.
> 
> > 2)It only indicates an ATTEMPT, not a succes.
> 
> Tags are part of what I use to see what the outcome of the attmept
> is.  Great feature :)

Yes, it's great for investigation. (i currently use a modified version
of demarc) But the cmd.exe and alike rules fill up our database to
200,000 events or more in a couple of days. :-)

> >
> > For succesfull attacks we offcourse have the "attack response
> > rules".  (very usefull indeed!)  infected. This way, once a server
> > is infected, there's no way to detect there is something wrong!
> > (because they don't generate those nice attack responses anymore)
> 
> Right but part of IDS is the initial attack recognition. It's not
> everything.  The tags help use it more as a passive psuedo
> vulnerabilty detector.

Offcourse, but the network i'm monitoring is already existing and may
contain some hosts that are already infected. Some of the hosts are ADSL
connections for normal homeuse, and we also want to be alerted if these
people are infected in someway.

> >
> > But we are lucky! The worm wants to spread itself, so it's doing
> > "cmd.exe access" and similar thing to other ip adresses. The problem is
> > that this will NOT be logged by rules like the one I mentioned above.
> > Therefore I currently turning some rules around:
> >
> > alert tcp $HTTP_SERVERS  any -> $EXTERNAL_NET 80 (msg:"P-1-INFECTED worm
> > cmd.exe access"; flags: A+; content:"cmd.exe"; nocase;
> > classtype:attempted-user; sid:1002; rev:1;)
> 
> Right - I do tons of outgoing rules for things like this and popular
> exploits.  Since the rule is different thgouh, you should use a
> different sid.
> 
> We probably should try and add a lot more "outgoing rules" to the
> default ruleset to make life easier on people.

That's where I'm waiting for :)

> >
> > This rule almost never generates false positives and should be able to
> > detect infected servers.
> 
> Save for one of cnet's business channels that created a false positves
> has a cgi named "admin-cmd.exe" or something like that.  Complained to
> the address on the webpage it came up on and bounced around for a
> while but I Don't think they ever renamed it.
> 
> To lower this type of false positive rate, I have thought about
> creating something like "passcontent: admin_cmd.exe" that would work
> after a rule is going to be decalred successful and work at a certain
> depth/offset ( preferably with lots of knowledge about the previous
> content rule so that it doesn't create a gaping hole in your ruleset
> that easily ).
can't you use the dynamic rules for this? (they don't seem to be used
very often it seems)

> 
> > Everytime a worm on one of your servers try's to spread it self, it
> > should be triggered.  Is my story correct, or am I missing something
> > here? If it is, then why nobody else thought about it?? :-)
> 
> Lots of people have.  Perhaps there should be an "outgoing.rules" in
> snort that contains the list of popular attacks but it does fall into
> one of the things that is part of tuning your own ruleset b/c thats
> probably too many rules for a lot of folks to run.

Well maybe it is, but it's hard to investigate every rule and find out
if it needs to be "turned-around" somehow, and perhaps needs a higher
priority.
It would be nice if these rules would already exists and that people can
disable them at will. (demarc is very good in doing this)

> 
> > btw. I'm security engineer at a large(?) ISP and snort has to analyse a
> > continuous flow of 3Mbytes/s. So you can imagine that rules with too
> > much false positives just aren't usefull to me.
> 
> What I do is mine out what I Don't want and only investigate a certain
> subset.  It's not really setup to be tied to my pager though :>
Well I defined some "code-reds" that are going to be tied to my gsm. :-)

> 
> --
> Chris Green <cmg at ...26...>
> "Yeah, but you're taking the universe out of context."

--                            __________________
                             /\ ___/          
Edwin Eefting               /- \ _/  Business Internet Trends BV
                           /--- \/           __________________




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