[Snort-sigs] snort rule tuning and weeding out false positives

Alex Kirk akirk at ...435...
Thu Mar 17 08:57:55 EDT 2011

Tuning is always one of the more interesting parts of setting up Snort. The
good news is, you've already essentially stated the guiding principle of the
process: use only what you actually need.

The easy first step - beyond using PulledPork to start out with one of the
VRT-recommended default policies - is to turn off entire categories of rules
that don't apply; for example, if you've got no Oracle servers to defend,
oracle.rules stays off. From there, depending on how confident you are in
your environment's patching process, you can go through and start turning
off older rules; as a rule of thumb, anything more than 5 years old is
pretty much a case of "default is off, until proven otherwise". You can also
go ahead and disable individual rules for software you don't have.

That said, as for the particular event you're getting hits on - if you want
to send some sample events over to me directly, I'd be curious to see if
they look like legit attacks or if we need to tune the rule.

On Thu, Mar 17, 2011 at 8:30 AM, Youngquist, Jason R. <jryoungquist at ...3574......
> wrote:

>  Hi.
> I’m new to Snort and am trying to weed out false positives and ignore
> events that are purely informational.  I was wondering if anyone could
> recommend any good howtos/docs/methodology on how to go about Snort rule
> tuning.
> For example, I have this rule, sid 17441, which is “CHAT MSN Messenger and
> Windows Live Messenger Code Execution attempt” and have gotten it several
> times from multiple student machines.  It appears that this was an issue
> back in 2007 with Microsoft MSN Messenger 6.2, 7.0, and 7.5.  Right now,
>  Windows Live Messenger 2011 seems to be the current version.  So assuming
> that this is a false positive and I can ignore this rule.  Thoughts?
> Thanks.
> Jason Youngquist
> Information Technology Security Engineer
> Technology Services
> Columbia College
> 1001 Rogers Street, Columbia, MO  65216
> (573) 875-7334
> jryoungquist at ...3572...
> http://www.ccis.edu
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Alex Kirk
AEGIS Program Lead
Sourcefire Vulnerability Research Team
alex.kirk at ...435...
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