Re: Views and Correlation in Intrusion Detection

["Stephen P. Berry" <spb () meshuggeneh net>]

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DAVID MARKLE writes:

> An IDS alert is ONLY relevant if the 
> firewall permits the traffic through.  To further the comment, and 
> attack signature tripped for (known attack) xyz, is ONLY relevant when 
> the attacked host is vulnerable to xyz.  This is the ultimate job of 
> correlation.  If the above surrounding conditions are true, the 
> severity of the attack becomes increased to critical, otherwise it is 
> informational only.

I disagree.  If you see six packets from a single source and five of them
match five discrete attack signatures and your NIDS doesn't tell you
anything about the sixth, the smart money says that someone just tried
five attacks you know about and one you don't.  If you're ignoring the
five (because you know you're safe from them), you just missed the sixth
(which is the one you're going to get paged about a couple hours later)[0].

If you're about to suggest that disambiguating this sort of situation
isn't something that most NIDS products do well (or, indeed, at all),
ya got me there.  But, alas, we have not yet found a way to convince
the blackhats to only attack us in such ways as we find convienient
to monitor[1].  The question you've got to ask yourself is what your
NIDS is there for:  to behave such that it does not inconvienience your
incident analysts, or to behave in such a way as to catch the maximum
number of bad guys[2].







- -spb

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0       Random aside:  try to express what I've just described using the
        IDMEF.
1       Certainly not in the general case.  Honeynets and such things
        undoubtedly work for some situations.
2       Of course this is a min/max problem, and neither extreme is
        optimal.

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