Re: Preventing layer 3/4 evasions

> I'm curious about the market status quo and trends in the area of how 
> network IDS/IPS products are dealing with layer 3/4 evasion techniques 

As far as I've been able to determine, it's in fact difficult to discern
just what IDS/IPS products do about evasion and how effectively.  Many of
them state that they're evasion-resistant, but there aren't enough particulars
to understand how strong the claims might be.  To this end, I'm currently
working with Christian Kreibich and some colleagues on developing a framework
for testing an IDS/IPS for its vulnerability to a variety of layer 3/4/7
evasions, as I think this problem remains under-addressed by vendors and
underappreciated by customers.  If we can work towards some community
evasion benchmarks, this will help provide market pressures to strenghen
products with better evasion resilience.  (Some IDS tests already include
evasion evaluations, but to my knowledge the tests are proprietary and so
it's difficult externally to gauge the significance of the results they
produce.)

> ... The Handley/Paxson/Kreibich paper from Usenix01 lists 
> three approaches (not counting "use a host-based IDS" :-) ):
> 1. inline normalization
> 2. profiling the intranet and using target-specific algorithms
> 3. bifurcating analysis

Note, scheme #3 (as noted in the paper) is fundamentally limited.  There's
also a 4th approach, which is to have the end system work in conjunction
with the NIDS in real-time.  See for example our paper

	H. Dreger, C. Kreibich, V. Paxson and R. Sommer, Enhancing the
	Accuracy of Network-based Intrusion Detection with Host-based
	Context, Proc. Conference on Detection of Intrusions and Malware
	and Vulnerability Assessment (DIMVA) 2005.

	http://www.icir.org/vern/papers/dimva05.pdf

>  From what I've read, Snort is going route #2, with the Sourcefire RNA 
> system doing the profiling.

By the way, we also have a paper on this approach:

	U. Shankar and V. Paxson, Active Mapping: Resisting NIDS Evasion
	Without Altering Traffic, Proc. IEEE Symposium on Security and
	Privacy, May 2003.

	http://www.icir.org/vern/papers/activemap-oak03.pdf

One significant difficulty is the mapping information becoming out
of date due to churn.

> - Does Snort's decision indicate any sort of consensus that #2 is the 
> best approach, or would that be considered controversial?

I would certainly say (speaking from the ivory tower) that there isn't
consensus for #2, and my own leaning is towards #1.

		Vern

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