bugtraq - Re: Verified evasion in Snort

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Date: 1 Feb 2006 21:22:01 -0000
From: mwatchinski@...rcefire.com
To: bugtraq@...urityfocus.com
Subject: Re: Verified evasion in Snort

This and other target base fragmentation evasions are the reason we re-wrote the fragmentation engine in Snort.

If you look at Judy Novak's Frag3 Development paper, Snort's latest fragmentation engine (frag3) supports target-based fragmentation policies for overlaps, ttl evasions, and timeouts. This can be configured on a per IP basis to allow exact emulation of how the end host handles fragmentation reassembly.

Here is a sample configuration that could be used for frag3. This configuration would handle the evasion outlined in the advisory.  This configuration is based on the 5 second timeout used in the PoC code provided.

	preprocessor frag3_engine: policy first \
	bind_to 10.2.1.0/24 \
	timeout 5 \
	detect_anomalies

>From our testing, Windows XP actually has a 1 minute timeout for fragments. The actual configuration to handle this evasion would be the following:

	preprocessor frag3_engine: policy first \
	bind_to 10.2.1.0/24 \
	timeout 60 \
	detect_anomalies

For the VRT's detailed analysis of the PoC tool and the advisory please see:

http://www.snort.org/rules/docs/vrt/evasion_snort_v233.html

Cheers,
Matthew Watchinski
Director, Vulnerability Research
Sourcefire, Inc.