Methodologies to avoid detection by IDS involve one of three types of techniques: evasion, insertion and denial of service. These are defined and described in the landmark paper "Insertion, Evasion, and Denial of Service:Eluding Network Intrusion Detection", released back in 1998 by Thomas Ptacek and Timothy Newsham. A new intrusion analyst would do well to read this paper, mostly still relevant after 13 years, and keep a copy for reference.
An insertion attack is where an IDS accepts a packet that the destination host rejects.
An evasion attack is where an IDS rejects a packet that the destination host will accept.
An example of an insertion attack would be where an IDS does not calculate TCP checksums, or does not calculate them correctly. An attacker sends a series of packets containing an exploit to host A. Within that stream of packets is one with a bad TCP checksum. If IDS B does not detect that the checksum failed, it will use that packet when it reassembles the stream and check it against it's signatures and processors. The match will fail because of the extra payload data contained in the bad packet, and the IDS will miss the attack.
Host A, however, will drop the packet with a bad checksum, reassemble the packets, and the exploit will be launched. The IDS never saw the attack. This is an unlikely scenario in 2011, but the next one I'll discuss involves at least one modern IDS that is still vulnerable to an evasion attack outlined over a decade ago.
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