Cyber Risks · Cisco Talos

Inline Normalization with Snort 2.9.0

Summary: Snort 2.9.0 adds a new preprocessor for inline deployments that normalizes packets and streams so Snort models end systems more accurately. The examples show TTL normalization raising a packet's TTL from 4 to 8 when the minimum is set to 5, and TCP stream normalization overwriting overlapping retransmitted segments so the server reassembles the stream only one way.

Bottom line: Without stream normalization, overlapping segments at relative sequence numbers 6 through 10 would reach the server twice, and what the application receives depends on that server's implementation-specific TCP reassembly policy - a difference an attacker can exploit and a defender cannot control.

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