Passive NIDS can be evaded via three fundamental classes of ambiguity: incomplete protocol analysis (none of the four commercial systems tested by Ptacek and Newsham in 1998 correctly reassembled IP fragments), divergent end-system behavior (different OS stacks resolve overlapping TCP retransmissions differently), and topology uncertainty (low-TTL packets may not reach the victim end-system, so the NIDS cannot determine which packets are delivered).

An attacker can conduct stealth port scans against a victim without revealing their own IP by exploiting a 'patsy' host whose OS uses a globally incrementing IP Identifier: the attacker observes ID increments of 2 (rather than 1) in the patsy's traffic when the victim sends a RST to the patsy in response to a spoofed SYN, revealing open ports. Choosing a different patsy for each port makes the scan very hard to detect.

§5.1 detection

The user-level norm normalizer processes a realistic 100,000-packet trace (88% TCP) at approximately 101,000 pkts/sec (397 Mb/s) with all normalizations enabled on a $1,000 AMD Athlon 1.1 GHz PC, compared to a memory-copy-only baseline of 727,270 pkts/sec; the authors conclude a kernel implementation could sustain a bidirectional 100 Mbps access link with sufficient headroom to weather high-speed small-packet flooding attacks.

§7.2 evaluation

TCP RSTs are delivered unreliably and different OS stacks apply different validity rules, so a NIDS cannot safely tear down connection state on RST alone; a 'reliable RST' scheme — sending a keep-alive ACK behind every forwarded RST and tearing down state only upon observing a confirming RST from the trusted side — resolves this without violating end-to-end semantics. The cold-start problem (state loss on restart) can be addressed statelessly by stripping payload from unknown-connection packets from untrusted hosts and probing the trusted endpoint with a keep-alive before instantiating state.

§6.1–6.2 defense

A traffic normalizer placed inline ('bump in the wire') can eliminate over 70 IP/TCP packet-level ambiguities before a NIDS inspects traffic — including fragment reassembly, TTL restoration, DF flag clearing, IP option removal, and cryptographic IP ID scrambling — leaving the classifier with an unambiguous byte stream and removing the degrees of freedom an attacker needs to evade detection.

§3, Appendix A defense

Russian TSPU devices directly block ECH by dropping ClientHello messages that contain both an ECH extension and the outer SNI hostname "cloudflare-ech.com" — the static outer SNI Cloudflare advertises in all its ECH configurations. Blocking affects both TLS and QUIC. ECH connections to servers with Cloudflare ECH support but outside Cloudflare's official IP ranges are NOT blocked. TCP segmentation alone or TLS record fragmentation alone did NOT bypass TSPU ECH blocking, but combining both techniques did circumvent it. TSPU has also added TCP reassembly capabilities that defeat previously effective fragmentation-only bypasses.

2025-niere-encrypted §5, Figure 5 dpirst-injectionmiddlebox-interference

The GFI's HTTP and HTTPS filters are now stateful (requiring initial SYN packet with matching sequence numbers) and have been activated on all TCP ports—not only standard ports 80 and 443 as reported by prior studies. This is a significant departure from previous work that found stateless HTTP/HTTPS blocking limited to standard ports. The HTTP filter injects a 403 Forbidden blockpage (not RST packets as used by the GFW), while HTTPS injects a single RST+ACK packet. The GFI also exhibits TCP non-compliance (not requiring a full three-way handshake to trigger filtering), enabling outside-in measurement without in-country servers.

2025-tai-irblock §2.1, §3.2 dpirst-injectionpacket-injectionmiddlebox-interference

The Henan Firewall is stateless in two exploitable ways: (1) it requires the TCP header to be exactly 20 bytes—enabling any TCP option (e.g., TCP Timestamps, which Windows disables by default) to bypass it entirely; (2) it does not perform TCP reassembly, so splitting a TLS ClientHello across two TCP segments such that the SNI extension straddles the boundary bypasses the censor. Both bypasses require only client-side changes and have already been implemented in Xray, GoodbyeDPI, and Shadowrocket. TLS record fragmentation (splitting the ClientHello across multiple TLS records within one TCP segment) also defeats both the Henan Firewall and the GFW, since neither performs TLS reassembly.

2025-wu-regional-censorship §4.3 / §6 sni-blockingdpimiddlebox-interference

Longitudinal GFWeb data spanning 20 months shows the GFW actively patched previously-published evasion findings during the measurement period: overblocking bugs reported in academic papers were fixed, and fragmented-packet reassembly failures that researchers used to bypass blocking were corrected. This demonstrates that the GFW operator monitors published research and iterates on the system in response to disclosed vulnerabilities.

2024-hoang-gfweb Abstract, §5.4, §6 dpimiddlebox-interference

The SAT Móvil app (Mexico's official tax service, 1M+ downloads) consistently fetches its 'Chat' page over cleartext HTTP, exposing citizen ID numbers (CURP, RFC), passwords, and tax documents to any in-path attacker. None of the four major Latin American telco apps (MiTelcel, MiTigo, MiClaro, MiMovistar) implement HSTS on SMS-delivered external links, making all of them uniformly vulnerable to SSL strip downgrade attacks.

2024-kujath-analyzing §7.2, Table 3 dpimiddlebox-interference

China's Great Firewall showed anomalous inconsistency: 13 test vectors produced mixed outcomes—TCP RST injection on some executions and a clean server response on others—with circumvention rates between 10% and 35% across 100 executions per vector. The authors attribute this to heterogeneous GFW infrastructure components applying different HTTP parsing logic, a departure from the GFW's usual consistency.

2024-m-ller-turning §5.2 dpirst-injectionkeyword-filteringmiddlebox-interference