Of 4,488 total HTTP Request Smuggling test vectors, 2,015 (44.9%) were accepted by at least one web server. CL*/TE vectors had a 99.0% acceptance rate (1,103/1,114); TE*/CL had 76.0% (859/1,130); CL/TE* had only 4.7% (53/1,130); and TE/CL* had 0%. Nginx 1.25.2 accepted 1,315 vectors while Apache 2.4.57 accepted only 11, reflecting HRS countermeasures added in Apache 2.4.25 and 2.4.52.

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.

§5.2 detection

HTTP Request Smuggling—a web-security vulnerability that exploits CL/TE header parsing ambiguities between a front-end (censor) and back-end (web server)—can be systematically repurposed as a censorship circumvention technique. By hiding a censored Host in the body of a benign outer request, the censor parses only the uncensored outer request while the destination server processes both, successfully bypassing HTTP censorship in China (19 vectors), Iran (254 vectors), and Russia (all 2,015 vectors) from the evaluated vantage points.

§3 / §8 defense

Iran's censor contains an implementation bug: when the Content-Length header carries an invalid (non-integer) value and a Transfer-Encoding header is also present, the censor gracefully skips the invalid CL value and attempts to parse subsequent traffic, but fails to correctly interpret the TE header—causing it to pass the smuggled (censored) request. This bug enabled 254 of 2,015 evaluated test vectors to bypass Iranian censorship, all using the CL*/TE or CL/TE* vector types.

§5.2 / §5.3 detection

Russia's censor (at the Moscow/ASN-50867 vantage point) inspects only the first HTTP packet of the first TCP segment per TCP stream and never analyzes subsequent HTTP requests—whether in the same TCP packet or a later one. This caused all 2,015 accepted test vectors to successfully evade censorship, and the bypass is achievable with standard-compliant HTTP (e.g., whitespace or case variations in header names, which HTTP/1.1 explicitly permits).

§5.2 / §5.3 detection

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

TCP-compliant packet alphabets are insufficient for modeling stateful firewall evasion. Including non-TCP-compliant traffic — specifically flipped-direction SYNs, out-of-window seq/ack numbers, and packets that form a parallel TCP connection in the reverse direction — is what unlocks discovery of deep attack paths. Prior model-inference work (Alembic) that restricted itself to compliant sequences produced models incapable of generating any of the 6,000+ attacks Pryde found.

2024-moon-pryde §4.1–§4.2 middlebox-interferencedpi