Web security vulnerabilities whose exploitation depends on parser divergence between two co-located systems are structurally isomorphic to censorship circumvention attacks, where the censor acts as the frontend parser and the destination server as the backend. The authors demonstrated this by directly converting all HRS test vectors from prior security research into circumvention probes with no modification, showing that censorship-circumvention techniques can be systematically constructed from existing vulnerability corpora.

China's GFW exhibited unusually inconsistent HTTP censorship behavior: 13 of the evaluated HRS test vectors circumvented the GFW in some executions but not others, with per-vector success rates between 10% and 35% across 100 executions per domain. The authors attribute this to two distinct parts of GFW infrastructure employing different HTTP censorship mechanisms, a departure from the GFW's typical consistency.

§5.2 (China paragraph), §5 intro detection

HTTP request smuggling (HRS) vectors that exploit CL/TE header parsing divergence between a censor-as-middlebox and a destination web server can circumvent HTTP censorship in China, Iran, and Russia. Of 4,488 test vectors derived from prior HRS research, 2,015 (44.9%) were accepted by at least one web server; CL*/TE vectors achieved a 99.0% web-server acceptance rate while TE/CL* vectors achieved 0%.

§3, §5.1, Table 1 defense

Iran's censor injects an HTTP block page consistently but contains an implementation bug: it fails to parse the TE header when a CL header with an invalid (non-integer) value is present, causing it to pass subsequent traffic. 254 of the evaluated test vectors circumvented Iran's censor; the 'Wrapping' CL*/TE strategy (e.g., 'Content-Length: <len>\u00FF\x0aX: X') was especially effective, exploiting this graceful-degradation fault.

§5.3 (Wrapping strategy, Iran discussion) detection

The Russian censor at the tested Moscow vantage point (ASN 50867, China Unicom-equivalent private ISP) inspects only the first HTTP packet of the first TCP segment in a TCP stream and never blocks a second HTTP request, whether coalesced in the same TCP packet or sent in a subsequent one. All 2,015 web-server-accepted test vectors evaded Russian censorship, including standard-compliant whitespace-injection vectors (e.g., 'Content-Length\x20: <len>\x20').

§5.2 (Russia paragraph) 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

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