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.

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

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

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.

§3, §8 (Conclusions) defense

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.

2024-m-ller-turning §3 / §8 dpikeyword-filteringpacket-injection

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.

2024-m-ller-turning §5.2 / §5.3 dpipacket-injectionkeyword-filtering

FilterMap identified 90 blockpage clusters from 90 vendors and actors across 103 countries using 374 million measurements from ~45,000 vantage points against 18,736 sensitive domains; 87 of these signatures were previously unknown. Commercial filters were detected in 36 out of 48 countries rated 'Not Free' or 'Partly Free' by Freedom House, with Fortinet alone present in at least 60 countries.

2020-raman-measuring §V dpikeyword-filteringpacket-injection

During the June 2025 shutdown, Iranian authorities blocked international One-Time Password (OTP) SMS delivery, preventing new sign-ins to foreign secure-messaging platforms and VPN services. This forced users toward government-approved domestic platforms that lack security and privacy protections. The blockade of OTPs effectively weaponized account-recovery flows as a secondary shutdown layer, disproportionately affecting users who needed to activate new circumvention tools during the crisis.

2025-iran-shutdown-measurement §Human Rights Implications port-blockingdpikeyword-filtering

Input blocking in Chinese LLM services (DeepSeek, Qwen, Kimi, Doubao) is overwhelmingly consistent: all four services persistently block the exact same queries across all 5 measurement samples in both Simplified and Traditional Chinese. Output blocking is far less consistent, with only 29 out of 349 output-blocked queries blocked across all 5 samples. Baidu-Chat is exceptional: it performs almost no input blocking but instead relies heavily on post-search and output blocking (78.6% of blocks are output-phase).

2026-ablove-characterizing §VI-B keyword-filteringdpi

Iran's censorship of refraction-networking proxies (Conjure via Psiphon) is not monolithic: different ISPs independently deploy different techniques and timelines. Over 800 million logged Conjure connections from July 2023–February 2025 across 10+ Iranian ASes show TCI (AS58224, ~33% of traffic) uses packet injection, while MCCI/Hamrah-e Avval (AS197207, ~22%) applies IP-based blocking, and some ASes (Parsonline AS16322, Shatel AS31549) show no proxy blocking at all.

2025-alaraj-iran-refraction §4 rst-injectionip-blockingpacket-injectiondpi