TCP Window Reduction (Strategy 8)—reducing the SYN+ACK TCP window to 10 bytes and stripping wscale options, forcing the client to segment its request—achieves 100% evasion success against HTTP in India and Kazakhstan, 100% against HTTP and HTTPS in Iran, and 100% against SMTP in China, because none of these censors can reassemble TCP segments. The strategy is compatible with all 17 tested client OS versions when implemented without SYN+ACK payloads, making it the most broadly deployable server-side strategy found.

All 25 applicable client-side Geneva strategies failed when mechanically translated to server-side analogs against China's GFW, even when the only structural difference was which endpoint sent the insertion packet. Experiments with the server placed inside China and client outside also failed, indicating the GFW tracks connection initiator identity and processes client versus server packets asymmetrically—meaning server-side circumvention requires a completely independent discovery approach.

§3 evaluation

China's GFW uses distinct, co-located censorship boxes—each with its own independent network stack implementation and bugs—for each application-layer protocol it censors. TCP-level strategies that exploit transport-layer bugs show dramatically different success rates per protocol: Strategy 1 (Simultaneous Open + Injected RST) achieves 89% for DNS but only 14% for HTTPS; Strategy 8 (TCP Window Reduction) achieves 100% for SMTP but only 2–3% for DNS, HTTP, and HTTPS. TTL-limited probes confirm all protocol boxes are co-located at the same network hop.

§6, Table 2 detection

The paper identifies three distinct GFW resynchronization-state triggers with protocol-specific behavior: (1) a server payload on any non-SYN+ACK packet causes resync on the next SYN+ACK or client ACK-flagged packet for all protocols; (2) a server RST causes resync on the next client packet for all protocols except HTTPS; (3) a SYN+ACK with a corrupted acknowledgment number triggers resync only for FTP. Strategy 1's 50% per-attempt success rate for HTTP is confirmed to result from the 50% probability of the GFW entering the resynchronization state on an injected RST, consistent with Wang et al. [36].

§5.1 detection

The paper presents 11 purely server-side censorship evasion strategies requiring zero client-side software, successfully bypassing censorship in China, India, Iran, and Kazakhstan across DNS-over-TCP, FTP, HTTP, HTTPS, and SMTP. All strategies manipulate only TCP handshake packets (primarily the SYN+ACK) and were verified against 17 versions of 6 client operating systems (Windows XP–Server 2018, MacOS, iOS, Android, Ubuntu, CentOS) with unmodified clients.

§1, §5, Table 2 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