TSG's Psiphon3 signature is a multi-stage stateful match (IP, then protocol/SNI, then a negative/NOT condition) where the NOT condition is only evaluated once, explicitly at the 8th packet of a session. Sessions carrying fewer than 8 packets never reach that evaluation point and are never flagged as Psiphon, regardless of actual protocol. Confirmed in production (E21) as the root cause of a customer-reported partial bypass.
受限于执行Not的时机（一个会话只能执行一次Not）... 当前特征受描述能力限制，对于小于8个包的会话，无法有效执行带Not条件的psiphon3特征
Defense implications
- Any multi-stage/stateful signature that relies on a NOT-condition evaluated only at a fixed packet index is structurally blind to shorter sessions -- a general technique class (not Psiphon-specific): keeping negotiation/handshake exchanges under whatever packet count a NOT-based rule fires at is a reusable evasion shape against this engine's rule language, not just this one signature.
Related findings
Side-by-side internal performance comparison across Xinjiang (China Mobile carrier deployment, hostname cmcc-xj-server1-sapp-244), Fujian (domestic), and Ethiopia (E21) explicitly references the same 'app_sketch_maat' diagnostic log format at all three sites, confirming the identical AppSketch app-fingerprinting stack is used for domestic Xinjiang surveillance and for the exported Ethiopia deployment.
Direct dated evidence (July 2023) that the Ethiopia (E21) customer actively toggles application-level blocking as a live policy lever: lifting the block on YouTube plus four other apps caused a documented traffic surge and device alerts at the BOL-IGW site.
TSG's QUIC-parsing layer decrypts QUIC ClientHello payloads and parses a user-agent-like parameter from the decrypted content, deployed at the Ethiopia (E21) DIR-IGW site. A missing bounds check on this field caused a watchdog-timeout crash, confirmed via a MESA_Platform/quic GitLab commit (git.mesalab.cn/MESA_Platform/quic).
Ethiopia customer (E21/E-site) explicitly requested TSG blocking be extended beyond the baseline (Psiphon 3) to a named list of commercial VPNs: Freegate, CyberGhost, Torguard, NordVPN, IPVanish, VPN Unlimited, ExpressVPN, Surfshark, Windscribe, Hotspot Shield, Ivacy, Atlas VPN, PureVPN, ProtonVPN, Norton Secure VPN. Engineering confirmed delivery of NordVPN and Hotspot Shield signatures with successful field tests.
TSG integrates a licensed third-party DPI engine (versioned separately from TSG/App Sketch DB releases) that repeatedly segfaulted across many E21 (Ethiopia) NPB nodes over Oct-Dec 2023 when processing specific packet-encapsulation stacks (Ethernet->MPLS->IPv4->UDP, VLAN->IPv4->UDP), requiring an App Sketch DB version bump to resolve.
Root-caused an Ethiopia SNI-block-failure incident to the detection mechanism itself: APP_SKETCH's FQDN-scanning module -- which exists specifically to identify Psiphon3 and Freegate via top-N SNI matching -- was CPU-expensive enough to saturate cores, triggering sapp's fail-open DDoS bypass and letting some target connections through uninspected. A signature-structure fix roughly doubled throughput (33K/s to 73K/s new connections) and disabling the fail-open bypass restored reliable blocking.