TapDance's non-blocking asymmetric design leaves the overt connection open but abandoned, enabling an active censor to inject a TCP ACK carrying a stale sequence number; the overt server responds with its true TCP state, exposing the discrepancy and confirming decoy routing. The attack requires no clean-path routing capability: the injected packet is forwarded through the tainted path by the non-blocking TapDance station itself.

Schuchard et al. demonstrated that latency differences caused by a decoy routing proxy communicating with a distant covert destination are sufficient not only to detect the use of decoy routing but also to fingerprint which specific censored webpage the client accessed. All prior decoy routing systems (Telex, Cirripede, Curveball, TapDance, Rebound) remained vulnerable to this attack at time of publication.

§4.1 detection

Slitheen replaces only 'leaf' HTTP resources (images, video) in overt-site responses with covert content, reusing all TCP/IP headers verbatim and forwarding packets immediately on arrival. This forces every observable feature—packet size, direction, inter-arrival timing—to be identical to a genuine access of the overt page, eliminating the censor's ability to apply latency analysis, website fingerprinting, or protocol fingerprinting to distinguish decoy sessions from normal traffic.

§3.1, §4.1 defense

Measurement of the Alexa top 10,000 TLS sites showed that the fraction of traffic replaceable by a Slitheen relay varies from 0% (Facebook, due to large TLS records preventing leaf replacement) to 100% (Wikipedia, Yahoo). For representative sites: Reddit achieved 70% ±10% of leaf bytes replaced (19% ±3% of total page bytes), Gmail 87.7% ±0.2% of leaf bytes (23% ±9% total), and Quora 99% ±5% of leaf bytes (20% ±10% total), as reported in Table 2.

§6.1, Table 2 evaluation

Table 1 shows Slitheen is the first decoy routing system to simultaneously defend against latency analysis, website fingerprinting, and protocol fingerprinting attacks, while also resisting TCP replay and Crazy Ivan active attacks. This security is achieved at the cost of requiring symmetric flows and inline blocking—requirements previously considered prohibitive—which the authors argue are increasingly met by commercial DPI traffic-shaping appliances (e.g., Sandvine) already deployed by ISPs.

§4.5, Table 1 evaluation

Re-testing in 2025 on a Pixel 10 Pro XL running Android 16 with October 2025 security updates confirmed that blind in/on-path VPN inference attacks remain fully viable despite CVE-2019-9461, CVE-2019-14899, and CVE-2024-49734 having been formally closed. All three core attack primitives—VPN-assigned internal IP discovery, active connection inference, and TCP reset injection via sequence/acknowledgment window scanning—succeeded across OpenVPN, WireGuard, and NordLynx.

2026-tolley-architectural §5.1–5.3 traffic-shaperst-injectionactive-probing

Six widely deployed VPN and circumvention tools—OpenVPN, WireGuard/NordLynx, NordWhisper, Orbot (Tor on Android), Lantern, and Psiphon—all failed to block internal IP inference, connection-state detection, and TCP reset injection under identical adversarial conditions on fully patched Android 16. Application-layer obfuscation in Lantern and Psiphon did not prevent TCP-layer disruption; Orbot's VPN-style encapsulation of Tor traffic was bypassed via the same tunnel-level side channels.

2026-tolley-architectural §5.2, §5.4–5.5 traffic-shaperst-injectionactive-probing

The paper proposes an Internet Freedom vulnerability registry with five design principles: persistent cross-vendor tracking under shared identifiers (e.g., IF-ARCH-2025-001) as long as a risk remains reproducible; human-centered impact ratings anchored to harm potential for journalists and dissidents rather than CVSS-style exploitability scores; timestamped re-verification hooks with linked PCAPs and minimal reproduction scripts; a structured media interface to counter vendor narrative capture; and open public APIs for integration into risk dashboards so that users of tools like Orbot or Lantern can directly query their configuration's exposure to known metadata-based attacks.

2026-tolley-architectural §6.1–6.5 traffic-shaperst-injectionactive-probing

By 2018 the GFW shifted from blocking Tor bridges by (IP, port) tuples to blocking the entire IP address. A blocked bridge remains inaccessible for exactly 12 hours; the block renews to 12 hours if any additional Tor connection attempt is made during that window, after which the GFW re-scans and removes the IP from the blacklist if Tor is no longer running.

2018-dunna-analyzing §4.2 dpiactive-probingip-blockingrst-injection

Because TapDance does not block client-to-server packets, a censor can inject a TCP packet with a stale acknowledgment number directly to the true decoy server; the server will reply with its actual TCP sequence state, which will differ from the sequence numbers the TapDance station has been using — confirming the flow is proxied. This active packet-injection attack is qualitatively easier to execute against TapDance than against Telex or Cirripede, which used inline blocking to prevent such probes from reaching the server. Table 1 in the paper confirms that TapDance, unlike Telex, lacks replay/preplay attack resistance and has no traffic-analysis defense.

2014-wustrow-tapdance §5.2, Table 1 active-probingpacket-injectionrst-injection

Default Tor connections to a private bridge inside China were detected by the Great Firewall via active probing: an initial connection succeeded, followed by a probe from a Chinese IP address approximately 15 minutes later that performed a TLS handshake and then blacklisted the (IP, port) combination. Subsequent connection attempts resulted in a successful SYN followed by spoofed TCP RSTs terminating both the client and bridge connections.

2013-dyer-protocol §6 active-probingrst-injection