VMess servers exhibit inconsistent TCP connection-draining behavior depending on error type: a first-seen (Encryption IV, Encryption Key) pair waits for more data before closing, while a replayed pair closes immediately. This timing asymmetry allows a prober to distinguish VMess servers from non-VMess servers with a three-connection probe sequence (M1, M2, M2 replay), as documented by @nametoolong in June 2020.

Frolov et al. (2020) found that obfs4, Shadowsocks Outline, Psiphon's OSSH, and Lantern's Lampshade are all identifiable by TCP flag and timing patterns when servers close connections on error, because each tool's timeout value and FIN/ACK behavior are distinct. Their recommended mitigation—'forever read' on errors so the prober always closes first—forces the server to terminate with FIN/ACK consistently across all code paths.

§Our comments defense

V2Ray clients emitted TLS ClientHello messages with a hardcoded, rarely-seen ciphersuite (fingerprint ID 8c48b95f67260663 on tlsfingerprint.io) that allowed a machine-learning classifier to identify V2Ray TLS traffic with 0.9999 accuracy; the same classifier could not accurately identify the traffic after the fingerprint was changed. The blocking rule based on the unique ciphersuite could be expressed in a single iptables line.

§Unique TLS ClientHello Fingerprints detection

V2Ray's HTTP obfuscation mode prepends an HTTP header only to the first TCP payload per connection and uses a hardcoded HTTP 500 response for all failure cases, making the mimicry trivially detectable: legitimate HTTP servers send headers on every response, and do not return 500 for protocol errors a real HTTP server would never encounter.

§Failed to Mimic the HTTP Server detection

VMess authentication uses a timestamp-based credential with a maximum 120-second (average ~60-second) expiration window, allowing an attacker to replay a captured legitimate request within that window. By making 16 connections with altered Encryption Key bytes that enumerate all 16 possible Margin P padding-length values, a prober can confirm a VMess server by observing a non-repeated set of connection-close byte counts spanning a delta of 15.

§Replay Attacks against the VMess Protocol detection

CensorLess's function refresher automatically retires serverless bridges and deploys fresh ones in batches across diverse regions; the expected time until a bridge is identified and blocked in practice is 2 days (per Fifield et al.), while Tor bridges in China are discovered within 2–36 days. The old bridge is only removed after all clients have completed live migration to a new URL, maintaining uninterrupted connectivity.

2026-kang-censorless-serverless §4.5 active-probingip-blocking

CensorLess's threat model explicitly relies on a rational-censor assumption: the censor will not block entire cloud-provider IP ranges or domain namespaces because the collateral damage to legitimate business services would be politically and economically unacceptable. AWS Lambda's inherent IP-address ephemerality (new IPs on each invocation, function lifetime up to 15 minutes) means even censors willing to attempt enumeration face a continuously shifting target distributed across the cloud provider's global address space.

2026-kang-censorless-serverless §4.2 ip-blockingasn-blackholingactive-probing

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

Obscura proxies resist active probing by never exposing open ports or accepting incoming connections; combined with a large ephemeral volunteer pool (analogous to Snowflake's scale), the vast IP address space and rapid proxy rotation make exhaustive enumeration infeasible without causing sufficient collateral damage to deter the censor — consistent with the absence of observed blind-blocking campaigns against Snowflake.

2026-vilalonga-obscura-enabling-ephemeral §5.1 active-probingip-blocking