Obfsproxy (predecessor to obfs4) listens on randomized ports as an explicit defense against discovery by comprehensive Internet-wide scanning, because an adversary must scan all 65,535 ports to locate bridges rather than a single known port — multiplying scan cost by roughly 65,000× relative to a single-port sweep.

High-speed Internet-wide scanning enables a censor or attacker to locate every publicly reachable host vulnerable to a newly disclosed flaw within hours of disclosure; in a concrete example, 3.4 million UPnP-vulnerable devices were identified in under 2 hours — faster than network operators could apply patches — with a 150-SLOC probe module written in approximately 4 hours.

§4.3 detection

Comprehensive Internet-wide scanning enables cross-IP tracking of users and devices by correlating stable cryptographic identifiers — TLS certificates or SSH host keys presented by home routers and cable modems — with public geolocation data across DHCP lease changes, defeating the anonymity assumption behind dynamic IP addresses.

§4.6 detection

By scanning ports 443 and 9001 and fingerprinting responses with Tor's TLS v1 cipher-suite handshake pattern, ZMap identified 79–86% of all allocated Tor bridge fingerprints in a single scan, demonstrating that bridges whose protocol is distinguishable are largely discoverable through comprehensive Internet-wide scanning even though their addresses are not publicly listed.

§4.4 detection

ZMap completes a single-port scan of the entire public IPv4 address space in under 45 minutes from a commodity machine with a gigabit Ethernet connection, over 1,300 times faster than the most aggressive Nmap configuration. A single-probe scan achieves approximately 97.9% coverage of live hosts, rising to 98.8% with two probes and 99.4% with three probes.

§3, §3.2, §3.4, Table 1 evaluation

The June 2025 Iran shutdown—carried out during the Iran-Israel war beginning ~June 19—did not use BGP route withdrawals as in 2019. Instead, authorities applied service-level restrictions at the national border: DNS poisoning of foreign destinations, protocol whitelisting permitting only pre-approved domestic services, and DPI to block circumvention-tool traffic. Iran's international traffic fell roughly 90% while the country's BGP routes remained advertised, making the shutdown invisible to BGP-based monitoring systems. OONI measurement volume, which totalled 121,333 in June 2025, collapsed to under 200 submissions on June 19-20.

2025-iran-shutdown-measurement Executive Summary / §2 dpidns-poisoningport-blockingip-blocking

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

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

VPN search demand in Iran spiked approximately 707% during the June 2025 stealth blackout, as measured by Top10VPN analytics, making it one of the highest-documented circumvention-demand spikes associated with a single shutdown event. Despite this demand, many VPN connections failed because the protocol whitelist eliminated non-HTTPS tunneling methods and HTTP-level filters could detect known VPN signatures on port 443.

2025-aryapour-stealth-blackout §1, §5 dpiport-blockingtraffic-shape