2025-syverson-onion-location-measurements-fingerprinting
findings extracted from this paper
Scanning 0.91B unique SANs extracted from 3.7B certificates across 17 CT logs revealed 3,330 unique .onion addresses configured by 26,937 domains. After six months, only 2,101 onions (63%) remained reachable, of which 1,505 (72%) had matching clearnet index pages, constituting the effectively enumerable target set for a targeted OLF adversary.
Local onion association—periodically downloading the full set of onion associations from a CT-log-based API and performing each lookup locally—produces a traffic pattern from the guard's perspective that is indistinguishable from generic onion service access, eliminating both the OLF fingerprint and the DNS-based Website Oracle attack vector. This approach requires no per-connection clearnet exit circuit and imposes negligible overhead given the current ~1,500 stable O-L site count.
OLF reduces an adversary's target anonymity set from roughly 10,000 active onionsites to the ~1,500 stably available O-L sites—nearly an order of magnitude. Because O-L requires an exit circuit with a DNS lookup, a DNS-based Website Oracle further collapses the false-positive rate, making OLF effectively a closed-world attack on the enumerated O-L site list.
Circuit fingerprinting from a guard-relay position achieves ≥99.9% accuracy with FPR ≤0.1% for all four Tor circuit types (general, HSDir, introductory, rendezvous) using the Deep Fingerprinting classifier on the first 512 cells, despite Tor's deployed partial defenses. Onion-Location fingerprinting (OLF) combining these circuit classifiers then achieves 98.81–99.87% accuracy (FPR 0.16–1.23%) distinguishing O-L sessions from ordinary clearnet or onion-only visits.
Automatic Onion-Location redirect was disabled in Tor Browser 13.0.12 as a direct result of this research, because automatic redirect forces the distinguishable clearnet-then-onion circuit pattern on every visit without user awareness. Manual O-L remains in Tor Browser but is still fingerprintable with the same near-perfect accuracy since the exit→onion circuit sequence is identical whether the redirect is automatic or manually triggered.