Tor onion services can be deanonymized using long-lived circuits
The Cost of Stability: Deanonymizing Onion Services Long-Lived Introduction Circuits
Cryptography and Security
Summary
Tor helps keep online activity private by routing data through several relays, so it's hard to trace back to users or services. However, onion services in Tor use special circuits that stay active for a long time, which makes it easier for an attacker watching just one relay to figure out the service’s location. The authors show how this vulnerability can uncover the path between a user and an onion service in about two hours. They also suggest a way to reduce this risk by changing these long-lived circuits more often.
What this means in practice
- •For tor network operators: Improve Tor’s security by periodically rebuilding onion service introduction circuits to reduce anonymity risks from long-lived connections.
- •For network security engineers: Assess anonymity weaknesses in services that use long-lasting network circuits and develop defenses against traffic analysis attacks.
Authors
Nicolas Constantinides, Mahdi Rahimi
Abstract
Tor is a widely used anonymity network that provides network privacy by routing client communications through a sequence of relays to their destinations. An adversary observing a single Tor relay cannot readily link clients to their destinations, as doing so requires identifying the other relays along the client circuit. Identifying these relays from traffic observed at the monitored relay alone is challenging: each relay carries traffic for many circuits simultaneously, and client circuits typically last only about 10 min, limiting observations of a given circuit. In contrast, Tor onion services use introduction circuits that remain fixed for 18 to 24 h. We develop an intersection attack that exploits this design, allowing an adaptive adversary observing one Tor relay at a time to identify the network location of an onion service. We evaluate the attack on the live Tor network against a self-operated onion service carrying genuine third-party traffic. Across nine end-to-end experiments, the attack reconstructs the complete Tor circuit in every run, with an estimated median time of 2.2 h. To mitigate this attack, we propose a mechanism that periodically rebuilds internal introduction circuits to limit evidence accumulation without changing publicly advertised information.