New attack on Tor can deanonymize hidden services with surprising accuracy

Computer scientists have devised an attack on the Tor privacy network that in certain cases allows them to deanonymize hidden service websites with 88 percent accuracy.

Such hidden services allow people to host websites without end users or anyone else knowing the true IP address of the service. The deanonymization requires the adversary to control the Tor entry point for the computer hosting the hidden service. It also requires the attacker to have previously collected unique network characteristics that can serve as a fingerprint for that particular service. Tor officials say the requirements reduce the effectiveness of the attack. Still, the new research underscores the limits to anonymity on Tor, which journalists, activists, and criminals alike rely on to evade online surveillance and monitoring.

"Our goal is to show that it is possible for a local passive adversary to deanonymize users with hidden service activities without the need to perform end-to-end traffic analysis," the researchers from the Massachusetts Institute of Technology and Qatar Computing Research Institute wrote in a research paper. "We assume that the attacker is able to monitor the traffic between the user and the Tor network. The attacker’s goal is to identify that a user is either operating or connected to a hidden service. In addition, the attacker then aims to identify the hidden service associated with the user."