Tor Project working to fix weakness that can unmask anonymous users

Developers of Tor software believe they’ve identified a weakness that was scheduled to be revealed at the Black Hat security conference next month that could be used to de-anonymize Tor users.

The Black Hat organizers recently announced that a talk entitled “You Don’t Have to be the NSA to Break Tor: Deanonymizing Users on a Budget” by researchers Alexander Volynkin and Michael McCord from Carnegie Mellon University’s Computer Emergency Response Team (CERT) was canceled at the request of the legal counsel of the university’s Software Engineering Institute because it had not been approved for public release.

“In our analysis, we’ve discovered that a persistent adversary with a handful of powerful servers and a couple gigabit links can de-anonymize hundreds of thousands Tor clients and thousands of hidden services within a couple of months,” the CERT researchers had written in the abstract of their presentation. “The total investment cost? Just under $3,000.”

In a message sent Monday to the Tor public mailing list, Tor project leader Roger Dingledine said that his organization did not ask Black Hat or CERT to cancel the talk. Tor’s developers had been shown some materials about the research in an informal manner, but they never received details about the actual content of the planned presentation, he said. The presentation was supposed to include “real-world de-anonymization case studies.”

Despite the lack of details, Dingledine believes that he has figured out the issue found by CERT and how to fix it. “We’ve been trying to find delicate ways to explain that we think we know what they did, but also it sure would have been smoother if they’d opted to tell us everything,” he said in a subsequent message on the mailing list.

Dingledine suggested that the issue affects Tor relays, the Tor network nodes that route user connections in a way that’s meant to hide the traffic’s origin and destination from potential network eavesdroppers.

“Based on our current plans, we’ll be putting out a fix that relays can apply that should close the particular bug they found,” he said. “The bug is a nice bug, but it isn’t the end of the world. And of course these things are never as simple as ‘close that one bug and you’re 100% safe’.”

Tor—originally called The Onion Router—started out as a project of the U.S. Naval Research Laboratory, but is now developed and maintained by a nonprofit organization called The Tor Project. The software allows users to access resources on the Internet without revealing their real IP (Internet Protocol) addresses, a feature appreciated by privacy conscious users as well as criminals.

According to media reports last year based on documents leaked by former U.S. National Security Agency contractor Edward Snowden, both the NSA and the U.K.’s Government Communications Headquarters targeted Tor and had some success in de-anonymizing limited numbers of users.