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Is the Tor Network Broken?


Is the popular Tor network broken? In the recent months, after Snowden’s revelations, many security experts have started to investigate on the possibility that the US intelligence, and possibly others, has found a way to compromise the Tor network. Last week I published a blog post that commented a report issued by the German broadcaster ARD in which is confirmed that the NSA XKeyscore was used to target two Germany-based Tor Directory Authority servers.

In the recent months it was also discussed about the possibility to track selected users on Tor networks with a technique known as Traffic Correlation Attack defined by the researchers led by Aaron Johnson of the Naval Research Laboratory.

Today I want to report to you that two hackers have announced a technique to de-anonymize Tor users easily, and the two experts Alexander Volynkin and Michael McCord will present the results of their study at the next Black Hat 2014.

In the presentation they have announced, titled “YOU DON’T HAVE TO BE THE NSA TO BREAK TOR: DEANONYMIZING USERS ON A BUDGET“, the experts will show how to identify Tor users with a very small budget, just $3,000.

Tor Network Presentation Black hat

“There is nothing that prevents you from using your resources to de-anonymize the network’s users instead by exploiting fundamental flaws in Tor design and implementation. And you don’t need the NSA budget to do so. Looking for the IP address of a Tor user? Not a problem. Trying to uncover the location of a Hidden Service? Done. We know because we tested it, in the wild…”  “In this talk, we demonstrate how the distributed nature, combined with newly discovered shortcomings in design and implementation of the Tor network, can be abused to break Tor anonymity,” are the statements used by the two researchers to describe their work.

I confess you to be very excited to see the presentation of the two experts. If it is possible to track users on Tor network, let’s imagine the capabilities of  a persistent attacker with much more computational capability and with a “couple gigabit links”. According to the two experts 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.”

Many readers will be very happy to read about the this discovery, in fact, Tor network is used by many criminals for illicit activities, like botnet controlsale of drugs and weapons, and distribution of child pornography. I personally hope that the researchers will carefully disclose the results of their study, notifying to Tor’s developers how to fix potential flaw in the Tor network design.

The Tor network is also precious tool to avoid censorship and oppressive governments and the discovery of the two researchers could be abused. This is probably already happening. Tor users that adopt the anonymizing network and other technologies to avoid control and Internet monitoring are tracked by the NSA for example.

Stay tuned … if the discovery is confirmed, millions of Tor network users are at risk.


  1. Tor is down.

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