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Computer scientists found almost 20 exit relays in the Tor anonymity network that attempted to spy on users’ encrypted traffic using man-in-the-middle techniques.




Even though connections between Tor relays are encrypted, traffic is returned to its original state when it leaves the network. This means that if it’s not using SSL or another secure transport protocol, Tor exit relays can inspect it. That’s why the Tor Project recommends the use of HTTPS—HTTP with SSL encryption—with all websites that support it, even if using Tor.




The relays engaged in HTTPS sniffing used self-signed certificates which lowered the attack’s success rate because this triggered browser certificate errors that users would have had to manually dismiss. The Tor Project maintains and distributes a software package called the Tor Browser Bundle that contains a browser based on Mozilla Firefox and other components needed to access the Web over Tor.

The researchers believe the relays doing HTTPS or SSH traffic interception were operated by the same individual or group of individuals because they used very similar self-signed certificates, almost all of them were located in Russia on the network of a virtual private server (VPS) hosting provider and all of them ran an old version of Tor——that’s uncommon among relays. Only two benign relays that used this same Tor version were identified during the scans.


They also only appeared to target connections to particular websites. For example, Facebook.com was targeted, but other sites in the Alexa top ten list or popular Russian social media sites were not.


”We don’t know how many users were actually tricked, but we don’t think it’s many because people wouldn’t expect a certificate error if they go to Facebook,” Winter said. “We tried to figure out what they were actually doing with this, so we set up a fake Facebook profile and logged into it through one of those malicious relays. We didn’t see anyone logging in after us.”

”I’m not even sure if they captured passwords,” the researcher said. “Maybe it was just an experiment. It didn’t seem like a very sophisticated and serious attack to us.”

There’s a possibility that the man-in-the-middle attacks happened upstream of the exit relays, on their ISP’s network, for example. However, the researchers believe that’s unlikely because there was one malicious relay with the same particularities that was located in the U.S., so in a different region and network. That suggests that the relays themselves were the source of attack.



However, this approach does have limitations. For example, if an attacker controls a large number of exit relays and they’re all set up for MitM HTTPS sniffing, the chances of detecting an attack by comparing certificates received over two exit relays decreases.

Also, the extension wouldn’t detect MitM attacks that use fraudulently obtained certificates signed by trusted certificate authorities, as those certificates would automatically be trusted by browsers and wouldn’t result in errors.



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