Pierluigi Paganini January 23, 2014

Researchers Winter and Lindskog identified 25 nodes of Tor network that tampered with web traffic, decrypted the traffic, or censored websites.

Two researchers, Philipp Winter and Stefan Lindskog of Karlstad University in Sweden, presented the results of a four-month study  conducted to test Tor network exit nodes for sneaky behavior, it has been discovered that a not specified Russian entity is eavesdropping nodes at the edge of the Tor network.

The researchers used a custom tool for their analysis and they discovered that the entity appeared to be particularly interested in users’ Facebook traffic.

Winter and Lindskog identified 25 nodes that tampered with web traffic, decrypted the traffic, or censored websites. On the overall nodes compromised, 19 were tampered using a man-in-the-middle attacks on users, decrypting and re-encrypting traffic on the fly.

Tor network anonymizes user’s web experience, under specific conditions, bouncing encrypted traffic through a series of nodes before accessing the web site through any of over 1,000 “exit nodes.”

The study proposed is based on two fundamental considerations:

• User’s traffic is vulnerable at the exit nodes. For bad actors, the transit through an exit node of the traffic exposes it to eavesdrop. Very popular was the case of WikiLeaks, that was initially launched with documents intercepted from Tor network eavesdropping on Chinese hackers through a bugged exit node.
• Tor nodes are run by volunteers that can easily set up and taken down their servers every time they need and want.

The attackers in these cases adopted a bogus digital certificate to access traffic content, for the remaining 6 cases it has been observed that impairment resulted from configuration mistakes or ISP issues.

The study revealed that the nodes used to tamper the traffic were configured to intercept only data streams for specific websites, including Facebook, probably to avoid detection of their activity.

The researchers passive eavesdropped on unencrypted web traffic on the exit nodes, by checking the digital certificates used over Tor connections against the certificates used in direct “clear-web sessions”, they discovered numerous exit nodes located in Russia that were used to perform man-in-the-middle attacks.

The attackers control the Russian nodes access to the traffic and re-encrypt it with their own self-signed digital certificate issued to the made-up entity “Main Authority.” It appears as a well-organized operation, the researchers noted that blacklisting the “Main Authority” Tor nodes, new ones using the same certificate would set-up by the same entity.

It is not clear who is behind the attack, Winter and Lindskog believe that the spying operation was conducted by isolating individuals instead government  agency because the technique adopted is too noisy, the attackers used a self-signed certificate that causes browser warning to Tor users.

“It was actually done pretty stupidly,” says Winter.

It must be also considered that Intelligence agencies,  including the NSA are spending a great effort to infiltrate the  Tor network, one of the documents leaked by Edward Snowden on the US surveillance expressly refers a project, codenamed Tor Stinks, to track profiles in the deep web.

Despite the high interest in Tor networks and its potentialities, the network is considerably the best way to protect user’s anonymity online, and governments don’t want this.

Pierluigi Paganini

(Security Affairs –  Tor network, Russia)

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