Some Tor Exit Nodes Attempt to Spy on Encrypted Traffic, Researchers Find

19 Tor exit relays used self-signed certificates to launch man-in-the-middle attacks against HTTPS and SSH connections

By Lucian Constantin
Wed, January 22, 2014

IDG News Service — 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.

The research was carried out over a period of four months by Philipp Winter and Stefan Lindskog, researchers in the PriSec (Privacy and Security) group at Karlstad University in Sweden, who recently published a paper with their findings.

The Tor network is designed to provide anonymity for users and bypass Internet censorship attempts. This is achieved by encrypting user traffic and routing it through a series of computers that act as relays and are run by volunteers before sending it to its intended destination on the Internet.

Computers that handle the final hop in the Tor network are known as exit relays. According to statistics from the Tor Project, there are about 1,000 such relays as of this month.

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.

However, their man-in-the-middle (MitM) position allows Tor exit relays to even tamper with HTTPS connections, using techniques like SSL stripping or impersonating the destination website using a rogue certificate.

The researchers built a scanning tool called exitmap that can identify exit relays behaving maliciously or abnormally and ran it on the Tor network. Over a four-month period they identified 25 bad relays that were subsequently reported to the Tor Project and blacklisted.

Fourteen relays engaged in man-in-the-middle HTTPS traffic sniffing using fake certificates, four relays did both HTTPS and SSH sniffing and one attempted only SSH sniffing. Two other relays used the sslstrip tool to force HTTPS connections over plain HTTP, one relay injected HTML code in HTTP traffic and three relays engaged in Internet censorship by blocking access to certain websites at the DNS level, intentionally or because of misconfiguration.

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.

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