The Tor Project has released an upgrade to its software that blocks some recently revealed critical vulnerabilities in the Tor network’s protection of user anonymity. The Tor Project’s Erinn Clark detailed the fixes in a blog entry, and urged all users to update immediately.

Tor, originally called “The Onion Router,” anonymizes user visits to websites and other Internet traffic by passing it through a series of relay servers around the Internet. It also can be used to bypass national firewalls like China’s Great Firewall, or Internet “shutdowns” like the one imposed by the Mubarak regime in Egypt earlier this year, through “bridge relays” that connect to the global Internet through dialup or satellite connections or other connections that bypass normal Internet routing. However, an attack has been developed that could be used to track individuals using the Tor network, and discover hidden bridges, potentially putting them at risk.

These fixes are totally unrelated to the vulnerabilities in Tor alleged by the research team of Eric Filiol, head of the Operational Cryptography and Computer Virology lab at ESIEA in Paris. Filiol is presenting his research on an exploit he said can be used to take control of parts of the Tor Network at the Hackers To Hackers conference in São Paulo this weekend.

The privacy of Tor users could be exposed by the attack because the previous version of the software used the same Transport Layer Security (TLS) certificate when connecting to different Tor network relays. An attacker who has identified the user’s client key could use the Tor network’s protocols to probe relays to see if the user’s key was connected to it. “Each client or bridge would use the same cert chain for all outgoing [Onion Router] connections until its IP address changes,” wrote Clark, “which allowed any relay that the client or bridge contacted to determine which entry guards it is using.”

The new version of Tor, version 0.2.2.34, no longer sends the TLS certificate chain as part of connections over the Tor network. It also changes the router code to block probes to test them for clients that are still transmitting TLS certificates.

One of the vulnerabilities that makes these attacks possible has been well-known for years. In 2005, Steven Murdoch and George Danezis of Cambridge University’s Computer Laboratory demonstrated a traffic analysis method that could allow attackers to figure out which nodes in the Tor network were being used to relay traffic from a specific site. Since Tor clients randomly select three relay sites at connection as their “guards” for privacy, the guards selected by a client could be used as a fingerprint for the user.

That vulnerability, which would allow a malicious website to discover the Tor relays for a specific user, had not yet been addressed, and Clark wrote that traffic analysis attacks “remain as open research problems.”