The Tor Project has begun blacklisting exit nodes which it believes are vulnerable to the Heartbleed vulnerability. Researcher Collin Mulliner, with the Systems Security Lab at Northeastern University in Boston, published the results of an experiment he conducted using a publicly disclosed Heartbleed proof-of-concept exploit against 5,000 Tor nodes.

Mulliner said that 1,045 nodes, or a little more than 20 percent, were vulnerable and could enable communications to be hacked. He listed Tor exit nodes which were leaking plaintext user traffic, including host names, credentials and web content. Tor Project leader Roger Dingledine said on the Tor mailing list said that 380 vulnerable exit keys were being rejected. Heartbleed was discovered on April 7 and uses a vulnerability lies in the heartbeat function in OpenSSL 1.0.1 to 1.0.1f which publicly leaks 64 KB of memory to any client or server pinging a web server running the vulnerable crypto library.

The memory leaks can reveal anything from user credentials to private server keys if the attack is repeated enough. Mulliner said he did not try to extract private keys from Tor and did not think it was possible. 

He used a random list of 5,000 Tor nodes from the Dan.me.uk website for his research; of the 1,045 vulnerable nodes he discovered, he recovered plaintext traffic that included Tor plaintext announcements, but a significant number of nodes leaked user traffic in the clear.”

Heartbleed allows attackers to avoid having to set up their own exit node and waiting for traffic to pass through it. All a hacker would have to do is query a vulnerable exit node to obtain traffic, Mulliner said.