The Tor Project is in the process of rejecting exit nodes vulnerable to the Heartbleed OpenSSL vulnerability after researcher Collin Mulliner discovered more than 1,000 leaking plaintext traffic.

The Tor Project has begun blacklisting exit nodes vulnerable to the Heartbleed vulnerability in OpenSSL.

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 to the bug.

Mulliner said only Tor exit nodes were leaking plaintext user traffic, including host names, credentials and web content. Mulliner conducted his experiment for three days last Friday through Sunday, and his results are a point-in-time snapshot. A post yesterday from Tor Project leader Roger Dingledine on the Tor mailing list said that 380 vulnerable exit keys were being rejected.

Heartbleed was publicly reported on April 7. The 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 disclose in plaintext anything from user credentials to private server keys if the attack is repeated enough. Several researchers have already managed to retrieve private SSL keys in an online challenge from vendor CloudFlare. Speculation is that intelligence agencies and/or hackers may have been exploiting it since November. Mulliner said he did not try to extract private keys from Tor, nor did he think it was possible.

Tor promises anonymity to its users by using proxies to pass encrypted traffic from source to destination. Mulliner said 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.”

“I found a significant amount of plaintext user traffic, complete Web traffic, session IDs; everything you would find if you ran Heartbleed against a normal Web server,” Mulliner said.

Heartbleed saves attackers the work of setting up their own exit node and waiting for traffic to pass through it. Using Heartbleed, all a hacker would have to do is query a vulnerable exit node to obtain traffic, Mulliner said.

Dingledine yesterday published the first list of rejected exit nodes and said those nodes will not be allowed back on the network.

“I thought for a while about trying to keep my list of fingerprints up-to-date (i.e. removing the !reject line once they’ve upgraded their openssl), but on the other hand, if they were still vulnerable as of yesterday, I really don’t want this identity key on the Tor network even after they’ve upgraded their OpenSSL,” Dingledine wrote. He added that he hopes others will add to this list as other vulnerable relays are discovered.

Tor acknowledged some of its components were vulnerable to Heartbleed in a post to its blog on April 7.

Mulliner said it was a fairly straightforward process to write a script to run a Heartbleed proof of concept.

“Anybody who can get the Python script can play around with it,” Mulliner said, adding that there are likely fewer vulnerable Tor nodes now than when he ran his scans last week since some have likely been patched and Tor has begun blacklisting. “The data is dated, but it’s a good picture of that point in time.”