Unknown hackers hijacked more than 300 Tor exit nodes and reconfigured them to favor unencrypted email traffic and redirect SSH connections through servers they controlled, a researcher reported Tuesday.

The hijacking began in late August, when, according to Tor volunteers, the attacker used a valid password that had been reused to gain access to a Tor management server. The attacker then installed Ebury, a backdoor written for Linux that was discovered 12 years ago.

Mysteries remain

Then the attacker connected from the management server to a second Tor server, called Switzerland2, through SSH, using an RSA key that, for unknown reasons, investigators couldn’t find among Switzerland2’s authorized keys. With administrative control of Switzerland2, the attackers installed Ebury there as well. Finally, the attacker reconfigured settings on two exit nodes that Switzerland2 hosted.

Ultimately, 333 exit relays were involved in the SSH and email interception campaign, although investigators haven’t established how the attackers gained control of all the underlying servers. The attackers then reconfigured the servers. One set of changes redirected SSH traffic traversing Tor to one of 12 attacker-controlled interceptor servers. Tor users who ignored warnings that the SSH host key had changed would then enter their SSH passwords.

A second set of changes caused the exit nodes to block ports 25, 465, 587, 993, and 995—used to accept various forms of encrypted email—while continuing to allow ports for receiving unencrypted email. If email services downgraded the connections, the attackers used their access to the exit nodes to obtain unencrypted messages. It’s unclear whether any such downgrade attempts succeeded. The hijacking occurred from October 2 to October 5.

Tor volunteers partially outlined the initial compromise of the management server and Switzerland2 and the attempt to intercept unencrypted email. Tuesday’s report revealed many new details, including how many exit nodes were compromised and how the attackers used them to redirect SSH traffic.

“If you happen to get a circuit that exits over one of those 333 infected exit nodes, you won’t notice anything at first,” Kaba, the name of the Tor volunteer who authored the post, wrote. “Encrypted web traffic is forwarded as usual, but if you attempt an SSH connection over Port 22, you’re unknowingly redirected to one of 12 SSH interception servers. That’s exactly what happened to me on Friday morning.” Kaba then received the following warning: