tor -- diffie-hellman handshake flaw

A tor advisory reports

Tor clients can completely loose anonymity, confidentiality,
and data integrity if the first Tor server in their path is
malicious. Specifically, if the Tor client chooses a
malicious Tor server for her first hop in the circuit, that
server can learn all the keys she negotiates for the rest of
the circuit (or just spoof the whole circuit), and then read
and/or modify all her traffic over that circuit.