Tor before 0.1.1.20 allows remote attackers to identify hidden services via a malicious Tor server that attempts a large number of accesses of the hidden service, which eventually causes a circuit to be built through the malicious server.

Tor 0.1.0.13 and earlier, and experimental versions 0.1.1.4-alpha and earlier, does not reject certain weak keys when using ephemeral Diffie-Hellman (DH) handshakes, which allows malicious Tor servers to obtain the keys that a client uses for other systems in the circuit.