http://sysadmin.oreilly.com/news/silverman_1200.html the SSH-1 public-key user-authentication method is also vulnerable; the attacker may use the signed authenticator from the client side to access the client's account on the server. However, the SSH-2 public-key and host-based client authentication methods are MITM-proof: the authenticators involved are bound to session identifiers that are forced to be different on either side of the attacker.