On *nix systems, check your sshd_config and ssh_config files. In both files, the Protocol line should read “Protocol=2″ and NOT “Protocol=2,1″ or similar values that include protocol version 1 as an option. Putty should be configured to use only protocol version 2 as well.
Failure to check your SSH configuration can lead to a downgrade attack, where user credentials and the entire SSH session are recovered in the clear. If you are using SSH protocol version 1, your SSH session is no more secure than Telnet.