4.18.4 ‘SSH protocol version’

This allows you to select whether to use SSH protocol version 2 or the older version 1.

You should normally leave this at the default of ‘2’. As well as having fewer features, the older SSH-1 protocol is no longer developed, has many known cryptographic weaknesses, and is generally not considered to be secure. PuTTY's protocol 1 implementation is provided mainly for compatibility, and is no longer being enhanced.

If a server offers both versions, prefer ‘2’. If you have some server or piece of equipment that only talks SSH-1, select ‘1’ here, and do not treat the resulting connection as secure.

PuTTY will not automatically fall back to the other version of the protocol if the server turns out not to match your selection here; instead, it will put up an error message and abort the connection. This prevents an active attacker downgrading an intended SSH-2 connection to SSH-1.