Players can login as other players(on premium servers)

1. Get the player you want to login as to connect to a 'special' server
2. After the handshake packet, the 'special' server sends a handshake packet to the real one(with the player's name)
3. The special server gets the server hash from the real one, and sends it to the client
4. The client now sends an http request to http://session.minecraft.net/game/joinserver.jsp making the server hash valid
5. The special server finishes the login to the real one