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In SSH-2, every 'channel' within the SSH connection (i.e. a single shell session, forwarded port, etc) specifies a maximum amount of data that can be sent for that channel within a single data packet. If the other end has more data to send than that, it will break the data up into smaller packets.
If an SSH server advertises a maximum packet size of zero, this creates a problem. PuTTY responded to that problem by going into a tight loop, consuming CPU time, and preventing the SSH connection from doing anything further useful.
This just about qualifies as a vulnerability, since it can be used as a DoS attack in the sense of wasting your computer's time and power. (And effectively terminating your SSH connection, although the server could do that anyway by other means.) However, it can't be used to do anything worse.
Also, SSH-2 connection channels do not appear until after key exchange and host key checking, so a MITM can't do this to you.