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When a cipher is selected using OpenSSH's protocol extension for encrypt-then-MAC mode, if an SSH server sends the largest possible size of packet that PuTTY will accept, it can cause PuTTY to overrun its buffer for the incoming packet by four bytes.
This condition can only happen after encryption is enabled and the server's host key is checked. So a MITM can't trigger it. Only the server you really meant to connect to, if it is malicious, or compromised.
We don't know exactly how exploitable this is. An overrun by 4 bytes only allows the attacker to corrupt the heap data structures following the allocated block. On the other hand, the data it writes comes directly from the network, so it can be precisely controlled by the attacker.
Thanks to Yukihiro Nakamura for discovering this vulnerability.