The remote host is vulnerable to a buffer overflow.
The remote host is using a vulnerable version of PuTTY, a SSH client built for Linux and UNIX variants as well as Microsoft Windows operating systems. It has been reported that PuTTY does not safely handle SSH2_MSG_DEBUG messages. As a result, an attacker may be able to exploit it by setting up a malicious SSH server.
Upgrade to PuTTY 0.56 or higher.