The issue is related to insufficient authentication data verification in the implementation of the SSH protocol in PuTTY. This can allow a remote attacker to gain unauthorized access to protected information. Specifically, PuTTY proceeds with establishing an SSH session even if it has never sent a substantive authentication response, making it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt. This can be used to capture credential data for purposes undesired by the client user.