PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-36367
• https://git.tartarus.org/?p=simon/putty.git;a=commit;h=1dc5659aa62848f0aeb5de7bd3839fecc7debefa
• https://www.chiark.greenend.org.uk/~sgtatham/putty/changes.html
• https://git.tartarus.org/?p=simon/putty.git%3Ba=commit%3Bh=1dc5659aa62848f0aeb5de7bd3839fecc7debefa
• https://www.debian.org/security/2023/dsa-5588
• https://lists.debian.org/debian-lts-announce/2024/04/msg00016.html