Accept any private key for authentication
Configuring an SSH server to accept any password would be easy with PAM — put pam_permit
on the auth
stack, and voilà. The possibility of misconfiguring such an open system is inherent to the flexibility of PAM — since it lets you chain as many tests as you want, the possibility of doing 0 tests is unavoidable (at least without introducing weird exceptions that wouldn't cover all cases).
Key authentication doesn't go through PAM, and there's no configuration setting for “accept any key”. That would only be useful in extremely rare cases (for testing or honeypots), so it isn't worth providing it as an option (with the inherent risk of misconfiguration).