Remember this MacOS bug? Letting you login to any computer as a root user by typing "root" as the username with no password.

My IT department had a blast with that one, pure disbelief that it worked on all of our systems

https://arstechnica.com/information-technology/2017/11/macos...

What about Hotmail's "eh" flaw of 1999? I'd say a two-letter password is practically "zero auth".