The commit attempts to avoid a timing attack against PAM. Some attacker can know valid user names by timing how quickly PAM returns an error, so the fix is to use an existing user in the system we always validate against to ensure a consistent timing. But which user is always present on a Linux system? root!

The code does not check if root has any valid passwords set. An invalid user would fail, loop over to root and try validate. root has no password. It’s blank. We have nullok set. And we have pam_permit.so. The invalid user is authenticated. We have enough information to do a quick POC.

1 + 1 = 3.

source: L