Using macOS High Sierra? Make sure to give your root user a password, because of this serious security issue:
Dear @AppleSupport, we noticed a *HUGE* security issue at MacOS High Sierra. Anyone can login as "root" with empty password after clicking on login button several times. Are you aware of it @Apple?
— Lemi Orhan Ergin (@lemiorhan) November 28, 2017
Update: Check the Mac App Store for Apple’s official fix. It’s being automatically pushed to your Mac as well.