The bug was discovered by Lemi Orhan Ergin who tweeted about it this morning:
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?
The bug has been verified and it is a massive security risk. It appears that an attempt to login as root with no password will enable the root user if it's not already enabled and give you access to the device. It appears to work on macOS 10.13, 10.13.1, and 10.13.2 beta.
To try the bug for yourself:
● Open System Preferences
● Choose Users & Groups from the System Preferences window
● Click the lock at the bottom left of the window
● Enter root as the username and hit enter or click Unlock.
It's believed that the first time you click Unlock the root account is enabled and the second time you click unlock you gain access. However, some users report needing to click a few more times. Others report needing to move the cursor into the password field first.
There have been reports of the bug working in Apple Script, in Installers, from the login window, from a guest account, via shared services login, and more.
Until Apple releases a fix, you should enable the root user on your machine and give it a non-blank password. You can find instructions on how to do so below. This will prevent any malicious login attempts.
● How to Enable the Root User and Change the Root User Password in macOS High Sierra
Apple says its working on a software update which should be available soon. Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.