A trivial-to-exploit flaw in macOS High Sierra, aka macOS 10.13, allows users to gain admin rights, or log in as root, without a password.
The security bug is triggered via the authentication dialog box in Apple’s operating system, which prompts you for an administrator’s username and password when you need to do stuff like configure privacy and network settings.
If you type in “root” as the username, leave the password box blank, hit “enter” and then click on unlock a few times, the prompt disappears and, congrats, you now have admin rights. You can do this from the user login screen.
The vulnerability effectively allows someone with physical access to the machine to log in, cause extra mischief, install malware, and so on.
You should not leave your vulnerable Mac unattended until you can fix the problem. And while obviously this situation is not the end of the world – it’s certainly far from a remote hole or a disk decryption technique – it’s just really, really sad to see megabucks Apple drop the ball like this.
But there is a workaround for now. If you have a root account enabled and a password for it set, the above blank password trick will not work.
So, keep the account enabled and set a root password right now.