Update 11/29/17: Apple has released an official fix for the issue via a security update. You can install the update by launching the App Store app, and then click on Updates. Press Command-R to reload the Updates page to see new updates. It will appear as “Security Update,” and you can click on the Update button to install it. Your Mac does not need to restart.
On Tuesday, a macOS 10.13.1 security issue was revealed—a flaw that allows root access to a Mac without the need for a password. Developer Lemi Orhan Ergin tweeted that anyone can log into a Mac by entering the user name
root without a password. The first time you try to login, it won’t work. But if you try it again, you will be granted access. Here’s Erign’s tweet: