Recently, a security bug was discovered in macOS High Sierra
that allows anyone to root access into your Mac without entering any passcode.
Now Apple has fixed this vulnerability in macOS High Sierra update. 
If you are
running macOS High Sierra 10.13.1 and want to be safe then go ahead and install
the latest version. 
To install the update, click the Updates tab in Mac App
Store and update it or you can update through different method by going to  Apple menu () from top left corner > About this Mac > Software Update.

Apple has fixed this vulnerability in macOS High Sierra update that allows anyone to root access into your Mac without entering any passcode.
Besides that Apple apologizes to Mac users about this bug
and says its customer’s to
deserve better.
If you are running previous macOS version and stay on that version, then you should apply this fix at earliest.

How to enable Root User to fix macOS High Sierra Security bug

How to See your Mac Version

Choose Apple menu () from top left corner > About this Mac > Overview

Apple has fixed this vulnerability in macOS High Sierra update that allows anyone to root access into your Mac without entering any passcode.

For detailed information about the security content of this update see https://support.apple.com/kb/HT201222.

Also See

MAC Tips and Tricks