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.
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 See your Mac Version
Choose Apple menu () from top left corner > About this Mac > Overview
For detailed information about the security content of this update see https://support.apple.com/kb/HT201222.