If you’re running macOS High Sierra 10.13.1, run, don’t walk, to the App Store and install Apple Security Update 2017-001 right now.

If you want to download the update to manually install, or push out with Munki or your favourite managed software update system, then head over to the Download Page for Security Update 2017-001.

This update fixes a critical issue in macOS High Sierra that lets anyone with physical access to your computer take complete control over it, without needing to know your admin password. Once it’s been exploited, it also enables the root account (the highest level system administration account on the computer) to be able to log in remotely with no password. Yes, this is pretty serious. Please make sure you’re updated ASAP.

Apple have made the update available in the App Store under Updates and they will begin automatically and silently pushing this update out to all Macs running 10.13.1 later today.

The update is quick and easy to install and, in my limited testing, doesn’t need a reboot.