To be more specific, this impacts Macs running the latest version of High Sierra — 10.13.1 (17B48).
Here's an article on TechCrunch with some more info, including a temporary solution from Apple: https://techcrunch.com/2017/11/28/astonishing-os-x-bug-lets-anyone-log-into-a-high-sierra-machine/
TLDR; DON'T LEAVE YOUR MAC UNATTENDED UNTIL THIS IS FIXED.