Greg Barbosa, writing for 9to5Mac:
A newly discovered macOS High Sierra flaw is potentially leaving your personal data at risk. Developer Lemi Orhan Ergin publicly contacted Apple Support to ask about the vulnerability he discovered. In the vulnerability he found, someone with physical access to a macOS machine can access and change personal files on the system without needing any admin credentials.
Users who haven’t disabled guest user account access or changed their root passwords (likely most) are currently open to this vulnerability. We’ve included instructions on how to protect yourself in the meantime until an official fix from Apple is released.
Incredibly embarrassing and dangerous screwup for a company as devoted to security as Apple. They’re working on a fix, and in the meantime you should follow these steps to change your root password (thankfully, I had guest user access disabled, so the bug didn’t affect my machine).
See also: Rene Ritchie’s explainer.