If you're running macOS High Sierra, don't let anyone near your Apple Mac.
It's possible for anyone to login to the Mac and get the admin level of access to change passwords, get access to all data on the main account and lock the original user out. Fortunately, there's a fix that should solve the problem, even as Apple works to patch.
First, the bug. In what may go down as one of the most embarrassing vulnerabilities in Apple history, all a "hacker" needs to do is sign in as an "Other" user, type in "root" for a username and no password. Then they're in. Experts tested the vulnerability and found it wide open, allowing a change of passwords for other accounts on the Mac. The initial finding came from Lemi Orhan Ergin, founder of Software Craftsmanship Turkey, who disclosed the bug via Twitter.
Patrick Wardle, a security researcher with Synack with a history of Apple hacks, tweeted how simple (and baffling) the bug really is.
Whilst it would normally require physical access, and won't work if the Apple Mac is rebooted and has disk encryption enabled (and therefore requires another password), the attack opens up some serious issues. Thieves will now have an easy way into Apple Macs they've stolen, whilst the government can now quickly login to any devices they couldn't get into before.
Wardle and other researchers have noted it's possible to launch attacks without physical access to devices. But hacks from afar will only work where screen sharing features are turned on. It's possible to check if they're on (and turn them off) by going to the Sharing section of System Preferences.
There's a fix!
Solutions to the issue are coming in. One that expert has tested appears to work. It involves opening a terminal and typing the command sudo passwd root. Then type the user's normal password, which will then allow them to add a password to the root account. No longer will an unauthorized user be able to get in without that additional credential.
Wardle told the fact the fix worked confirmed what is happening: if root isn't set up on macOS High Sierra, Apple will enable a new admin-level account without a password when prompted as per the attack above.
An Apple spokesperson said in an emailed statement: "We are working on a software update to address this issue. In the meantime, setting a root password prevents unauthorized access to your Mac. To enable the Root User and set a password, please follow the instructions here: https://support.apple.com/en-us/HT204012. If a Root User is already enabled, to ensure a blank password is not set, please follow the instructions from the ‘Change the root password’ section."
Mac users would also be wise to add full disk encryption in the meantime.