Apple is urgently working to fix what has been called a "huge" security issue in the latest version of its Mac operating system.
Turkish software developer Lemi Orhan Ergin tweeted the tech giant to say he had discovered that anyone could log in to a computer running MacOS High Sierra without a password via system preferences.
In the tweet, he said: "Dear @AppleSupport, we noticed a *HUGE* security issue at MacOS High Sierra. Anyone can login as "root" with empty password after clicking on login button several times. Are you aware of it @Apple?"
He went on to say: "You can access it via System Preferences>Users & Groups>Click the lock to make changes. Then use "root" with no password. And try it for several times. Result is unbelievable!"
Videos posted online show people in the users and groups box typing the username "root" at the login screen, leaving the password field empty, and appearing to get unrestricted access to the machine.
One Twitter user called Mike Hanley said: "This is not the password-less future we all had in mind."
In a statement, Apple said it was "working on a software update to address this issue".
On Apple's support website, it advises users to set a password for the root user as a temporary workaround.
Hackers cannot exploit the bug remotely, so until Apple creates an update to fix it, Mac owners are advised to keep their computers in a secure place.