There are hackable security flaws in software. And then there are those that don't even require hacking at all—just a knock on the door, and asking to be let in. Apple's macOS High Sierra has the second kind.

On Tuesday, security researchers disclosed a bug that allows anyone a blindingly easy method of breaking that operating system's security protections. When anyone hits a prompt in High Sierra asking for a username and password before logging into a machine with multiple users, installing an application or changing settings, they can simply type "root" as a username, leave the password field blank, click "unlock" twice, and immediately gain full access.

In other words, the bug allows any rogue user that gets the slightest foothold on a target computer to gain the deepest level of access to a computer, known as "root" privileges. Malware designed to exploit the trick could also fully install itself deep within the computer, no password required.

"We always see malware trying to escalate privileges and get root access," says Patrick Wardle, a security researcher with Synack. "This is best, easiest way ever to get root, and Apple has handed it to them on a silver platter."

As word of the security vulnerability rippled across Twitter and other social media, a few security researchers found they couldn't replicate the issue, but others captured and posted video demonstrations of the attack, like Wardle's GIF below, and another that shows security researcher Amit Serper logging into logged-out account. WIRED also independently confirmed the bug.

The fact that the attack could be used on a logged-out account raises the possibility that someone with physical access could exploit it just as easily as malware, points out Thomas Reed, an Apple-focused security researcher with MalwareBytes. They could, for instance, use the attack to gain root access to a logged-out machine, set a root password, and then regain access to a machine at any time. "Oooh, boy, this is a doozy," says Reed. "So, if someone did this to a Mac sitting on a desk in an office, they could come back later and do whatever they wanted."