As a result, even when attackers succeed in executing malicious code, they're unable to get the malware permanently installed or to access sensitive parts of the OS. A key protection found in virtually all OSes is to restrict the privileges given to running software. Such escalation-of-privilege exploits have become increasingly valuable over the past decade as a way to defeat modern OS defenses. Of more concern is that malicious hackers can exploit this vulnerability to give their malware unfettered control over the computer and OS. Locking a screen with a password also appeared to protect a computer while it's unattended. The upshot of all of this: as long as someone has filevault turned on, their files are most likely safe from this exploit as long as their Mac is turned off before an attacker gets hold of it. The behavior observed in Ars tests and reported on social media was extremely inconsistent, so results are likely to vary widely. Even on Macs that have filevault turned on, the bypass can also be used to make unauthorized changes to the Mac System Preferences (including disabling filevault), or the bypass can be used to log in as root after logging out of an existing account but not turning off the machine.
Exploiting the vulnerability was also not possible when a Mac was turned on and the screen was password protected. When full-disk encryption is turned off, an untrusted user can turn on a Mac that's fully powered down and log in as root. The password bypass can be exploited in a variety of ways, depending on the way the targeted Mac has been set up. The flaw isn't present on previous macOS versions.
Ars reporters were able to replicate the behavior multiple times on three Macs. With that-after a few tries in some cases-the latest version of Apple's operating system logs the user in with root privileges. The bypass works by putting the word "root" (without the quotes) in the user name field of a login window, moving the cursor into the password field, and then hitting enter button with the password field empty. In one of Apple's biggest security blunders in years, a bug in macOS High Sierra allows untrusted users to gain unfettered administrative control without any password. What follows is the story as written before the patch was available. Installing the patch immediately is the best way for Mac users to protect themselves and supersedes any mitigation advice. Update 9:47 AM California time: Apple patched the flaw on Wednesday morning.
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