Update: 9:29 am PT: Apple has now released a fix for the bug described here. That fix is part of Security Update 2017-001, which is available from the Mac App Store, in the Updates tab, with the label “Install this update as soon as possible.” (Somewhat confusingly, there have already been previous Security Update 2017-001 releases, for unrelated issues, for Sierra, El Capitan and Yosemite.) This update should be installed as soon as possible, and does not require a restart.

On Tuesday afternoon, a tweet about a vulnerability in macOS High Sierra set off a firestorm of commentary throughout the Twitterverse and elsewhere.

It turns out that the issue in question works with any authentication dialog in High Sierra. For example, in any pane in System Preferences, click the padlock icon to unlock it and an authentication dialog will appear. Similarly, if you try to move a file into a folder you don’t have access to, you’ll be asked to authenticate:

Enter “root” as the username, and leave the password field blank. Try this a few times, and it may work on the first try, but more likely you’ll have to try two or a few more times.

When the authentication window disappears, whatever action you were attempting will be done, without any password required.

Let’s take a step back for just a moment and consider what this means. On a Unix system, such as macOS, there is one user to rule them all. (One user to find them. One user to bring them all and in the darkness bind them. /end obligatory nerdy Lord of the Rings reference>)

That user is the “root” user. The root user is given the power to change anything on the system. There are some exceptions to that on recent versions of macOS, but even so, the root user is the single most powerful user with more control over the system than any other.

Being able to authenticate as the root user without a password is serious, but unfortunately, the problem gets worse. After this has bug has been triggered, it turns out you can do anything as root on the first try, without a password.

The root user, which has no password by default, is normally disabled. While the root user is disabled, it should not be possible for anyone to log in as root. This is how macOS has worked since day one, and it has never been an issue before, but this vulnerability causes the root user to become enabled… with no password.

Unfortunately, this means that anyone will be able to log into your Mac using user “root” and no password!

Note that this does not require that the login window be set to always ask for a username and password. If you have it set to display a list of user icons instead, after triggering this vulnerability, there will be an “Other…” icon that will be present on the login screen. Clicking that will allow you to manually enter “root” with no password.

Remote access

This bug does not appear to be exploitable through some of the remote access services that can be enabled in the Sharing pane of System Preferences. Remote Login, which enables access via SSH, does not appear to be exploitable in our testing, nor does File Sharing. Even after triggering the bug and, thus, enabling the root user with no password, we were not able to connect to the vulnerable Mac through these methods.

Unfortunately, it looks like Screen Sharing, which allows you to view and remotely control the screen of your Mac, is vulnerable to this bug. In fact, it can actually be used to trigger this bug, without needing to rely on the root user already having been enabled!

In the screen sharing authentication window on a remote Mac, the same technique can be used. We were able to connect via screen sharing, using “root” as the username and no password, on the second attempt. At that point, the root user was enabled on the remote Mac, and we were able to log in to the root account via screen sharing without any blatant indication that we were doing so appearing on the screen shown to the logged in user on the target Mac. (An icon does appear in the menu bar on the target Mac, but it is not immediately obvious what that icon means. The average user will likely never notice the new icon.)

Unforeseen consequences

Once someone is logged into your Mac as root, they can do whatever they want, including accessing your files, installing spyware, you name it. So, in other words, if you were to leave your Mac unattended for 30 seconds, someone could backdoor it and have a very powerful way in later.

Suppose that you are Suzy, an average office worker in a cubicle farm. You step away from your desk for a moment to grab a cup of coffee. You’ll only be gone for about a minute, and don’t bother locking your screen. While you’re gone, Bob from the next cubicle comes over and “roots” your computer.

Later, you go to lunch. You’re gone for an hour, and Bob knows this because he’s familiar with your routine. He uses the root user to log into your Mac and install spyware—perhaps something to peep through the webcam, hoping to catch you in a compromising position later on when you’ve taken your MacBook Pro home with you.

Of course, all that’s even easier if you have screen sharing turned on, and he can install the spyware remotely, without ever touching your Mac.

Creeped out yet?

Fortunately, if you have your Mac’s hard drive encrypted with FileVault, this will prevent the attacker from having a persistent backdoor. In order to log in, the attacker would have to know the password that will unlock FileVault. Not even the all-powerful root user can access an encrypted FileVault drive without the password.

It’s also worth pointing out that a well-prepared attacker with access to your unlocked Mac could install spyware in less than a minute without relying on this vulnerability and without needing an admin password of any kind (depending on what the spyware does). Some spyware can be installed with normal user privileges.

Further, with a longer interval of unsupervised physical access to any Mac that doesn’t have FileVault turned on, an attacker can install spyware of any kind without needing an admin password.

Avoiding an attack using this vulnerability is actually fairly trivial. Just turn on FileVault, and always lock your Mac’s screen or log out when you’re away from it. While you’re at it, set a firmware password. And, to prevent remote access, turn off all services in System Preferences -> Sharing as a precaution.

Still, this is a very serious vulnerability, which Apple needs to address as quickly as possible. We contacted Apple for comment, but by the time of this writing, had not heard back.

Undoing the damage

If you, like many, have tried this out on your own Mac, you’ve opened up a potential backdoor. Fortunately, closing that door isn’t particularly hard, if you know the door is there and that it’s open.

First, open the Directory Utility application. It’s buried deep in the system where it’s hard to find, but there’s an easy way to open it. Just use Spotlight. Click the magnifying glass icon at the right side of the menu bar, or press command-space, to invoke Spotlight. Then start typing Directory Utility in the search window. Once the application is found, simply double-click it in the list to open it. (Or, even easier, press return once it’s selected in the search results.)

Once Directory Utility opens, click the lock icon in the bottom left corner of the window to unlock it. Then, pull down the Edit menu.

If you see an item reading Enable Root User, as shown in the screenshot above, you’re good. Whatever you did, the root user wasn’t enabled. Quit Directory Utility, and go about your business.

If, instead, you see an item reading Disable Root User, choose that. The root user will be disabled again, as it should be, and it will no longer be possible to log in as the root user from the login screen. Just be aware that this does nothing to protect against the vulnerability, so the root user could easily be enabled again.

Be sure to take the other measures described above to secure your system against unauthorized physical access. Namely,  turn on FileVault, always lock your Mac’s screen or log out when you’re away from it, set a firmware password, and turn off all services in System Preferences -> Sharing.