But the first security update has already come out, and we suggest you apply it urgently.
The update is called High Sierra 10.13 Supplemental Update, detailed in the security advisory APPLE-SA-2017-10-05-1.
There are two bugs fixed; the facepalming one is described thus:
[BUG.] A local attacker may gain access to an encrypted APFS volume. If a [password] hint was set in Disk Utility when creating an APFS encrypted volume, the password was stored as the hint.
To explain.
APFS is short for Apple File System, Apple’s new way of organising hard disks that replaces the old (but still supported) HFS Plus, a 20-year-old filing system itself derived from Apple’s Hierarchical Filing System, or HFS, that dates back to the 1980s.
By some accounts, APFS was long overdue: HFS Plus dated from the early days of Mac OS, and wasn’t really designed for the Unix core that was introduced in OS X (now macOS).
For example, HFS Plus can’t deal with dates after 2040, and doesn’t allow multiple processes to access the filesystem at the same time, making it more sluggish and less future-proof than other widely-used filing systems such as NTFS on Windows and ext4 on Linux.
New drivers, new utilities
APFS was introduced as Apple’s default and preferred filing system in High Sierra.
This means new drivers inside the operating system to support disks formatted with the new system, and new features in Apple’s disk management utilities to prepare APFS disk volumes for use.
There are two main disk management tools in macOS – the easy-to-use graphical tool Disk Utility, and the super-powerful but arcane command line program diskutil.
It turns out that the APFS support in the High Sierra version of Disk Utility has feet of clay, as we’ll show here.
We erased a USB disk and created a new APFS (Encrypted) volume on it.
Disk Utility prompted us for a password (twice) and an optional hint.
We entered keepthisSecret as the password and The hint should be shown as the hint.
Disk Utility created the encrypted volume and mounted it automatically.
We unplugged the USB disk and then plugged it back in, and macOS asked for the password. We entered keepthisSecret and the disk was unlocked and mounted, showing that the password had been set as expected.
So far, so good, until we unplugged the device and plugged it back in:
Again, macOS asked for the password. This time, we clicked the [Show Hint] button before entering the password.
The password dialog revealed that keepthisSecret has been set as the hint as well as the password.
The text The hint should be shown had, it seemed, simply been thrown away.
In other words, if you set a password hint as suggested, anyone who stole your disk could “hack” the password simply by using Disk Utility’s [Show Hint] button!
What to do?
If you haven’t created any new APFS encrypted volumes since upgrading to High Sierra, you are OK. If you created an APFS encrypted volume but didn’t specify a hint, you are OK. If you created an AFPS encrypted volume using diskutil you are OK (the bug is in Disk Utility, not the operating system itself).
If you upgraded to High Sierra from an earlier version of macOS, your disk will have been converted to APFS, but any hint you had before is left untouched (so
far as we can tell), so you are OK.
Apply the APPLE-SA-2017-10-05-1 Supplemental Update as soon as you can.
By the way, you can blank out the password hint on any APFS volume, just in case, with the following diskutil command in a terminal window:
$ diskutil apfs hint /Volumes/[YOURNAME] -user disk -clear
Removing any hint from cryptographic user XXXXXXXX on APFS Volume diskYsZ
$
If there wasn’t a hint, no harm is done, but you’ll see an error message like this, so by repeating the above command until you provoke the error message, you can verify that any hint was indeed scrubbed:
Error editing cryptographic user on APFS Volume:
Unable to set APFS crypto user passphrase hint (-69554)
Alternatively, you can overwrite the existing password hint by using the command line option -hint, instead of -clear, like this:
$ diskutil apfs hint /Volumes/[YOURNAME] -user disk -hint "Your hint here"
Setting hint "Your hint here" for cryptographic user XXXXXXXX on APFS Volume diskYsZ
$
Whatever you do, though, don’t follow the suggestions of Apple’s own diskutil help text, which offers this terrible advice:
$ diskutil apfs hint help
[. . . .]
Set a passphrase hint for an existing cryptographic user; you can specify
"disk" for the "Disk" user. Specifying "-clear" will remove any hint.
Ownership of the affected disks is required.
Example: diskutil apfs setPassphraseHint disk5s1 -user disk -hint NameOfMyPet
$
Pets’ names makes a dreadful passwords, because they’re usually neither secret nor hard to guess, and setting a hint to tell a crook that you have made a dreadful password choice just makes a bad thing worse.
Of course, if you had set a hint with Disk Utility, then for all you know someone who knew the [Show Hint] trick might have seen your password, so you ought to change it.
You can update the passphrase on an APFS Encrypted volume quickly and easily as follows:
$ diskutil apfs changepassphrase /Volumes/[YOURNAME] -user disk
Old passphrase for user XXXXXXXX: ..........
New passphrase: ..........
Repeat new passphrase: ..........
Changing passphrase for cryptographic user XXXXXXXX on APFS Volume diskYsZ
Passphrase changed successfully
$
A bad look for Apple, letting a buggy system utility like that into a production release…
…but a creditable response by Apple in getting a fix out quickly.
