If you're a Mac user and you downloaded this week's Mac OS X update (Mavericks 10.9.3), you may have noticed that your computer's Users/ and Users/Shared/ folders have disappeared. It turns out the disappearance is due to a security flaw in iTunes 11.2, which was part of the OS X Mavericks 10.9.3 update. The flaw is only really serious on iMacs and MacBooks with multiple user accounts, as it lets one account compromise the other accounts on the same computer. Apple has already patched the flaw with iTunes 11.2.1, which users can download via the Software Updater or from the Apple website.

Apple explained in a support note that the bug had to do with how the computer handles the permissions of separate users on the same machine: "Upon each reboot, the permissions for the /Users and /Users/Shared directories would be set to world-writable, allowing modification of these directories [by any user]." The issue also occurred on some Mavericks computers that had iTunes 11.2 installed separately without upgrading to Mavericks 10.9.3.