Elcomsoft, a Russian firm that has created tools to break into iPhones, discovered the vulnerability as it worked to update its phone breaker tool. It found that backups saved after a user updates to iOS 10 uses a new "password verification mechanism" that skips several security checks, according to a blog post.
The attack targets password-protected backups made by iOS 10. If an attacker managed to get one of those backup files without the associated password, Elcomsoft’s new attack would allow it to crack the encryption "approximately 2500 times faster compared to the old mechanism used in iOS 9 and older." Where the company could process 2,400 passwords per second under iOS 9, it can run 6 million passwords per second in iOS 10.
The weakness of the iTunes backups appears to be a weak link in security for the iPhone — but only for iOS 10 users. Elcomsoft noted that trying to break into the physical phone or into iCloud has gotten incredibly difficult, but accessing a backup stored on a computer allows for some access. "Forcing an iPhone or iPad to produce an offline backup and analyzing resulting data is one of the very few acquisition options available for devices running iOS 10."
"We're aware of an issue that affects the encryption strength for backups of devices on iOS 10 when backing up to iTunes on the Mac or PC. We are addressing this issue in an upcoming security update. This does not affect iCloud backups," a spokesperson said. "We recommend users ensure their Mac or PC are protected with strong passwords and can only be accessed by authorized users. Additional security is also available with FileVault whole disk encryption."
In the meantime, it might be best to wait for an updated version of iOS before you back your phone up.