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You may want to rethink using iMessage… unless you’ve updated to iOS 9.3

In its release of iOS 9.3 on Monday, Apple has included a patch that’s meant to repair a serious flaw in its iMessage encryption system. The fix comes in response to a possible attack revealed by researchers at Johns Hopkins University, who privately informed Apple of the problem last November.

If attackers can obtain encrypted messages, they can impersonate an Apple server (as far as the recipient’s phone is concerned) and repeatedly send different versions of the encrypted file and key, each one with a tiny portion of the message altered. How the phone responds to the attacker’s purposeful deformations—whether it accepts the form of the message or rejects it as invalid—reveals tiny hints about the contents. After about 130,000 of those attempts, the attacker can determine the entire key and decrypt the file. And because the server gives the phone an invalid download location of the target file that causes it to ultimately ignore every request, that entire interaction with the intended recipient’s phone isn’t revealed in messages popping up on his or her screen.

“The user never sees it, the phone never displays anything,” says Ian Miers, one of the graduate researchers who developed the attack. “But the [recipient’s] computer has tried to reach out and grab the file, and we get to observe that and see whether we crafted the message correctly.”

So who is affected by this? First, the good news: iOS 9.3, which Apple released yesterday along with a parallel update Apple is releasing for the desktop version of iMessage, fixes the flaw. But now the bad news: anyone who doesn’t install the update to both their iPhone and their OSX iMessage client could still potentially have files that are sent to them decrypted using the technique. And it’s important to note that the recipient, not the sender, is the one whose devices must be patched to fully prevent the attack.

The Johns Hopkin researchers’ work represents a rare and deep crack in Apple’s encryption protections. But Miers says that the average iPhone owner shouldn’t panic: For more recent versions of iOS, at least, the technique requires hacking Apple’s server infrastructure. Even so, he advises that everyone should update immediately, not just those concerned with highly motivated hackers.

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