Everything you need to know about the iMessage security flaw patched by iOS 9.3

Security researchers discovered a number of weaknesses in iMessage's encryption system. Apple's patches are already slated to appear.

iMessage has flaws in how it protects messages, researchers at Johns Hopkins University explain in a paper being released today, which can lead to effective, offline decryption of intercepted messages. The researchers disclosed their work to Apple in November, and today’s release of iOS 9.3 and OS X 10.11.4 remove some exploits and make others dramatically harder to take advantage of.

A story by the Washington Post appeared early, leading to inadvertent disclosure ahead of time. The story was quickly pulled but later re-published after that become clear. We agreed with the researchers to withhold some details from the paper ahead of Apple’s patches.

Here’s what you need to know.

Did they break iMessage encryption?

Yes and no. iMessage as a label is a wrapper around several different kinds of message and file transfer that are wrapped in different sorts of encryption. Apple doesn’t disclose more than a surface description of its system, and has been criticized for years about not providing more detail, which would allow “white hat” hackers—like academic researchers and those within Apple—to more effectively probe quietly for weaknesses, which could be fixed before a malicious party or government agency could take advantage of them.

While the researchers found many avenues of exploration, some of which we can imagine that governments and criminals have already separately discovered and potentially exploited, the paper focuses on being able to decrypt attachments to iMessages, like images and other files.

However, so far, the fundamental mechanism that prevents any but the intended recipients of a text message from being able to access the descrambled text remain intact. Also, the exploited used requires extraordinary, but not impossible, access to bypass one level of security.

How does the attack work?

The researchers found the weakest point in iMessage, which has to do with how it handles messages above a certain length, which the paper refers to as “long iMessages,” and can include runs of text and attachments, like images.

Effectively, they can intercept encrypted data intended to be sent to iMessage server from iOS and OS X, and then perform an enormous number of operations to extract information that then lets them decrypt the attachment in a reasonable amount of time using off-the-shelf equipment.

(We agreed to withhold further details until Apple has released the patches.)

Are my messages at immediate risk?

Not for most people. The exploits can’t be exploited broadly, but would be useful for parties intending to pinpoint information from a small set of people or individuals because of the complexity of obtaining the source data.

Is there anything I can do to improve iMessage security?

No, all the changes have to be made by Apple in its client software for iOS and OS X and its server operations.

Has Apple fixed the problems?

Yes, Apple has fixed all the problems the researchers specifically identified through one set of updates performed quietly a few months ago, and another set that appear in iOS 9.3 and OS X 10.11.4. (We’ll discuss the specifics in a later update to this post.)

However, there are larger problems that the researchers identified that they recommend Apple fix, alongside more comprehensive disclosures to put more eyes on the problem and identify weaknesses.

Is this related to the FBI/DOJ court order?

No, although two things are absolutely likely. First, it’s likely that the National Security Agency and other governments’ code-cracking divisions were well aware of this. Despite the cleverness of the researchers’ approach, it’s low-hanging fruit that such departments would likely have probed and discovered.

Second, Apple may be bundling other security improvements to iMessage and other software in the latest iOS and OS X releases in advance of any potential change in U.S. law or ruling, as it isn’t under any publicly known order at the moment to halt its advances in security.

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