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iOS 7 Email Attachment Vulnerability Real but Limited

Security researcher Andreas Kurtz has identified a vulnerability that leaves email attachments downloaded by iOS 7’s Mail app unprotected by Apple’s Data Protection technologies. In short, data protection enhances the built-in hardware encryption by protecting the hardware encryption keys with your passcode. Apple specifically notes that this “provides an additional layer of protection for your email messages attachments, and third-party applications.”

Apple has confirmed the vulnerability officially for us, and says it will fix it in a future update, but such a fix didn’t make it into the recent iOS 7.1.1 update.

How Data Protection Works — Data Protection ensures that even if a bad guy gains physical control of your device, he can’t access protected files without knowing your passcode, even if he can break the rest of the iOS device’s security. This is especially useful to thwart attackers (or law enforcement) who connect to a device and extract a copy of the entire file system, after which they attempt to decrypt it offline. If you don’t enable Data Protection by configuring a passcode, your iOS file system is encrypted in a way that is easy to circumvent by tethering your device to a computer.

Adding a passcode engages both additional hardware security to protect your device from unapproved physical connections, and it encrypts application data storage (including email) using both your passcode and a hardware key that’s both unique to your device and nearly impossible to extract or copy. Even if someone has a complete copy of your file system they must brute force both of these codes together to see your files, which is nearly impossible to do off the device.

The alternative is to try to brute force your passcode on the device (through a tethered connection to a computer), but the encryption chips are designed to slow down this kind of attack to make it far less effective. In fact, it’s nearly impossible if you have a passcode longer than 6 to 8 characters. Data Protection is extremely effective, although older iOS devices (before the iPhone 4S and iPad 2) lack some of the special hardware and are thus more vulnerable.

Limitations to the Attack — Although Kurtz says that he was able to access the filesystem using “well-known techniques,” these techniques require technical know-how and some of the tools are compatible only with the iPhone 4 and earlier, as mentioned. Plus, we’re already in the territory of the attacker needing full physical access to the device, so this isn’t the sort of thing that could be used broadly via malware or a network connection.

An attacker either needs your passcode (in which case they have everything anyway), or he needs a jailbreak that works without a passcode, allowing him access to the file system. That’s how Kurtz was able to attack an iPhone 4. It’s unclear how he was able to reproduce on an iPhone 5s and iPad 2 running iOS 7.0.4, since more recent devices running iOS 7 aren’t susceptible to a jailbreak without the passcode. It’s possible that Kurtz had already jailbroken his iPhone 5s and iPad 2, so they weren’t as protected as a normal device would be. The bug means that email attachments still aren’t encrypted on those devices, but there isn’t a way to get to them.

Regardless, the practical upshot is that unless you receive highly sensitive information in email attachments and are at risk for being targeted in person by those interested in your data, there’s little to worry about here. Enterprise admins will want to alert users still relying on the iPhone 4, since email attachment data on those could be exposed if an attacker were to gain physical access.

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