Apple’s T2 security chip has an unfixable flaw

2014 Mac mini and 2012 Mac mini

Enlarge / The 2014 Mac mini is pictured here alongside the 2012 Mac mini. They looked the same, but the insides were different in some key—and disappointing—ways.

A recently released tool is letting anyone exploit an unusual Mac vulnerability to bypass Apple’s trusted T2 security chip and gain deep system access. The flaw is one researchers have also been using for more than a year to jailbreak older models of iPhones. But the fact that the T2 chip is vulnerable in the same way creates a new host of potential threats. Worst of all, while Apple may be able to slow down potential hackers, the flaw is ultimately unfixable in every Mac that has a T2 inside.

In general, the jailbreak community hasn’t paid as much attention to macOS and OS X as it has iOS, because they don’t have the same restrictions and walled gardens that are built into Apple’s mobile ecosystem. But the T2 chip, launched in 2017, created some limitations and mysteries. Apple added the chip as a trusted mechanism for securing high-value features like encrypted data storage, Touch ID, and Activation Lock, which works with Apple’s “Find My” services. But the T2 also contains a vulnerability, known as Checkm8, that jailbreakers have already been exploiting in Apple’s A5 through A11 (2011 to 2017) mobile chipsets. Now Checkra1n, the same group that developed the tool for iOS, has released support for T2 bypass.

On Macs, the jailbreak allows researchers to probe the T2 chip and explore its security features. It can even be used to run Linux on the T2 or play Doom on a MacBook Pro’s Touch Bar. The jailbreak could also be weaponized by malicious hackers, though, to disable macOS security features like System Integrity Protection and Secure Boot and install malware. Combined with another T2 vulnerability that was publicly disclosed in July by the Chinese security research and jailbreaking group Pangu Team, the jailbreak could also potentially be used to obtain FileVault encryption keys and to decrypt user data. The vulnerability is unpatchable, because the flaw is in low-level, unchangeable code for hardware.

“The T2 is meant to be this little secure black box in Macs—a computer inside your computer, handling things like Lost Mode enforcement, integrity checking, and other privileged duties,” says Will Strafach, a longtime iOS researcher and creator of the Guardian Firewall app for iOS. “So the significance is that this chip was supposed to be harder to compromise—but now it’s been done.”

Apple did not respond to WIRED’s requests for comment.

There are a few important limitations of the jailbreak, though, that keep this from being a full-blown security crisis. The first is that an attacker would need physical access to target devices in order to exploit them. The tool can only run off of another device over USB. This means hackers can’t remotely mass-infect every Mac that has a T2 chip. An attacker could jailbreak a target device and then disappear, but the compromise isn’t “persistent”; it ends when the T2 chip is rebooted. The Checkra1n researchers do caution, though, that the T2 chip itself doesn’t reboot every time the device does. To be certain that a Mac hasn’t been compromised by the jailbreak, the T2 chip must be fully restored to Apple’s defaults. Finally, the jailbreak doesn’t give an attacker instant access to a target’s encrypted data. It could allow hackers to install keyloggers or other malware that could later grab the decryption keys, or it could make it easier to brute-force them, but Checkra1n isn’t a silver bullet.

“There are plenty of other vulnerabilities, including remote ones that undoubtedly have more impact on security,” a Checkra1n team member tweeted on Tuesday.

In a discussion with WIRED, the Checkra1n researchers added that they see the jailbreak as a necessary tool for transparency about T2. “It’s a unique chip, and it has differences from iPhones, so having open access is useful to understand it at a deeper level,” a group member said. “It was a complete black box before, and we are now able to look into it and figure out how it works for security research.”

The exploit also comes as little surprise; it’s been apparent since the original Checkm8 discovery last year that the T2 chip was also vulnerable in the same way. And researchers point out that while the T2 chip debuted in 2017 in top-tier iMacs, it only recently rolled out across the entire Mac line. Older Macs with a T1 chip are unaffected. Still, the finding is significant because it undermines a crucial security feature of newer Macs.

Jailbreaking has long been a gray area because of this tension. It gives users freedom to install and modify whatever they want on their devices, but it is achieved by exploiting vulnerabilities in Apple’s code. Hobbyists and researchers use jailbreaks in constructive ways, including to conduct more security testing and potentially help Apple fix more bugs, but there’s always the chance that attackers could weaponize jailbreaks for harm.

“I had already assumed that since T2 was vulnerable to Checkm8, it was toast,” says Patrick Wardle, an Apple security researcher at the enterprise management firm Jamf and a former NSA researcher. “There really isn’t much that Apple can do to fix it. It’s not the end of the world, but this chip, which was supposed to provide all this extra security, is now pretty much moot.”

Wardle points out that for companies that manage their devices using Apple’s Activation Lock and Find My features, the jailbreak could be particularly problematic both in terms of possible device theft and other insider threats. And he notes that the jailbreak tool could be a valuable jumping off point for attackers looking to take a shortcut to developing potentially powerful attacks. “You likely could weaponize this and create a lovely in-memory implant that, by design, disappears on reboot,” he says. This means that the malware would run without leaving a trace on the hard drive and would be difficult for victims to track down.

The situation raises much deeper issues, though, with the basic approach of using a special, trusted chip to secure other processes. Beyond Apple’s T2, numerous other tech vendors have tried this approach and had their secure enclaves defeated, including Intel, Cisco, and Samsung.

“Building in hardware ‘security’ mechanisms is just always a double-edged sword,” says Ang Cui, founder of the embedded device security firm Red Balloon. “If an attacker is able to own the secure hardware mechanism, the defender usually loses more than they would have if they had built no hardware. It’s a smart design in theory, but in the real world it usually backfires.”

In this case, you’d likely have to be a very high-value target to register any real alarm. But hardware-based security measures do create a single point of failure that the most important data and systems rely on. Even if the Checkra1n jailbreak doesn’t provide unlimited access for attackers, it gives them more than anyone would want.

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What the newly released Checkra1n jailbreak means for iDevice security

What the newly released Checkra1n jailbreak means for iDevice security

It has been a week since the release of Checkra1n, the world’s first jailbreak for devices running Apple’s iOS 13. Because jailbreaks are so powerful and by definition disable a host of protections built into the OS, many people have rightly been eyeing Checkra1n—and the Checkm8 exploit it relies on—cautiously. What follows is a list of pros and cons for readers to ponder, with a particular emphasis on security.

The good

First, Checkra1n is extremely reliable and robust, particularly for a tool that’s still in beta mode. It jailbreaks a variety of older iDevices quickly and reliably. It also installs an SSH server and other utilities, a bonus that makes the tool ideal for researchers and hobbyists who want to dig into the internals of their devices.

“I expected it to be a little rougher around the edges for the first release,” Ryan Stortz, an iOS security expert and principal security researcher at the firm Trail of Bits, said in an interview. “It’s really nice to be able to install a new developer beta on your development iPhone and have all your tooling work out of the box. It makes testing Apple’s updates much much easier.”

Another benefit of Checkra1n is that it promises to work reliably on a wide array of hardware. Those models include devices from the iPhone 5s all the way to the iPhone X running iOS 12.3 or later. (At the moment, the Checkra1n beta doesn’t support the iPad Air 2, first generation iPad Pro, and fifth generation iPad. Users may also experience problems when running this beta on the iPhone 5s, iPad mini 2, and iPad mini 3. These incompatibilities will likely be fixed in time, as new Checkra1n updates become available.)

Also significant, Checkm8-based jailbreaks will work permanently on these devices. Unlike most jailbreaks, which exploit vulnerabilities in iOS, Checkm8 targets a flaw in the Boot ROM, which is the first code that runs when an iDevice is turned on. This code is burned into the hardware itself and can’t be patched. This is the reason Checkra1n will work with every new release of iOS over the lifetime of a vulnerable phone.

This means that people can continue to enjoy the benefits and security fixes available in new iOS releases without losing the ability to jailbreak their devices (new versions of iOS inevitably fix jailbreaking vulnerabilities). This is a far cry from jailbreaks over the past decade that forced users to run outdated versions of iOS. The last time a jailbreak targeted the Boot ROM was in 2010 when hacker George Hotz (aka Geohot) developed one for the iPhone 3GS and iPhone 4.

Checkra1n is also helpful because it makes it painfully obvious it has been used. A large Checkra1n logo displays during bootup. And the home screen will include the Cydia and Checkra1n apps, neither of which appear when an iDevice runs normally.

And like all Checkm8-based jailbreaks, Checkra1n requires physical access to the vulnerable device and a reboot, which means user data and Touch ID and Face ID are inaccessible until the next time a PIN is entered to unlock the device. This means that remote exploits aren’t possible.

The bad

Checkm8-based jailbreaks, including Checkra1n, come with some notable limitations that many jailbreaking enthusiasts consider deal-breakers. First, Checkm8 doesn’t work on iDevices introduced in the past two years, specifically those with A12 and A13 CPUs. That limits the jailbreak to older devices, most of which—but not all—are no longer sold in retail outlets.

The other major limitation is that Checkm8-based jailbreaks are “tethered,” meaning they don’t survive a reboot. Each time the device is restarted, it must first be connected to a Mac—eventually Windows versions of Checkra1n are expected—and jailbroken all over again. Untethered jailbreaks, by contrast, are much more popular because they allow iDevices to boot normally, without being connected to a computer each time.

Another drawback to any jailbreak is that it’s an inevitably risky thing, since it unbinds an iDevice from the protections and quality assurances Apple has painstakingly built into iOS over more than a decade. Apple warns here that jailbreaking can “cause security vulnerabilities, instability, shortened battery life, and other issues.” The stakes are raised further by the beta status of Checkra1n. The Checkra1n website warns: “This release is an early beta preview and as such should not be installed on a primary device. We strongly recommend proceeding with caution.”

Then there are the risks of error by inexperienced users who are drawn to Checkra1n’s reliability, robustness, and its promise to work—on older devices, anyway—in perpetuity.

“The biggest threat from Checkra1n is how easily a non-technical user can jailbreak their device, which then leaves it vulnerable to additional attacks,” Christoph Hebeisen, head of security research at mobile security provider Lookout, said. One protection that Checkra1n deactivates is the iOS sandbox, which cordons off sensitive parts of iOS from the apps it runs. The risk is heightened by the ability of jailbroken devices to run any app. Normally iPhones and iPads can run only apps that are available in the App Store, which vets submissions for security and stability before allowing them in.

One other warning: the site checkrain[.]com is an imposter site that installs a malicious profile onto the end-user device. Readers should steer clear.

The (more subtly) bad

There’s a more subtle threat posed by Checkra1n’s ease in almost completely unpairing a device from the protections that have made iOS arguably the world’s most secure OS. As noted earlier, it would be hard for someone to use this jailbreak maliciously against someone else. But Stortz, the iOS security expert at Trail of Bits, said that Checkra1n’s release demonstrates just how powerful it could be should its capabilities fall into the wrong hands.

“The threat is more real now because a sophisticated exploit is available to everyone,” he said. He went on to theorize cases of attackers reverse-engineering Checkra1n and combining its jailbreaking capabilities with rootkits or other malicious code. All attackers might need to use this malicious Checkra1n-derived jailbreak is very brief access to an iDevice. This type of attack could covertly steal text messages, login credentials, cryptographic keys, and all kinds of other sensitive data. These attacks would be particularly effective against iPhones and iPads that don’t use fingerprints or face scans for unlocking. He explained:

Checkm8 allows someone to undermine the trust of the iOS secure boot chain. Checkra1n makes it easy to do. It’s true that checkra1n puts a nice logo on it and installs development tools, but that doesn’t need to happen. Someone will modify checkra1n to remove the logo and install a rootkit instead. [In that scenario] having a PIN-only passcode is a poor choice. You’ll pick up your phone [after Checkra1n is surreptitiously installed] and unlock it, allowing the rootkit full access to your personal data.

It has been possible to create this type of malicious jailbreak since late September, when the Checkm8 exploit became public. But that kind of attack required huge amounts of time and skill. Now, Stortz said, “no one would do that when Checkra1n exists and is so well done.”

The take-away from the sort of scenario Stortz hypothesizes is this: for journalists, dissidents, and other high-value targets who use iOS devices and can afford to, it’s best to use hardware that has an A12 or higher CPU. An iDevice introduced in the past two years will ensure that it’s safe from Checkra1n-derived attacks at border crossings, in hotel rooms, or in other situations that involve brief separations.

For iOS users who can’t afford a newer iPhone or iPad, using Touch ID or Face ID can lower the chances of malicious jailbreaks since users can be tipped off that something is amiss if the iDevice unexpectedly requires a PIN. And whenever the device has been out of the user’s control, even briefly—or users suspect anything else is amiss—they should reboot it.

This level of scrutiny is probably overkill for most users of vulnerable iPhones and iPads. Unfortunately, for users belonging to more targeted groups, these precautions are a natural consequence of a post-Checkra1n era.