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Apple's T2 custom secure boot chip is not only insecure, it cannot be fixed without replacing the silicon

Which means your new Mac is vulnerable to 'evil maid' attacks, if that's something you worry about

Apple's T2 security chip is insecure and cannot be fixed, a group of security researchers report.

Over the past three years, a handful of hackers have delved into the inner workings of the custom silicon, fitted inside recent Macs, and found that they can use an exploit developed for iPhone jailbreaking, checkm8, in conjunction with a memory controller vulnerability known as blackbird, to compromise the T2 on macOS computers.

The primary researchers involved – @h0m3us3r, @mcmrarm, @aunali1 and Rick Mark (@su_rickmark) – expanded on the work @axi0mX did to create checkm8 and adapted it to target the T2, in conjunction with a group that built checkm8 into their checkra1n jailbreaking software. Mark on Wednesday published a timeline of relevant milestones.

The T2, which contains a so-called secure enclave processor (SEP) intended to safeguard Touch ID data, encrypted storage, and secure boot capabilities, was announced in 2017. Based on the Arm-compatible A10 processor used in the iPhone 7, the T2 first appeared in devices released in 2018, including MacBook Pro, MacBook Air, and Mac mini. It has also shown up in the iMac Pro and was added to the Mac Pro in 2019, and the iMac in 2020.

The checkm8 exploit, which targets a use-after-free() vulnerability, allows an attacker to run unsigned code during recovery mode, or Device Firmware Update (DFU) mode. It has been modified to enable a tethered debug interface that can be used to subvert the T2 chip.

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So with physical access to your T2-equipped macOS computer, and an appropriate USB-C cable and checkra1n 0.11, you – or a miscreant in your position – can obtain root access and kernel execution privileges on a T2-defended Mac. This allows you to alter macOS, loading arbitrary kernel extensions, and expose sensitive data.

According to Belgian security biz ironPeak, it also means that firmware passwords and remote device locking capabilities, instituted via MDM or the FindMy app, can be undone.

Compromising the T2 doesn't dissolve macOS FileVault2 disk encryption but it would allow someone to install a keylogger to obtain the encryption key or to attempt to crack the key using a brute-force attack.

Currently, there's no known way to conduct this attack remotely, so the T2's fragility is mainly a matter of concern to those traveling with macOS laptops who want to protect their data at border crossing and in areas like hotel rooms that may be subject to "evil maid" attacks. The chip's weakness also gives law enforcement another forensic option for probing cyber-crime.

While such threats are quite uncommon, the T2's shortcomings represent a black eye for Apple, which developed the chip at no small cost specifically to enhance device security.

Unfortunately, it appears the T2 cannot be fixed. "Apple uses SecureROM in the early stages of boot," explained Rick Mark in a blog post on Monday. "ROM cannot be altered after fabrication and is done so to prevent modifications. This usually prevents an attacker from placing malware at the beginning of the boot chain, but in this case also prevents Apple from fixing the SecureROM."

While Apple cannot fix the flaw in its T2, Mark says it should be possible to restore a compromised device that's still bootable into DFU by attaching it to a trustworthy second device.

Apple did not respond to a request for comment. ®

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