Apple’s M1 chips have an "unpatchable" hardware vulnerability that could allow attackers to break through its last line of security defenses, MIT researchers have discovered.
The vulnerability lies in a hardware-level security mechanism utilized in Apple M1 chips called pointer authentication codes, or PAC. This feature makes it much harder for an attacker to inject malicious code into a device's memory and provides a level of defense against buffer overflow exploits, a type of attack that forces memory to spill out to other locations on the chip.
Researchers from MIT’s Computer Science and Artificial Intelligence Laboratory, however, have created a novel hardware attack, which combines memory corruption and speculative execution attacks to sidestep the security feature. The attack shows that pointer authentication can be defeated without leaving a trace, and as it utilizes a hardware mechanism, no software patch can fix it.
The attack, appropriately called “Pacman," works by “guessing” a pointer authentication code (PAC), a cryptographic signature that confirms that an app hasn’t been maliciously altered. This is done using speculative execution — a technique used by modern computer processors to speed up performance by speculatively guessing various lines of computation — to leak PAC verification results, while a hardware side-channel reveals whether or not the guess was correct.
What’s more, since there are only so many possible values for the PAC, the researchers found that it's possible to try them all to find the right one.
In a proof of concept, the researchers demonstrated that the attack even works against the kernel — the software core of a device's operating system — which has "massive implications for future security work on all ARM systems with pointer authentication enabled," says Joseph Ravichandran, a PhD student at MIT CSAIL and co-lead author of the research paper.
"The idea behind pointer authentication is that if all else has failed, you still can rely on it to prevent attackers from gaining control of your system," Ravichandran added. "We've shown that pointer authentication as a last line of defense isn't as absolute as we once thought it was."
Apple has implemented pointer authentication on all of its custom ARM-based silicon so far, including the M1, M1 Pro and M1 Max, and a number of other chip manufacturers, including Qualcomm and Samsung, have either announced or are expected to ship new processors supporting the hardware-level security feature. MIT said it has not yet tested the attack on Apple's unreleased M2 chip, which also supports pointer authentication.
“If not mitigated, our attack will affect the majority of mobile devices, and likely even desktop devices in the coming years,” MIT said in the research paper.
The researchers — which presented their findings to Apple — noted that the Pacman attack isn’t a "magic bypass" for all security on the M1 chip, and can only take an existing bug that pointer authentication protects against.
When reached prior to publication, Apple would not comment on the record. After publication, Apple spokesperson Scott Radcliffe provided the following: "We want to thank the researchers for their collaboration as this proof of concept advances our understanding of these techniques. Based on our analysis as well as the details shared with us by the researchers, we have concluded this issue does not pose an immediate risk to our users and is insufficient to bypass operating system security protections on its own."
In May last year, a developer discovered an unfixable flaw in Apple’s M1 chip that creates a covert channel that two or more already installed malicious apps could use to transmit information to each other. But the bug was ultimately deemed "harmless" as malware can't use it to steal or interfere with data that's on a Mac.
Updated with on-the-record comment from Apple.