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Understanding the First Vulnerability Discovered in Apple Silicon and Mobile Chips

May 16, 2022

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Researchers at three universities have unearthed a vulnerability in Apple Silicon used in Macs, iPhones, and iPads. While any vulnerability is a cause for alarm, the researchers state that the risk is “not that bad”. The weakness has been dubbed the “Augury Apple Silicon vulnerability”. Discover what it is, and why it matters. 

Augury Apple Silicon vulnerability is detailed in new research

A team of researchers from the universities of Illinois, Tel Aviv, and Washington have demonstrated the first Data Memory-Dependent Prefetcher (DMP) vulnerability, titled “Augury”, that exclusively affects Apple processors. This means that the first security breach affecting the manufacturer’s proprietary chips has been identified. The Augury Apple Silicon microarchitectural flaw has been demonstrated to leak data at rest, but doesn’t appear to be “that bad” at this point.

This weakness allows attackers to access data at will. To work, this security loophole makes use of the DMP function of Apple processors. The latter works quite simply: It anticipates what files will be used and pre-loads them into memory so the user doesn’t have to wait for them—resulting in a smoother user experience.

This advantage can also be a disadvantage, because the processor can be tricked into pre-loading files that’s not needed. In this way, attackers who exploit this weakness will be able to access absolutely any file without any restrictions. It’s the same principle observed in the Spectre and Meltdown vulnerabilities. The difference between them and Augury is the number of files that can be accessed: Augury can access any file, whereas Meltdown and Spectre can only access pre-loaded files.

“Specifically, we found that the Apple M1, M1 Max, M1 Pro, and A14 processors possess an Array-of-Pointers (AoP) prefetcher that recognizes streaming and striding reads and dereferences over an array of pointers, and then prefetches the result of dereferencing future pointers […]. As the AoP DMP operates only on a stream of memory accesses, and does not have any concept of array bounds, this prefetcher can overshoot the legal set of pointers to access and attempt a prefetch of unrelated memory addresses up to its prefetch depth. This act of dereferencing the out-of-bounds pointer, potentially even if it is not actually a pointer, creates a memory side channel that an attacker can use to learn the pointer. In fact, we show that this pattern recognition is relatively robust, can operate at large strides, and can trigger even if all memory accesses are speculative and eventually squashed. Together, these capabilities enable the attacker to target and leak pointer values across much of memory”, the authors explained in their paper.

All Apple processors with DMP support are exposed

Processors that use DMP are the Apple A14 Bionic (found in the iPhone 12 and iPad Air 4th Gen), M1, and M1 Max. There is speculation that processors up to the A14 could also be vulnerable to Augury. The Pro and Ultra variants of the M1 could also suffer from this problem, but nothing has been proven yet. In fact, the only processor that researchers were able to exploit using this method was the M1 Max.

More interestingly, the same researchers said that Apple already knew about the problem. But as with any problem with Apple’s products, their lips are sealed and nobody outside the company knows if there is a fix, if one is being worked on, or if the company decided to otherwise address the issue. And since the American manufacturer places high value on security, security experts believe that they are already working on a solution. 

The flaw in the design of the Apple Silicon M1 chip allows any two applications running under iOS to covertly exchange data between them without using memory, sockets, files, or any other normal operating system features. This works between processes running as different users and under different privilege levels—creating a covert channel for surreptitious data exchange.

Why it matters

Hardware-based security flaws have been a serious issue for Intel and AMD since their discovery in 2018. Now, a flaw has been identified in Apple’s latest custom processors. Although not as serious as Meltdown and Spectre, it confirms that Apple Silicon isn’t immune to vulnerabilities. The researchers who discovered the vulnerability don’t think Augury is very dangerous, partly because it only prefetches valid virtual addresses. However, it can break address space layout randomization (ASLR)—which could be the first step in a serious exploit.