A group of academics has devised a novel side-channel attack dubbed iLeakage that exploits a weakness in the A- and M-series CPUs running on Apple iOS, iPadOS, and macOS devices, enabling the extraction of sensitive information from the Safari web browser.
“An attacker can induce Safari to render an arbitrary webpage, subsequently recovering sensitive information present within it using speculative execution,” researchers Jason Kim, Stephan van Schaik, Daniel Genkin, and Yuval Yarom said in a new study.
In a practical attack scenario, the weakness could be exploited using a malicious web page to recover Gmail inbox content and even recover passwords that are auto-filled by credential managers.
iLeakage, besides being the first case of a Spectre-style speculative execution attack against Apple Silicon CPUs, also works against all third-party web browsers available for iOS and iPadOS owing to Apple’s App Store policy that mandates all browser vendors to use Safari’s WebKit engine.
Apple was notified of the findings on September 12, 2022. The shortcoming impacts all Apple devices released from 2020 that are powered by Apple’s A-series and M-series ARM processors.
This is accomplished by means of a side-channel exploit that can be weaponized by a malicious actor to infer sensitive information through other variables like timing, power consumption, or electromagnetic emanations.
The side channel that acts as the foundation of the latest attack is a performance optimization mechanism in modern CPUs called speculative execution, which has been the target of several similar methods since Spectre came to light in 2018.
Speculative execution is designed as a way to yield a performance advantage by using spare processing cycles to execute program instructions in an out-of-order fashion when encountering a conditional branch instruction whose direction depends on preceding instructions and whose execution is not completed yet.
The cornerstone of this technique is to make a prediction as to the path that the program will follow, and speculatively execute instructions along the path. When the prediction turns out to be correct, the task is completed quicker than it would have taken otherwise.
But when a misprediction occurs, the results of the speculative execution are abandoned and the processor resumes along the correct path. That said, these erroneous predictions leave behind certain traces in the cache.
Attacks like Spectre involve inducing a CPU to speculatively perform operations that would not occur during correct program execution and which leak the victim’s confidential information via the microarchitectural side-channel.
In other words, by coercing CPUs into mispredicting sensitive instructions, the idea is to enable an attacker (through a rogue program) to access data associated with a different program (i.e., victim), effectively breaking down isolation protections.
iLeakage attack not only bypasses hardening measures incorporated by Apple but also implements a timer-less and architecture-agnostic method that leverages race conditions to distinguish individual cache hits from cache misses when two processes — each associated with the attacker and the target — run on the same CPU.
This gadget then forms the basis of a covert channel that ultimately achieves an out-of-bounds read anywhere in the address space of Safari’s rendering process, resulting in information leakage.
While chances of this vulnerability being used in practical real-world attacks are unlikely owing to the technical expertise required to pull them off, the research underscores the continued threats posed by hardware vulnerabilities even after all these years.