Hardware trojan (HT) attacks against CPUs typically assume threat scenarios where an attacker targeting a system with a trojanized CPU is able to execute arbitrary code (i.e. machine-level instructions) to reliably interact with the implanted trojan. On end-user devices (i.e., mobiles, laptops), achieving arbitrary code execution in practice requires software exploits tailored to each specific target. Such strong adversarial premises reduce the generality of existing threat models casting doubt on CPU trojan attacks as a pragmatic threat vector. To push the envelope on HT attacks against client devices, we introduce the SURF class of CPU-trojans that can be activated without arbitrary code execution. Our key insight is that integer operations expressed in a high-level language can be mapped to microarchitectural side-effects distinguishable by a SURF trigger circuit. This observation unlocks HT activation via runtime engines, constrained environments executing untrusted high-level code. We demonstrate a SURF trojan inside a RISC-V processor and exploit JavaScript-level memory indexing operations inside Google's V8 engine to perform a code injection attack. Importantly, we show that SURF trojans remain effective across multiple JavaScript engine versions, enabling long-term compromise of endpoint devices. To facilitate research, we opensource SURF's design and supporting software.

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