Embedded systems are deployed in security critical environments and have become a prominent target for remote attacks. Microcontroller-based systems (MCUS) are particularly vulnerable due to a combination of limited resources and low level programming which leads to bugs. Since MCUS are often a part of larger systems, vulnerabilities may jeopardize not just the security of the device itself but that of other systems as well. For example, exploiting a WiFi System on Chip (SoC) allows an attacker to hijack the smart phone's application processor.

Control-flow hijacking targeting the backward edge (e.g., Return-Oriented Programming--ROP) remains a threat for MCUS. Current defenses are either susceptible to ROP-style attacks or require special hardware such as a Trusted Execution Environment (TEE) that is not commonly available on MCUS.

We present µRAI, a compiler-based mitigation to emph{prevent} control-flow hijacking attacks targeting backward edges by enforcing the emph{Return Address Integrity (RAI)} property on MCUS. µRAI does not require any additional hardware such as TEE, making it applicable to the wide majority of MCUS. To achieve this, µRAI introduces a technique that moves return addresses from writable memory, to readable and executable memory. It re-purposes a single general purpose register that is never spilled, and uses it to resolve the correct return location. We evaluate against the different control-flow hijacking attacks scenarios targeting return addresses (e.g., arbitrary write), and demonstrate how µRAI prevents them all. Moreover, our evaluation shows that µRAI enforces its protection with negligible overhead.

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