When Memory Mappings Attack: On the (Mis)use of the ARM Cortex-M FPB Unit

Abstract: In recent years we have seen an explosion in the usage of low-cost, low-power microcontrollers (MCUs) in embedded devices around us due to the popularity of Internet of Things (IoT) devices. Although this is good from an economics perspective, it has also been detrimental for security as microcontroller-based systems are now a viable attack target. In response, researchers have developed various protection mechanisms dedicated to improve security in these resource-constrained embedded systems. We demonstrate in this paper these defenses fall short when we leverage benign memory mapped design-for-debug (DfD) structures added by MCU vendors in their products. In particular, we utilize the Flash Patch and Breakpoint (FPB) unit present in the ARM Cortex-M family to build new attack primitives which can be used to bypass common defenses for embedded devices. Our work serves as a warning and a call in balancing security and debug structures in modern microcontrollers.