Permanent Laser Fault Injection into the Flash Memory of a Microcontroller

The Flash memory of a Microcontroller Unit (MCU) is an important part of its attack surface as it contains its firmware and its security related data (e.g. passwords and cryptographic keys). Recent research works report the use of Laser Fault Injections (LFI) to corrupt the firmware at run time by targeting the Flash memory during its read operations (data reads from Flash were also faulted). These faults, induced on a single bit and following a bit-set fault model, were non-permanent: the data stored in Flash stayed unaltered while only their read copies were corrupted. We report an extension of this fault model on the Flash memory of a 32-bit MCU. Using LFI, we were able to induce permanent faults into its Flash. Single bit faults, that followed a bit-reset fault model, were induced during the Flash write operations. As a proof of concept, we describe how we were able to iteratively set to zero all the bits of a 32-bit password using a laser pulse with relatively undemanding settings (15 µm beam diameter, and 3 µs pulse duration).