The concept of Trusted Execution Environments has been broadly introduced to microcontrollers with ARM’s TrustZone-M. While much experience with TrustZone-A can be applied, architectural differences with ARMv8-M lead to a very different approach to configuration and transitions between secure and non-secure worlds. This talk will deep dive into how TrustZone-M works, where to look for weaknesses in implementations, and a detailed look into NXP LPC55S69’s implementation including discovering an undocumented peripheral that leads to a priviledge escalation vulnerability exploitable with TrustedFirmware-M. Finally, NXP PSIRT will be used as a case study in how _not_ to respond to a vulnerability report.