An issue was discovered in the Texas Instruments (TI) TM4C microcontroller series, such as the TM4C123. The eXecute-Only-Memory (XOM) implementation prevents code read-outs on protected memory by generating bus faults. However, single-stepping and using breakpoints is allowed in XOM-protected flash memory. As a consequence, it is possible to execute single instructions with arbitrary system states (e.g., registers, status flags, and SRAM content) and observe the state changes produced by the unknown instruction. An attacker could exploit this vulnerability by executing protected and unknown instructions with specific system states and observing the state changes. Based on the gathered information, it is possible to reverse-engineer the executed instructions. The processor acts as a kind of "instruction oracle."