Abstract: A protection unit of a subroutine stack accessible by a CPU controlled by one main software program, for storing and removing stack frame(s), the stack protection unit being coupleable to the stack and the CPU, comprising: a processor coupled to a first and a second address register; wherein, when a first stack frame is stored onto the stack and the execution of the main software program is suspended by the CPU due to the execution of a subroutine; the processing unit is adapted to set one access rule based on the first and second address registers, preventing: the ongoing subroutine, from accessing a hardware-protected region of the stack, comprising at least one stack frame associated with a return address from which the main software program resumes execution after termination of the execution of the subroutine. A processor, a method and a computer program are also claimed.