A method of detecting illegal execution of code sequences includes the steps of: setting an active identifier to a first sequence identifier of a first code sequence, executing at least part of the first code sequence, calling, from the first code sequence, a second code sequence having a second sequence identifier, providing (20) a caller sequence identifier and a callee sequence identifier, checking (21) whether the callee sequence identifier is the same as the second sequence identifier, checking (23) whether the caller sequence identifier is the same as the active identifier, and generating (27) an alarm signal if either of the checks provide incorrect results.