An apparatus for detecting fault injection includes functional circuitry and fault detection circuitry. The functional circuitry is configured to receive one or more functional input signals and to process the functional input signals so as to produce one or more functional output signals. The functional circuitry meets a stability condition that specifies that stability of a designated set of one or more of the functional input signals during a first time interval guarantees stability of a designated set of one or more of the functional output signals during a second time interval that is derived from the first time interval. The fault detection circuitry is configured to monitor the designated functional input and output signals, to evaluate the stability condition based on the monitored functional input and output signals, and to detect a fault injection attempt in response to detecting a deviation from the stability condition.