The inspiration for this work is the rising possibility of cyber attacks, making security a critical problem in many applications. This work addresses two key problems related to the security of a Finite State Machine (FSM): attack detection and localization. A composition of nominal plant and the attacker FSM is developed to analyze different security aspects, under actuator attacks, and necessary and sufficient conditions are developed for attack detection. In some cases, the intruder can make replacements in the sensor or actuator channel. From the security perspective, it is important to identify the attacked channel. The term localization is used, to detect the attacked channel, and necessary and sufficient conditions are provided that ensure localization.