In this paper, we investigate the control problem of a cyber-physical system modeled with labeled Petri nets under malicious cyber attacks in the discrete event system framework. Attacks aim to modify sensor readings and control commands in sensor and actuator communication channels. We use transitions in labeled Petri nets as the formalism to model these attacks. Because the real states of the plant may be different from the states estimated by the supervisor, we construct the attack monitor and supervisor monitor to describe the above two types of states. Based on the concurrent composition of the two monitors, we develop a general attack structure that can describe all possible attacks. By splitting the attack structure into two parts, we give conditions for the existence of a robust supervisor. Finally, a new method for designing a robust supervisor to guarantee the security of the plant is proposed.