This paper primarily focuses on designing stealthy false data injection attacks targeting two communication channels in cyber‐physical systems equipped with state estimators and attack detectors. It introduces the concept of perfect attacks, rendering the attack detector unable to detect the designed attack signals, thereby enabling attackers to further destabilize the system. Through the error model designed by the attacker, the designed attack does not require knowledge of the state estimator. By injecting a carefully designed perfect attack into the communication channel, it is possible to extend the state estimation error of a residual detector system based on the Kalman filter to infinity. Moreover, this designed attack can bypass the residual‐based detector, thus achieving a completely stealthy attack. Finally, the results of theoretical derivations are validated by a numerical example and a practical case.