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For networked cyber-physical systems to proliferate, it is important to ensure that the resulting control system is secure. We consider a physical plant, abstracted as a single input- single-output stochastic linear dynamical system, in which a sensor node can exhibit malicious behavior. A malicious sensor may report false or distorted sensor measurements. For such compromised systems, we propose a technique which ensures that malicious sensor nodes cannot introduce any significant distortion without being detected. The crux of our technique consists of the actuator node superimposing a random signal, whose realization is unknown to the sensor, on the control law-specified input. We show that in spite of a background of process noise, the above method can detect the presence of malicious nodes. Specifically, we establish that by injecting an arbitrarily small amount of such random excitation into the system, one can ensure that either the malicious sensor is detected, or it is restricted to add distortion that is only of zero-power to the noise entering the system. The proposed technique is potentially usable in applications such as smart grids, intelligent transportation, and process control.