A passive network sensor is a cybersecurity device or software that monitors and analyzes network traffic without actively interacting with or altering the flow of data. Unlike active sensors, passive sensors do not send probes or packets into the network, ensuring that operational systems remain undisturbed. They are typically deployed to detect threats, unauthorized activities, or policy violations by observing copies of network traffic through methods like port mirroring or network taps. This approach allows organizations to gain visibility into network activity, identify security incidents, and maintain compliance without introducing latency or risk to critical infrastructure.