Typical severity: Low. Likelihood of attack: High. An adversary engages in activity to detect the version or type of OS software in a an environment by passively monitoring communication between devices, nodes, or applications. Passive techniques for operating system detection send no actual probes to a target, but monitor network or client-server communication between nodes in order to identify operating systems based on observed behavior as compared to a database of known signatures or values. While passive OS fingerprinting is not usually as reliable as active methods, it is generally better able to evade detection.