NSS stands for National Security Systems: federal information systems used for intelligence, military, cryptologic, command-and-control, weapons, and other mission-critical work. They are a special cybersecurity category because compromise can affect classified operations, national defense, or public safety. For that reason, NSS environments often follow stricter rules than ordinary enterprise networks, including tighter access control, approved cryptography, and special approval paths for exceptions.
In real attacks, NSS targets may be sought for espionage, sabotage, or long-term persistence, so defenders focus on segmentation, strong identity controls, vetted hardware and software, and rapid incident reporting. NSS governance also matters in defense: it determines who can set security standards, authorize deviations, and coordinate response across agencies. In practice, the term appears in policy, compliance, and architecture discussions where the question is not just what controls exist, but who has authority over them.