Fail-safe design is an engineering approach that keeps a system in a safe state when a component fails, loses power, or behaves unexpectedly. Instead of continuing in an uncertain mode, the system defaults to the least dangerous condition, such as locking a door, shutting down a valve, or disabling a control path. In cyber security, this matters because attackers often target the systems that manage access, power, cooling, and remote administration. If those controls fail open or remain partially active, an intrusion or outage can spread quickly.
In practice, fail-safe design appears in building access controls, industrial control systems, and data-center operations. For example, a secure facility may require doors to lock on power loss, management interfaces to shut down when authentication is lost, and safety systems to preserve cooling or ventilation during abnormal conditions. Defenders use it to reduce blast radius and prevent unsafe states; attackers try to defeat it by abusing dependencies, logic flaws, or poorly segmented fallback paths.