Fail secure

Fail secure means that when a control loses power, crashes, or otherwise fails, it defaults to denying access: the electric lock stays locked, the firewall stops passing traffic, the vault door remains sealed. The failure state protects the confidentiality and integrity of whatever sits behind the control. It is also called fail closed, and it is the normal posture for controls guarding property and data, where it forms one deliberate layer of defense in depth.

Its opposite, fail safe, defaults to protecting people: fire exit doors unlock when power dies so occupants can evacuate. The same physical mechanism, an electric door lock, can be configured either way, and the choice is a question of what the failure must protect: people, property, or availability. On network gear the vocabulary shifts to fail open versus fail closed, a trade of availability against security, and it belongs to the same family of design decisions as the preventive control types.

Exam relevance: life safety always wins. If a scenario involves people who might need to evacuate, the answer is fail safe, no matter how valuable the asset behind the door; ISC2 will never accept trapping humans to protect property. If the question concerns an unmanned vault, a data centre cage, or a firewall guarding sensitive data, fail secure (fail closed) is correct. The classic trap pairs an expensive asset with an occupied room and dares you to pick the lock that stays shut.