Fail secure is a design principle where a system enters a protected state when it encounters an error, timeout, crash, or outage. Instead of keeping access paths open, it preserves restrictive defaults: deny access, lock sensitive functions, and avoid unsafe fallback behavior. In security engineering, this is often called fail closed.
It matters because many attacks exploit weak failure modes. If an authentication service, API gateway, or access-control component breaks and the system still grants permission, an ordinary fault can become a security breach. A fail secure design keeps secrets, permissions, and critical workflows protected until the system can verify state again. Defenders use it in firewalls, vaults, privilege checks, and safety controls so that disruption reduces availability, not confidentiality or integrity.