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Security Fundamentals & Crypto · Foundational

Fail-Secure

Failure mode where a system defaults to a locked or denied state, protecting assets when it fails.

Definition

A fail-secure (fail-closed) system maintains security when a component fails, typically by blocking access. In logical security, a firewall that stops passing traffic on failure is fail-secure. In physical security, fail-secure locks remain locked when power is lost, which can conflict with life-safety requirements.

Example

An electronic lock on a server room stays locked during a power outage.

Exam tip

Human safety takes priority: exterior emergency exits must be fail-safe, while data centers and vaults are typically fail-secure.

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