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Application & OT Security · Intermediate

Fail-Safe Defaults

Base access decisions on explicit permission rather than exclusion, so the default is no access.

Definition

Under fail-safe defaults, anything not explicitly allowed is denied, and errors result in denial. It underlies default-deny firewall rules and allowlist input validation. It is one of Saltzer and Schroeder's principles.

Example

An authorization check that throws an exception returns 403 rather than letting the request through.

Exam tip

Default deny is the answer; failure should not grant access.

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