+4.5 Defence in depth

4.5 Defence in depth

Principle (secure by design)	Apply multiple layers of security so that the failure or bypass of a single control does not result in full compromise.
Objective	Reduce the impact of security failures by layering independent controls that slow attackers, increase detection and prevent single points of compromise.

Checklist

Layer controls around critical assets	Identify the most sensitive assets (e.g. credentials, control functions, customer data). Ensure that more than one independent control protects each asset. Where critical data or commands cross trust boundaries, consider integrity and freshness controls that remain effective if another protection layer, such as transport security, is bypassed.
Assume control failure and design for containment	Design systems so that bypass of one control does not grant unrestricted access.
Enable detection at multiple layers	Generate security-relevant logs at key layers (identity, network, application, data). Ensure that failed and suspicious actions raise alerts and are not silently blocked.
Use diverse and independent controls	Avoid relying on multiple controls that share the same technology, identity source or trust assumption.
Slow attackers and enable response	Apply controls that increase attacker effort and time (rate limits, step-up authentication, staged access). Ensure that response mechanisms exist to contain or isolate compromised components when alerts are triggered.

Minimum evidence

Layered protection mapped: a simple table or diagram showing critical assets and the multiple security controls protecting each (stored in repo/wiki).
Independent controls confirmed: configuration or design notes showing that layered controls do not rely on the same identity source or enforcement point.
Multilayer logging enabled: log configuration or sample logs showing security events recorded across controls.
Alerts and response defined: alert rules or playbook reference showing how suspicious activity is detected and the associated containment or response action.

Release gate

Critical assets have more than one security control applied and the layering is documented.
Layered controls are independent; no single identity source or enforcement point is a single point of failure.
Security-relevant events are logged at multiple layers
Alerts are in place for suspicious activity and a response action exists

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