+4.4 Attack surface minimisation

4.4 Attack surface minimisation

Principle (secure by design)	Expose only what is strictly necessary. Remove or disable all unnecessary code, services, interfaces, protocols and dependencies by default.
Objective	Reduce the likelihood and impact of compromise by eliminating unnecessary entry points and limiting attacker options.

Checklist

List exposed interfaces	Identify all externally reachable APIs, ports, local interfaces (HMI), protocols, admin endpoints and update channels. Remove or disable anything without a clear, current justification.
Enforce default deny	Close all ports, APIs and interfaces by default. Explicitly allow only what is required for production.
Remove development and diagnostic functionality	Remove debuggers, test endpoints and debug modes from production builds. Remove development features.
Minimise dependencies	Remove unused libraries, SDKs and optional components from final builds. Remaining libraries should be verified for authenticity and integrity. Use the smallest possible operating system, runtime or firmware configuration that supports the product’s intended function.
Continuously monitor for legacy exposure	Continuously review exposed interfaces and dependencies. Update/remove deprecated APIs, old protocols and unused resources.
Minimise data collection, processing and retention	Collect, retain and store only the personal or sensitive data necessary for the product’s intended purpose and securely delete it when no longer required.

Minimum evidence

Exposure inventory exists: a maintained list of externally reachable interfaces (APIs, ports, protocols, admin interfaces, update channels) is stored in the repo or product documentation.
Default deny enforced: network rules, gateway configuration or device settings show that only explicitly required interfaces are enabled in production.
Minimal build verified: build configuration, manifest, package list or firmware contents show that only required components are included.
No dev or diagnostic tooling shipped that might enable exploitation: inspection of production artefacts confirms absence (or verification that they cannot impact security) of shells, debuggers, compilers, test interfaces and diagnostic utilities.
No legacy exposure: release notes, ticket or checklist entry confirms that exposed interfaces and dependencies were reviewed and any unused or legacy elements removed.
Data inventory and retention record: data inventory with purposes, retention periods and deletion methods is kept.

Release gate

Exposed interfaces reviewed; only required APIs, ports, protocols and management interfaces enabled
Default-deny posture confirmed; no unintended network paths or interfaces exposed
Production build is minimal; unused libraries, services and optional components removed
No development or diagnostic tools present in production unless there is evidence that it cannot impact security
Attack surface review completed for this release; deprecated or legacy interfaces updates/removed
Unnecessary data processing has been removed and required deletion mechanisms verified

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