ISO 27001 A.8.27: Secure system architecture and engineering principles

Document a set of secure engineering principles that all development must follow. The essentials:

Defence in depth - layer multiple controls so a single failure does not compromise everything. Least privilege - grant the minimum access needed for users, processes, and systems. Fail secure - when something breaks, it should deny access, not grant it. Separation of duties - split critical functions across different components and people. Economy of mechanism - keep security designs simple to shrink the attack surface. Complete mediation - check every access request rather than caching authorisation decisions. Secure defaults - systems should be locked down out of the box, requiring deliberate action to open up. Open design - security should not depend on keeping the design secret.

Turn these principles into practical reference architectures for your common system types (web applications, microservices, IoT). Include security patterns for authentication, authorisation, secure communications, error handling, and logging.

Bake secure engineering into the development workflow. Run security architecture reviews at the design stage. Use threat modelling to verify principles are properly applied. For significant projects, make security design review a gate before development starts.

Keep the principles alive. Review them annually and after significant security incidents. Fold in lessons from security testing and incident response. Update reference architectures as technology and threats evolve. Share best practices and case studies across teams.

Train all architects and developers on the principles and how to apply them in practice. Include secure architecture topics in training programmes. Provide mentoring and review support for complex security design decisions.