Lesson 4 of 6 · 8 min read
Least Privilege, Defence in Depth and Fail-Safe Defaults
Three design principles do most of the work in secure architecture. Least privilege limits what any one component, person or agent can do, so a compromise stays small. Defence in depth layers controls that fail independently, so another control may still block the path when one fails. Fail-safe defaults deny access unless something explicitly allows it. Use them to identify which consequences remain possible after a component is compromised.
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Practise it as an interviewer asks it
What is excessive agency, and how do you contain it?How do you evaluate a claim that prompt injection has been solved?Explain least privilege and defence in depth with an example where one layer failing does not cause a breach.Design layered defences against indirect prompt injection. Which layers are deterministic, and which are probabilistic?Design an agent's tool permissions: least privilege, scoped identities, read/write split and approvals.What are Linux capabilities, why is CAP_SYS_ADMIN called 'the new root', and how do you decide which capabilities a process gets?