What it means
Complete mediation is a classic secure-design principle. It means a system should validate each access to a protected object or operation when that access occurs. The principle prevents stale, bypassed, or overly broad decisions from silently continuing after permissions, context, or risk changes.
How it works
Practical implementation depends on trusted policy-enforcement points, reliable identity and session state, and authorization checks close to the resource. Caching can be safe when its scope, lifetime, invalidation, and failure behavior are deliberately controlled. The goal is not to repeat an expensive login for every click, but to prevent access from escaping current policy.
An application checks authorization when a file is requested instead of assuming that anyone who can see a link may download the file. If the user's role changes, the next request is evaluated under the new permissions.
Practical focus
- Place authorization checks at every protected operation and data boundary
- Invalidate sessions and cached decisions when privileges change
- Use server-side enforcement rather than relying on hidden user-interface controls
- Define safe behavior when the policy service is unavailable
Common mistakes
- Checking permission only when a user signs in
- Assuming possession of an object identifier proves authorization
- Caching access decisions without expiration or revocation
Certification relevance
This subject appears in or supports the following certification bodies of knowledge: