Trust but Verify

This principle teaches that we should always verify even when we trust an entity and its behaviour. An entity might be a user or a system. Verifying usually requires setting up proper logging mechanisms; verifying indicates going through the logs to ensure everything is normal. In reality, it is not feasible to verify everything; just think of the work it takes to review all the actions taken by a single entity, such as Internet pages browsed by a single user. This requires automated security mechanisms, such as proxy, intrusion detection, and intrusion prevention systems.