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Least privilege as a design and operational principle requires that any given system element (people or software-based) has the minimum level of authority and decision-making capability that the specifically assigned task requires, and no more. This means that designers must strictly limit the access to and control over information, by any subject involved in a process or task, to that minimum set of information that is required for that task and no more. Simply put, least privilege implements and enforces need to know.

A few examples illustrate this principle in action.

 A financial disbursements clerk, when generating payments against invoices from suppliers, has to access and use information about each supplier account as well as access his company's bank-related systems to make the payment take place. However, this clerk would not be expected to modify the information about where the payment should be sent, edit the invoice, or alter the amount of the payment. Nor would this clerk be expected to need any information about other employees, such as their payroll information, while generating payments to suppliers.

 A process control system that actively manages a chemical processing system for a paint manufacturer would not normally be expected to access the Internet or have a need to run web searches of any kind.

Each time you encounter a situation in which a person or systems element is doing something in unexpected ways—or where you would not expect that person or element to be present at all—is a red flag. It suggests that a different role, with the right set of privileges, may be a necessary part of a more secure solution.

Least privilege should drive the design of business logic and business processes, shaping and guiding the assignment (and separation) of duties to individual systems and people who accomplish their allocated portions of those overall processes. Driven by the Business Impact Analysis (BIA), the organization should start with those processes that are of highest potential impact to the organization. These processes are usually the ones associated with achieving the highest-priority goals and objectives, plus any others that are fundamental to the basic survival of the organization and its ability to carry on day-to-day business activities.