Security clearance

A security clearance is the trust level assigned to a subject stating the highest classification of information they may be given access to. For people it follows a formal determination, typically a background investigation and adjudication by the government or organisation that owns the information. Inside a system, the clearance is an attribute of the subject, and under mandatory access control it is compared with the security label of each object when access is requested.

A clearance is necessary but not sufficient. Classified environments commonly require need to know as well, and for compartmented information a formal access approval per compartment, so a Top Secret clearance does not open every Top Secret file. In the Bell-LaPadula model, the clearance sets the reading ceiling: the simple security property, no read up, forbids reading an object classified above it. Unlike a permission under discretionary access control, a clearance is not something an object’s owner can grant.

Exam relevance: a scenario in which a cleared user is refused a document at their own level is likely to turn on a missing need to know or compartment approval. Candidates are expected to keep the clearance (a property of the subject) apart from the label (a property of the object), and to recognise mandatory access control as the mechanism that compares them.