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A threat model states what is being protected, which adversarial actions are considered, which assumptions are trusted and which security properties must hold. A trust boundary marks where data or authority crosses between differently trusted parts. The model guides design and evaluation rather than treating every possible failure as equally relevant.^[ref]

For example, encrypting a database backup protects its contents from someone who obtains only the stored backup. That mechanism does not by itself stop a logged-in administrator from replacing live records, or an endpoint from disclosing data after decryption. Adding those capabilities to the attacker changes the model and may require authorization checks, integrity protection and audit evidence. An explicit model makes the scope of a security claim inspectable.

A trust boundary separates components or actors to which a threat model assigns different trust or authority. Data crossing it must be checked under the receiving side's rules. A boundary can exist between a browser and server, between two operating-system processes, or inside one application.^[ref]

For example, a browser sends a requested record identifier to a database-backed service. The service must check both its format and the requester's authorization to access that record. Hiding the identifier in a GUI widget does not establish either property: an attacker can send a different request. Identifying the trust boundary makes the required checks visible before choosing a security mechanism.