Security boundaries versus trust boundaries

Security boundary and trust boundary are often used as synonyms. Although similar, there is a subtle difference between these two terms. A trust boundary is where a system changes its level of trust. An execution trust boundary is where instructions need different privileges to run. For example, a database server executing code in /bin is an example of an execution crossing a trust boundary. Similarly, a data trust boundary is where data moves between entities with different trust levels. Data inserted by an end user into a trusted database is an example of data crossing a trust boundary.

Whereas a security boundary is a point of demarcation between different security domains, a security domain is a set of entities that are within the same access level. For example, in traditional web architecture, the user-facing applications are part of a security domain and the internal network is part of a different security domain. Security boundaries...