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]
Microsoft Threat Modeling Tool threats. Tampering: crossing a trust boundary; Elevation of privilege: context with greater privilege
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.
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A trust boundary is a critical concept in cybersecurity and software engineering that defines the limits within which a particular set of security controls and trust assumptions are applicable. It distinguishes between different levels of trust associated with various components of a system or network. Trust boundaries help identify where sensitive data might be exposed and where security measures must be implemented to mitigate risks effectively.