What are Trust Boundaries in threat modeling and why are they important?

Lines in the diagram where data crosses between different trust levels — exactly the points where an attacker can interject, so they're where you focus threat analysis.

Trust Boundaries mark where data flows cross between different trust levels.

Key concepts:

• Point/surface where an attacker can interject
• Add trust boundaries that intersect data flows in diagrams

Examples of trust boundaries:

• Machine boundaries - between different computers
• Privilege boundaries - between user and admin processes
• Integrity boundaries - between trusted and untrusted data

Important notes:

• Threads in a native process often share the same trust level (same privileges, rights, identifiers)
• Processes talking across a network always have a trust boundary (even with secure channels)
• Encrypting network traffic is an "instinctive" mitigation but doesn't address tampering or spoofing