Threat Boundary

What Is a Threat Boundary?

Also known as a trust boundary, a threat boundary is a dividing line or partition between areas of systems (whether internal or external) which may separate networks, applications, or organizations, as well as different security classifications of data. Threat boundaries define where security controls should be implemented between trust zones [link to glossary article] to guard against threats and mitigate vulnerabilities. .

A threat boundary can be thought of as a door between rooms, one of which contains valuable items. Properly credentialed individuals—i.e., those with the right key—can access that room, while anyone else cannot (although malicious actors seek to steal or duplicate keys to gain unauthorized access). In zero-trust security architectures, all networks, systems, and users are assumed to be untrusted, which means all traffic is verified through threat boundaries at every point of access.

Why Are They Important?

Understanding threat boundaries is central to effective security design. They are crucial as they demarcate where different security controls and protocols—such as encryption, firewalls, access controls, and authorization and authentication—should be used and why. Threat boundaries are essential to an effective multi-layered security strategy that reduces attack surfaces across perimeters and limits unauthorized access as well as the spread of malware.

What Are Some Key Considerations for Threat Boundaries?

Establishing threat boundaries not only helps improve security and reduce risk, but also provides businesses with much-needed visibility into data flows between systems, networks, and users, which helps meet regulatory requirements regarding sensitive data. Many security breaches occur as data moves between systems with many differing trust levels. However, identifying all the potential threat boundaries in complex systems can be difficult and time-consuming. Balancing security controls and usability requirements can also be challenging. 

How Are They Related to Threat Modeling?

Threat boundaries are a core component of threat modeling: They undergird threat models by identifying attack surfaces and aid in visualizing data flows and designing security controls. Threat boundaries also support risk prioritization based on the trust levels among systems and components. Threat modeling uses the information that threat boundaries provide to determine which security controls should be implemented and where, forming the basis of future iterative improvements.  

Closing

Threat boundaries are foundational to threat modeling, offering a structured view into the lines between various trust zones and data flows throughout a system. The sooner that security and development teams can establish where threat boundaries lie, the sooner they can ensure that gaps are closed and proper countermeasures are in place.