Data Flow Diagrams and Trust Boundaries
Mapping how data moves and where it crosses trust lines.
Why Diagram First
You cannot find threats in a system you do not understand. A Data Flow Diagram (DFD) is the foundation of threat modeling: it visually maps how data moves through your system.
The diagram makes implicit assumptions explicit. Once you can see where data goes and who touches it, threats become far easier to spot.
The Four DFD Elements
A DFD uses just four building blocks. Keep them simple:
- External entity — a person or system outside your control (user, third-party API)
- Process — something that transforms data (a service, a function)
- Data store — where data rests (database, file, cache)
- Data flow — the movement of data between elements (an arrow)
Almost any system can be represented with these four shapes.
All lessons in this course
- Why Threat Modeling Matters
- The STRIDE Framework
- Data Flow Diagrams and Trust Boundaries
- Attack Trees and Prioritizing Risk