6.2.2. Data Flow Diagrams

Data flow diagrams (DFDs) are typically used to graphically represent a system, but you can use a different representation (such as a UML diagram) as long as you apply the same basic method: decompose the system into parts and show that each part is not susceptible to relevant threats.

DFDs use a standard set of symbols consisting of four elements: data flows, data stores, processes, and interactors, and for threat modeling we add one more—trust boundaries.

Item	Symbol
Data flow	One way arrow
Data store	Two parallel horizontal lines
Process	Circle
Multi-process	Two concentric circles
Interactors	Rectangle
Trust boundary	Dotted line

See also:

https://www.owasp.org/index.php/Threat_Risk_Modeling
http://msdn.microsoft.com/en-us/magazine/cc163519.aspx
http://www.microsoft.com/security/sdl/process/design.aspx
http://youtu.be/wUt8gVxmO-0