Creating Your First System Diagram

Visual representation forms the foundation of effective threat modeling. Your first diagram doesn't need to be a work of art—clarity and accuracy matter more than aesthetics. Start with a simple data flow diagram (DFD) that shows how information moves through your system. This visual model helps identify potential security issues that might be overlooked in purely textual descriptions.

Begin by identifying the major components of your system. For a web application, these typically include users (browsers), web servers, application servers, databases, and any external services. Draw these as simple shapes—rectangles for processes, cylinders for data stores, and stick figures or squares for external entities. Don't worry about using formal notation initially; focus on capturing the system's essence.

Connect these components with arrows showing data flow. Label each flow with the type of data transmitted. For example, "login credentials" might flow from user to web server, while "user profile data" flows from database to application server. Include bidirectional flows where appropriate. As you draw, you'll likely discover flows you hadn't consciously considered, which is exactly why visual modeling proves so valuable.

Mark trust boundaries on your diagram using dotted lines or different colors. These boundaries might separate internet-facing components from internal systems, differentiate between authenticated and unauthenticated areas, or distinguish between different privilege levels. Each trust boundary represents a location where security controls are necessary. This visual identification of boundaries often reveals missing or inadequate controls.