Security in the Design Phase

Security must begin before any code is written, during the architectural and design phases of development. Threat modeling for XSS involves systematically identifying where user input enters your application, how it flows through various components, and where it might be displayed to other users. Create data flow diagrams that clearly show these paths, marking trust boundaries where data moves from untrusted to trusted zones. This visual representation helps teams understand where XSS protections are most critical.

Design decisions profoundly impact XSS resistance. Choosing frameworks with built-in XSS protections, designing APIs that separate data from code, and architecting clear boundaries between user content and application code all contribute to security. For example, designing a system where user content is always served from a separate domain eliminates many XSS risks through same-origin policy enforcement. Similarly, choosing a template engine with automatic escaping by default prevents many vulnerabilities from ever being introduced.

Security requirements should be as detailed and testable as functional requirements. Instead of vague statements like "the application should be secure against XSS," create specific requirements: "All user input displayed in HTML contexts must be HTML-encoded using the framework's built-in encoding function," or "Content Security Policy headers must be implemented with script-src restricting to self and specific trusted domains." These concrete requirements can be verified through testing and code review.