A security architect is building a threat model for a new customer-facing web application. She has produced a data flow diagram (DFD) showing the browser, a public web front end, an internal application server, and a back-end database. Before enumerating specific threats, she wants to identify the points in the diagram where data crosses between components operating at different privilege or trust levels. Which threat model component is she analyzing to prioritize where controls such as authentication and input validation are most critical?