The XSS vulnerability arises because user-controlled input from the 'q' parameter is embedded into error_message without proper escaping. The error.html template uses the |safe filter, which explicitly disables Jinja2's auto-escaping. While the direct code modification was in the template, the root cause lies in the interaction between the route handler (which passes raw user input to the template) and the template's unsafe rendering. The route handler function in app/routes.py (implied by references to line 448 in this file) is responsible for this unsafe data flow. The confidence is high because the vulnerability documentation explicitly links the parameter handling, template rendering, and safe filter usage.