The vulnerability exists in the /q endpoint's handling of the 'json' parameter. Since XSS occurs when input is reflected without sanitization, the servlet responsible for processing /q requests (likely QueryServlet) would read the 'json' parameter and include it in HTTP responses. The absence of proper HTML encoding in this output flow matches the described XSS pattern. Though exact code isn't available, the endpoint/parameter combination and CWE-79 classification strongly indicate improper output encoding in the request handler.