The vulnerability stems from the default error page for PageNotFoundException (and possibly other exceptions) not sanitizing user input. The EngineUtil class in Mayaa's codebase is a core component responsible for engine operations, and the handlePageNotFoundException() method would logically be involved in exception handling. Since the advisory explicitly implicates the default error page rendering for this exception, the function responsible for generating this page (which includes unsanitized user/exception data) is the most likely candidate. The lack of output encoding in this error rendering pathway directly enables the XSS vulnerability.