The core vulnerability stemmed from two key issues: 1) The default error handler (DefHandler.handle) failed to apply HTML escaping to error message/reason fields when rendering error pages, as shown by the patch adding xssFilter and escaper. 2) Multiple locations passed req.path(true) (raw path data) to error constructors, introducing unvalidated user input into error messages. The high-confidence vulnerable function is DefHandler.handle due to direct lack of output encoding, while the path-related functions are medium confidence as they contributed attack vectors but weren't the ultimate XSS sink.