The message argument of Apache Tomact's HttpServletResponse.sendError() call is not only displayed on the error page, but is also used for the reason-phrase of HTTP response. This may include characters that are illegal in HTTP headers. It is possible for a specially crafted message to result in arbitrary content being injected into the HTTP response. For a successful XSS attack, unfiltered user supplied data must be included in the message argument.

http://www.securiteam.com/unixfocus/5TP061FP5I.html