I have emailed this reporter about this already. Other than allowing characters such as " and >< in a user name, there is nothing vulnerable about this page. The characters are escaped properly on this page when there is an error. I have asked for more information about this issue both via email and on our own bug tracking system. I have received no reply so far. Phorum 5.1.18 did include a FIX for an XSS issue. But, it does not appear that this reporter is referring to that.
