A cross site scripting (XSS) vulnerability exists in Check_MK versions 1.2.8x prior to 1.2.8p25 and 1.4.0x prior to 1.4.0p9, allowing an unauthenticated attacker to inject arbitrary HTML or JavaScript via the output_format parameter, and the username parameter of failed HTTP basic authentication attempts, which is returned unencoded in an internal server error page.