The vCenter Perfcharts service must enable "ENFORCE_ENCODING_IN_GET_WRITER".
Some clients try to guess the character encoding of text media when the mandated default of ISO-8859-1 should be used. Some browsers will interpret as UTF-7 when the characters are safe for ISO-8859-1. This can create the potential for a XSS attack. To defend against this, enforce_encoding_in_get_writer must be set to true.