lib/setup.php in Moodle through 2.4.11, 2.5.x before 2.5.9, 2.6.x before 2.6.6, and 2.7.x before 2.7.3 does not provide charset information in HTTP headers, which might allow remote attackers to conduct cross-site scripting (XSS) attacks via UTF-7 characters during interaction with AJAX scripts.