Cross-site scripting (XSS) vulnerability in the user_get_user_details function in user/lib.php in Moodle through 2.6.11, 2.7.x before 2.7.9, 2.8.x before 2.8.7, and 2.9.x before 2.9.1 allows remote attackers to inject arbitrary web script or HTML by leveraging absence of an external_format_text call in a web service.