The MoodleQuickForm class in lib/formslib.php in Moodle through 2.1.10, 2.2.x before 2.2.10, 2.3.x before 2.3.7, and 2.4.x before 2.4.4 does not properly handle a certain array-element syntax, which allows remote attackers to bypass intended form-data filtering via a crafted request.