filter/mediaplugin/filter.php in Moodle through 2.5.9, 2.6.x before 2.6.7, 2.7.x before 2.7.4, and 2.8.x before 2.8.2 allows remote authenticated users to cause a denial of service (CPU consumption or partial outage) via a crafted string that is matched against an improper regular expression.