filter/urltolink/filter.php in Moodle through 2.5.9, 2.6.x before 2.6.9, 2.7.x before 2.7.6, and 2.8.x before 2.8.4 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.
