Open redirect vulnerability in the clean_param function in lib/moodlelib.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 redirect users to arbitrary web sites and conduct phishing attacks via vectors involving an HTTP Referer header that has a substring match with a local URL.