The time-validation implementation in (1) mod/feedback/complete.php and (2) mod/feedback/complete_guest.php in Moodle through 2.3.11, 2.4.x before 2.4.9, 2.5.x before 2.5.5, and 2.6.x before 2.6.2 allows remote authenticated users to bypass intended restrictions on starting a Feedback activity by choosing an unavailable time.