In Moodle 3.x, students can find out email addresses of other students in the same course. Using search on the Participants page, students could search email addresses of all participants regardless of email visibility. This allows enumerating and guessing emails of other students.
In Moodle 2.x and 3.x, a CSRF attack is possible that allows attackers to change the "number of courses displayed in the course overview block" configuration setting.