The core_enrol_get_enrolled_users web service in enrol/externallib.php in Moodle through 2.6.11, 2.7.x before 2.7.11, 2.8.x before 2.8.9, and 2.9.x before 2.9.3 does not properly implement group-based access restrictions, which allows remote authenticated users to obtain sensitive course-participant information via a web-service request.
https://moodle.org/mod/forum/discuss.php?d=323234
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-51861