mod/forum/externallib.php in Moodle 2.6.x before 2.6.6 and 2.7.x before 2.7.3 does not verify group permissions, which allows remote authenticated users to access a forum via the forum_get_discussions web service.
https://moodle.org/mod/forum/discuss.php?d=275159
http://www.securitytracker.com/id/1031215
http://openwall.com/lists/oss-security/2014/11/17/11
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-45303