The moodle_enrol_external:role_assign function in enrol/externallib.php in Moodle 2.0.x before 2.0.4 and 2.1.x before 2.1.1 does not have an authorization check, which allows remote authenticated users to gain privileges by making a role assignment.
http://openwall.com/lists/oss-security/2011/11/14/1
http://moodle.org/mod/forum/discuss.php?d=182738
http://git.moodle.org/gw?p=moodle.git%3Ba=commit%3Bh=d20f655d59cd486fd9b3a26ad353af13daafd1d3