report/outline/index.php in Moodle 2.2.x before 2.2.7, 2.3.x before 2.3.4, and 2.4.x before 2.4.1 does not properly enforce the moodle/user:viewhiddendetails capability requirement, which allows remote authenticated users to discover a hidden lastaccess value by reading an activity report.
https://moodle.org/mod/forum/discuss.php?d=220161
http://openwall.com/lists/oss-security/2013/01/21/1
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-33340