lib/filelib.php in Moodle through 2.2.11, 2.3.x before 2.3.10, 2.4.x before 2.4.7, and 2.5.x before 2.5.3 does not send "Cache-Control: private" HTTP headers, which allows remote attackers to obtain sensitive information by requesting a file that had been previously retrieved by a caching proxy server.