repository/s3/S3.php in the Amazon S3 library in Moodle through 2.2.11, 2.3.x before 2.3.9, 2.4.x before 2.4.6, and 2.5.x before 2.5.2 does not verify that the server hostname matches a domain name in the subject's Common Name (CN) or subjectAltName field of the X.509 certificate, which allows man-in-the-middle attackers to spoof SSL servers via an arbitrary valid certificate, related to an incorrect CURLOPT_SSL_VERIFYHOST value.
https://moodle.org/mod/forum/discuss.php?d=238393
http://www.openwall.com/lists/oss-security/2013/01/03/1
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-40615