The NTP implementation in Cisco IOS and IOS XE does not properly support use of the access-group command for a "deny all" configuration, which allows remote attackers to bypass intended restrictions on time synchronization via a standard query, aka Bug ID CSCuj66318.
https://exchange.xforce.ibmcloud.com/vulnerabilities/94420
http://www.securitytracker.com/id/1030549
http://www.securityfocus.com/bid/68463
http://tools.cisco.com/security/center/content/CiscoSecurityNotice/CVE-2014-3309