The monlist feature in ntp_request.c in ntpd in NTP before 4.2.7p26 allows remote attackers to cause a denial of service (traffic amplification) via forged (1) REQ_MON_GETLIST or (2) REQ_MON_GETLIST_1 requests, as exploited in the wild in December 2013.
https://h20564.www2.hpe.com/portal/site/hpsc/public/kb/docDisplay?docId=emr_na-c04790232
http://www.us-cert.gov/ncas/alerts/TA14-013A
http://www.securitytracker.com/id/1030433
http://www.oracle.com/technetwork/topics/security/linuxbulletinjul2016-3090544.html
http://www.kb.cert.org/vuls/id/348126
http://marc.info/?l=bugtraq&m=144182594518755&w=2
http://lists.opensuse.org/opensuse-updates/2014-09/msg00031.html
http://ics-cert.us-cert.gov/advisories/ICSA-14-051-04
http://aix.software.ibm.com/aix/efixes/security/ntp_advisory.asc