The broadcast mode of NTP is expected to only be used in a trusted network. If the broadcast network is accessible to an attacker, a potentially exploitable denial of service vulnerability in ntpd's broadcast mode replay prevention functionality can be abused. An attacker with access to the NTP broadcast domain can periodically inject specially crafted broadcast mode NTP packets into the broadcast domain which, while being logged by ntpd, can cause ntpd to reject broadcast mode packets from legitimate NTP broadcast servers.
The broadcast mode of NTP is expected to only be used in a trusted network. If the broadcast network is accessible to an attacker, a potentially exploitable denial of service vulnerability in ntpd's broadcast mode replay prevention functionality can be abused. An attacker with access to the NTP broadcast domain can periodically inject specially crafted broadcast mode NTP packets into the broadcast domain which, while being logged by ntpd, can cause ntpd to reject broadcast mode packets from legitimate NTP broadcast servers.
http://support.ntp.org/bin/view/Main/NtpBug3114
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