NTP BUG 3071: Client rate limiting and server responses

Last update: June 27, 2022 20:45 UTC (51d68a4aa)


Resolved 4.2.8p9 21 Nov 2016
References Bug 3071 CVE-2016-7426
Affects ntp-4.2.5p203, up to but not including ntp-4.2.8p9,
and ntp-4.3.0 up to, but not including ntp-4.3.94.
Resolved in 4.2.8p9
CVSS2 Score LOW 1.0 AV:L/AC:H/Au:S/C:N/I:N/A:P
CVSS3 Score LOW 1.6 CVSS:3.0/AV:P/AC:H/PR:H/UI:R/S:U/C:N/I:N/A:L


When ntpd is configured with rate limiting for all associations (restrict default limited in ntp.conf), the limits are applied also to responses received from its configured sources. An attacker who knows the sources (e.g., from an IPv4 refid in server response) and knows the system is (mis)configured in this way can periodically send packets with spoofed source address to keep the rate limiting activated and prevent ntpd from accepting valid responses from its sources.

While this blanket rate limiting can be useful to prevent brute-force attacks on the origin timestamp, it allows this DoS attack. Similarly, it allows the attacker to prevent mobilization of ephemeral associations.



This weakness was discovered by Miroslav Lichvar of Red Hat.