NTP BUG 3012(p12 update): Sybil vulnerability: ephemeral association attack

Last update: June 28, 2022 20:06 UTC (57417e17c)


Resolved 4.2.8p12 (Improve noepeer behavior.) 14 Aug 2018
References Bug 3012 CVE-2018-7170
While fixed in ntp-4.2.8p7 and with significant additional protections for this issue in 4.2.8p11, ntp-4.2.8p12 includes a fix for an edge case in the new noepeer support. Refer to CVE-2016-1549 for additional info.
Affects All ntp-4 releases up to, but not including 4.2.8p7, and 4.3.0 up to, but not including 4.3.94. Resolved in 4.2.8p11. Improved in 4.2.8p12 and 4.3.94.
CVSS2 Score LOW 3.5 AV:N/AC:M/Au:S/C:N/I:P/A:N
CVSS3 Score MED 5.3 CVSS:3.0/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N


ntpd can be vulnerable to Sybil attacks. If a system is set up to use a trustedkey and if one is not using the feature introduced in ntp-4.2.8p6 allowing an optional 4th field in the ntp.keys file to specify which IPs can serve time, a malicious authenticated peer – i.e. one where the attacker knows the private symmetric key – can create arbitrarily-many ephemeral associations in order to win the clock selection of ntpd and modify a victim’s clock. Two additional protections are offered in ntp-4.2.8p11. One is the noepeer directive, which disables symmetric passive ephemeral peering. The other extends the functionality of the 4th field in the ntp.keys file to include specifying a subnet range.



This weakness was originally discovered by Matthew Van Gundy of Cisco ASIG. The edge-case hole in the noepeer processing was reported by Martin Burnicki of Meinberg.