NTP BUG 3415: Provide a way to prevent authenticated symmetric passive peering

Last update: April 22, 2024 18:49 UTC (7e7bd5857)


Resolved 4.2.8p11 27 Feb 2018
References Bug 3415
also see: Bug 3012
also see: CVE-2016-1549
Affects All ntp-4 releases up to, but not including 4.2.8p7,
and 4.3.0 up to, but not including 4.3.92.
Resolved in 4.2.8p11.
CVSS2 Score LOW 3.5 AV:N/AC:M/Au:S/C:N/I:P/A:N
CVSS3 Score LOW 3.1 CVSS:3.0/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:L/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. Three additional protections are offered in ntp-4.2.8p11. One is the new noepeer directive, which disables symmetric passive ephemeral peering. Another is the new ippeerlimit directive, which limits the number of peers that can be created from an IP. The third extends the functionality of the 4th field in the ntp.keys file to include specifying a subnet range.



This weakness was reported as Bug 3012 by Matthew Van Gundy of Cisco ASIG, and separately by Stefan Moser as Bug 3415.