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PatchSiren cyber security CVE debrief

CVE-2016-1551 Ntp CVE debrief

CVE-2016-1551 describes a trust-confusion issue in ntpd and NTPsec where the software relies on the underlying operating system to block packets that impersonate reference clocks. According to the NVD description, reference clocks are handled like other peers and stored in the same structure, so a packet with a source IP matching a reference clock can be matched to that peer record and treated as trusted. In environments that do not block these packets with typical martian filtering, an attacker may be able to influence system time.

Vendor
Ntp
Product
CVE-2016-1551
CVSS
LOW 3.7
CISA KEV
Not listed in stored evidence
Original CVE published
2017-01-27
Original CVE updated
2026-05-13
Advisory published
2017-01-27
Advisory updated
2026-05-13

Who should care

Administrators and operators running affected NTP deployments, especially systems that depend on accurate time for authentication, logging, scheduling, or distributed coordination. Exposure is most concerning where the network path could admit spoofed packets and where martian packet filtering is not reliably enforced.

Technical summary

NVD lists affected CPEs for ntp 4.2.8p3 and NTPsec at commit a5fb34b9cc89b92a8fef2f459004865c93bb7f92. The issue is described as a reliance on the OS to filter packets that impersonate reference clocks. Because reference clocks are treated like peers, a packet sourced from a reference clock address (for example, 127.127.1.1) that reaches receive() may match the reference clock peer record and be accepted as trusted. NVD assigns CVSS 3.0 vector CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N and lists CWE-254.

Defensive priority

Medium for environments where time integrity is operationally important. The published CVSS severity is LOW, but successful time manipulation can still create security and reliability issues in dependent systems.

Recommended defensive actions

  • Confirm whether any deployed NTP or NTPsec instances match the affected versions listed by NVD.
  • Ensure network controls block spoofed or martian packets from reaching NTP services, especially packets that could imitate reference clock source addresses.
  • Review host and perimeter filtering so unauthorized traffic cannot reach UDP NTP listeners from untrusted networks.
  • Prefer vendor-maintained updates or configuration guidance referenced by the official advisories linked from NVD.
  • Audit systems that rely on NTP time for security-sensitive functions and validate that time sources are constrained and monitored.

Evidence notes

The debrief is based on the official CVE record, the NVD detail page, and the reference material enumerated in the NVD entry. The vulnerability description and affected versions come from the supplied NVD corpus. Timing context uses the CVE published and modified timestamps supplied with the record: published 2017-01-27T17:59:00.227Z and modified 2026-05-13T00:24:29.033Z.

Official resources

Publicly disclosed in the official record on 2017-01-27T17:59:00.227Z. The NVD entry was later modified on 2026-05-13T00:24:29.033Z; that modification date is not the issue date.