# CVE-2026-18321: NTPsec buffer overflow in Zyfer refclock

Source: https://junglewise.ai/threats/cve-2026-18321-ntpsec-buffer-overflow-in-zyfer-refclock  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-31. Updated: 2026-07-31  
License: CC BY 4.0 with a link back

CVE-2026-18321 · Severity: medium · CVSS 4.7 · Published 2026-07-31

A security vulnerability has been identified in NTPsec, a suite of tools used to synchronize time across computer networks. A local user with limited access could trigger a system crash by exploiting a flaw in how the software handles specific hardware clock data. This could lead to a denial of service, disrupting time-sensitive operations and network stability.

Technologies: [Ntpsec](https://junglewise.ai/threats/technologies/ntpsec). Vendors: [Ntpsec](https://junglewise.ai/threats/vendors/ntpsec).

## Quick answers

**Is CVE-2026-18321 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-18321 being exploited in the wild.

**How severe is CVE-2026-18321?** Medium severity, with a CVSS score of 4.7, published 31 July 2026.

**Which products and versions are affected?** Ntpsec < 1.2.5.

**Is there a fix for CVE-2026-18321?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-18321 attacked?** It is attacked with local access, in a software library.

**Am I affected by CVE-2026-18321?** Check whether you run Ntpsec at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A classic buffer overflow (CWE-120) exists in the Zyfer reference clock (refclock) driver within NTPsec. The vulnerability is caused by a buffer copy operation that does not properly check the size of the input data. A local attacker with low privileges can exploit this flaw to trigger a crash of the ntpd daemon. The attack requires specific conditions (high complexity) but results in a partial impact on system availability. The issue is addressed in NTPsec version 1.2.5.

## Affected products

- NTPsec ntpsec < 1.2.5

## Timeline

- 2026-07-31: disclosed
- 2026-07-31: advisory

## References

- [https://gitlab.com/NTPsec/ntpsec/-/work_items/890](https://gitlab.com/NTPsec/ntpsec/-/work_items/890)

## Related threats

- [NTPsec out-of-bounds read/write in NTS client extension parsing](https://junglewise.ai/threats/cve-2026-121506-an-out-of-bounds-read-and-write-in-nts-client-extension-field)
- [NTP and NTPsec reference clock impersonation vulnerability](https://junglewise.ai/threats/cve-2016-1551-ntp-and-ntpsec-reference-clock-impersonation-vulnerability)