---
id: "CVE-2026-121506"
kind: "vulnerability"
aliases: ["CVE-2026-121506"]
title: "NTPsec: crafted NTP replies corrupt memory and can bypass NTS time authentication"
severity: "high"
cvss: 7.4
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["linux-servers", "ntp", "ntpsec", "nts-client", "runtime"]
not_affected_if: ["managed-time-service", "no-nts-client", "other-ntp-implementation"]
published_at: "2026-11-03T15:17:37+00:00"
first_seen_at: "2026-11-03T15:22:01+00:00"
updated_at: "2026-11-03T15:22:54+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-121506", "first_seen_at": "2026-11-03T15:22:01+00:00"}]
---

# NTPsec: crafted NTP replies corrupt memory and can bypass NTS time authentication

NTPsec's NTS client parses extension fields without checking bounds, so a crafted server-mode NTP reply can corrupt heap memory and crash ntpd. Combined with a separate NTS-KE out-of-bounds read fixed in 1.2.4, an attacker can bypass NTS authentication and set the client's clock.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `ntpd --version` or `ntpq -c version` on each host and compare against the fixed release.
- Check whether NTS is enabled in the ntp configuration (look for `nts` directives in ntp.conf).

## Fix

- NTPsec `1.2.5` or later (the sources say fixed before 1.2.5; the 1.2.4 NTS-KE fix is also needed).

## Timeline

- 2026-11-03T15:22:01+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-121506

## From the sources

An out-of-bounds read and write in NTS client extension field parsing (extens_client_recv) in NTPsec before 1.2.5 allows a remote attacker who can send NTP server-mode replies to an NTS client to corrupt heap memory and crash ntpd, and, combined with the NTS-KE client out-of-bounds read fixed in 1.2.4, to bypass NTS authentication and set the client's time, via an NTS Authenticator and Encrypted Extension Fields field whose nonce length exceeds the field.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_