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

> Remote attackers can crash NSD DNS server child processes by sending crafted UDP packets with specific DNS Cookie options, causing service disruption.

- **Severity:** High 8.2 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 Aug 2026
- **Updated:** 26 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-19401, GHSA-v29p-5fvf-m7p8, UBUNTU-CVE-2026-19401

## What is CVE-2026-19401?

CVE-2026-19401 is a high-severity denial of service vulnerability in NLnet Labs NSD, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-617 (Reachable Assertion), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-19401?

It affects 1 product.


Products: Nlnetlabs Nsd.

## How do I fix CVE-2026-19401?

Upgrade to a fixed version.

## Is CVE-2026-19401 being exploited?

There is no known exploitation of CVE-2026-19401 in the wild and no public exploit code.

## Other vulnerabilities in NLnet Labs NSD

- [CVE-2026-12244](https://insights.sbomify.com/CVE-2026-12244/): Heap overflow and crash with crafted SVCB RR (high 8.7)
- [CVE-2026-12245](https://insights.sbomify.com/CVE-2026-12245/): Denial of DNS over TLS service by any DoT client (high 8.7)
- [CVE-2026-18664](https://insights.sbomify.com/CVE-2026-18664/): Wrong interpretation of ACL ranges (high 8.2)
- [CVE-2026-19538](https://insights.sbomify.com/CVE-2026-19538/): Bypass of BLOCKED ACL items on proxy protocol port over TCP or TLS (high 8.2)
- [CVE-2026-12490](https://insights.sbomify.com/CVE-2026-12490/): Bypass of client certificate verification with transfer over TLS (high 8.2)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-19401 and fix it if it is affected.

What it is: CVE-2026-19401 is a high-severity denial of service vulnerability in NLnet Labs NSD, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-19401 in the wild and no public exploit code.

Affected product: NLnet Labs NSD. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-19401/ (Markdown: https://insights.sbomify.com/CVE-2026-19401/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-19401
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-19401
- OSV: https://osv.dev/vulnerability/CVE-2026-19401

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-19401/
Summaries are generated and may contain mistakes; the official description above is the record.