# CVE-2026-18916: Remote TCP DoS by throttling the TCP receive window

> Remote attackers can crash NSD server children by throttling TCP receive windows, causing denial of service for TCP queries.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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-2026-18916, GHSA-m6mr-q7j3-3rc6, UBUNTU-CVE-2026-18916

## What is CVE-2026-18916?

CVE-2026-18916 is a medium-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 NSD serve child, by throttling the TCP receive window after a TCP query. By continuously crashing the serve childs, the remote client can denial all TCP service to this NSD instance.

Weakness: CWE-191 (Integer Underflow (Wrap or Wraparound))

## Which versions are affected by CVE-2026-18916?

It affects 1 product.


Products: Nlnetlabs Nsd.

## How do I fix CVE-2026-18916?

Apply the vendor's patch.

## Is CVE-2026-18916 being exploited?

There is no known exploitation of CVE-2026-18916 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-2026-19401](https://insights.sbomify.com/CVE-2026-19401/): Remote UDP DoS by sending multiple DNS Cookie options (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)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-18916 and fix it if it is affected.

What it is: CVE-2026-18916 is a medium-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-2026-18916 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-2026-18916/ (Markdown: https://insights.sbomify.com/CVE-2026-18916/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-18916
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-18916
- OSV: https://osv.dev/vulnerability/CVE-2026-18916

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-18916/
Summaries are generated and may contain mistakes; the official description above is the record.