# CVE-2026-40209: Denial of service via IXFR queries

> Attackers can cause denial of service in PowerDNS DNSdist by sending IXFR queries that exhaust backend connections or file descriptors.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Jun 2026
- **Updated:** 25 Jun 2026
- **Also known as:** DEBIAN-CVE-2026-40209, GHSA-c5m7-ppm5-wp5g, UBUNTU-CVE-2026-40209

## What is CVE-2026-40209?

CVE-2026-40209 is a medium-severity denial of service vulnerability in PowerDNS DNSdist, published on 25 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An attacker might be able to cause outgoing TCP connections to backend to be stuck until a timeout occurs instead of being released immediately, by sending IXFR queries. This could be used to cause a denial of service if there is a limit to the number of concurrent connections to this backend, or if the process runs out of file descriptors.

Weakness: CWE-772 (Missing Release of Resource after Effective Lifetime)

## Which versions are affected by CVE-2026-40209?


## How do I fix CVE-2026-40209?

Apply the vendor's patch.

## Is CVE-2026-40209 being exploited?

There is no known exploitation of CVE-2026-40209 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40209 and fix it if it is affected.

What it is: CVE-2026-40209 is a medium-severity denial of service vulnerability in PowerDNS DNSdist, published on 25 Jun 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-2026-40209 in the wild and no public exploit code.

Affected product: PowerDNS DNSdist. 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-2026-40209/ (Markdown: https://insights.sbomify.com/CVE-2026-40209/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40209
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40209
- OSV: https://osv.dev/vulnerability/CVE-2026-40209

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40209/
Summaries are generated and may contain mistakes; the official description above is the record.