# CVE-2026-42004: EDNS options smuggling

> Attackers bypass DNSdist filtering rules by smuggling EDNS options via crafted records that are rewritten during ECS insertion, exposing backend systems to unfiltered data.

- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **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-2026-42004, GHSA-q4qq-c3qm-82jj, UBUNTU-CVE-2026-42004

## What is CVE-2026-42004?

CVE-2026-42004 is a low-severity 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 can send a crafted EDNS OPT record that will be ignored by DNSdist’s filtering rules, but will be rewritten as a valid OPT record when EDNS Client Subnet is inserted, causing the backend to see the EDNS option(s) that DNSdist did not filter.

Weakness: CWE-115 (Misinterpretation of Input)

## Which versions are affected by CVE-2026-42004?


## How do I fix CVE-2026-42004?

Apply the vendor's patch.

## Is CVE-2026-42004 being exploited?

There is no known exploitation of CVE-2026-42004 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42004 and fix it if it is affected.

What it is: CVE-2026-42004 is a low-severity 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-2026-42004 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-2026-42004/ (Markdown: https://insights.sbomify.com/CVE-2026-42004/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42004
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42004
- OSV: https://osv.dev/vulnerability/CVE-2026-42004

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42004/
Summaries are generated and may contain mistakes; the official description above is the record.