# CVE-2026-42944: Heap overflow with multiple NSID, COOKIE, PADDING EDNS options

> An attacker can cause a heap overflow and crash the Unbound DNS resolver by sending queries with multiple EDNS options when specific features are enabled.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/U:Red`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 20 May 2026
- **Updated:** 26 Aug 2026
- **Also known as:** ALPINE-CVE-2026-42944, AZL-87042, DEBIAN-CVE-2026-42944, ECHO-7892-53ae-a3fb, GHSA-7mmq-q3m9-jrv7, ROOT-OS-DEBIAN-11-CVE-2026-42944, ROOT-OS-DEBIAN-12-CVE-2026-42944, UBUNTU-CVE-2026-42944

## What is CVE-2026-42944?

CVE-2026-42944 is a high-severity memory corruption vulnerability in Nlnetlabs Unbound, published on 20 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

NLnet Labs Unbound 1.14.0 up to and including version 1.25.0 has a vulnerability that results in heap overflow when encoding multiple NSID and/or DNS Cookie EDNS and/or EDNS Padding options in the reply packet. The relevant options ('nsid', 'answer-cookie', 'pad-responses' (default)) need to be enabled for the vulnerability to be exploited. An adversary who can query Unbound can exploit the vulnerability by attaching multiple NSID and/or DNS Cookie EDNS and/or EDNS Padding options to the query. A flaw in the size calculation of the EDNS field truncates the correct value which allows the encoder to overflow the available space when writing. Those two combined lead to a heap overflow write of Unbound controlled data and eventually a crash. Unbound 1.25.1 contains a patch with a fix to de-duplicate the EDNS options and a fix to prevent truncation of the EDNS field size calculation.

Weakness: CWE-131 (Incorrect Calculation of Buffer Size), CWE-787 (Out-of-bounds Write), CWE-197 (Numeric Truncation Error)

## Which versions are affected by CVE-2026-42944?

It affects 1 product.


Products: Nlnetlabs Unbound.

## How do I fix CVE-2026-42944?

Apply the vendor's patch.

## Is CVE-2026-42944 being exploited?

There is no known exploitation of CVE-2026-42944 in the wild and no public exploit code.

## Other vulnerabilities in Nlnetlabs Unbound

- [CVE-2026-81642](https://insights.sbomify.com/CVE-2026-81642/): Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY (critical 9.1)
- [CVE-2026-33278](https://insights.sbomify.com/CVE-2026-33278/): Possible arbitrary code execution during DNSSEC validation (critical 9.1)
- [CVE-2026-42959](https://insights.sbomify.com/CVE-2026-42959/): Crash during DNSSEC validation of malicious content (high 8.7)
- [CVE-2026-82717](https://insights.sbomify.com/CVE-2026-82717/): CNAME synthesis could lead to heap corruption (high 8.4)
- [CVE-2026-81634](https://insights.sbomify.com/CVE-2026-81634/): Possible heap buffer overflow during DNSSEC canonicalization (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42944 and fix it if it is affected.

What it is: CVE-2026-42944 is a high-severity memory corruption vulnerability in Nlnetlabs Unbound, published on 20 May 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-2026-42944 in the wild and no public exploit code.

Affected product: Nlnetlabs Unbound. 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-2026-42944/ (Markdown: https://insights.sbomify.com/CVE-2026-42944/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42944
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42944
- OSV: https://osv.dev/vulnerability/CVE-2026-42944

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42944/
Summaries are generated and may contain mistakes; the official description above is the record.