# CVE-2026-42959: Crash during DNSSEC validation of malicious content

> An attacker can crash the Unbound DNS resolver by sending malicious DNSSEC replies, causing a denial of service.

- **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:** 1 Sep 2026
- **Also known as:** ALPINE-CVE-2026-42959, AZL-87021, DEBIAN-CVE-2026-42959, ECHO-2fce-8b09-c05e, GHSA-5jp8-75xr-qrmf, ROOT-OS-DEBIAN-11-CVE-2026-42959, ROOT-OS-DEBIAN-12-CVE-2026-42959, UBUNTU-CVE-2026-42959

## What is CVE-2026-42959?

CVE-2026-42959 is a high-severity 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 up to and including version 1.25.0 has a denial of service vulnerability in the DNSSEC validator that can lead to a crash given malicious upstream replies. When Unbound constructs chase-reply messages for validation, the code uses the wrong counter to calculate write offsets for ADDITIONAL section rrsets. DNAME duplication could increase the ANSWER section count and authority filtering could decrease the AUTHORITY section count and create an uninitialized array slot. Combining these two, the validator later dereferences this uninitialized pointer, causing an immediate process crash. An adversary controlling a DNSSEC-signed domain can trigger this bug with a single query by configuring a DNAME chain with unsigned CNAMEs and a response containing unsigned AUTHORITY records alongside signed ADDITIONAL glue records. Unbound 1.25.1 contains a patch with a fix to use the proper counters to calculate the write offsets.

Weakness: CWE-824 (Access of Uninitialized Pointer)

## Which versions are affected by CVE-2026-42959?

It affects 1 product.


Products: Nlnetlabs Unbound.

## How do I fix CVE-2026-42959?

Upgrade to a fixed version.

## Is CVE-2026-42959 being exploited?

There is no known exploitation of CVE-2026-42959 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-2026-42944](https://insights.sbomify.com/CVE-2026-42944/): Heap overflow with multiple NSID, COOKIE, PADDING EDNS options (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-2026-42959 and fix it if it is affected.

What it is: CVE-2026-42959 is a high-severity 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-2026-42959 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-2026-42959/ (Markdown: https://insights.sbomify.com/CVE-2026-42959/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42959
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42959
- OSV: https://osv.dev/vulnerability/CVE-2026-42959

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42959/
Summaries are generated and may contain mistakes; the official description above is the record.