# CVE-2026-35501: Bypass of a configured island trust anchor underneath an insecure parent from an atacker-controlled NSEC record.


- **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
- **Fix available:** Yes
- **Published:** 21 Oct 2026
- **Updated:** 29 Oct 2026
- **Also known as:** AZL-109821, DEBIAN-CVE-2026-35501, ECHO-bc3c-40d7-60f2, GHSA-mh74-9w87-h7vx, UBUNTU-CVE-2026-35501

## What is CVE-2026-35501?

CVE-2026-35501 is a low-severity vulnerability in NLnet Labs Unbound, published on 21 Oct 2026. 

### Official description

When NLnet Labs Unbound 1.0.1 up to and including version 1.26.1 is configured with both a root trust anchor and an additional 'trust-anchor:' for an island-of-security zone whose parent is provably insecure under the root chain, a malicious actor with a forged NSEC reply with the SOA bit cleared can trick Unbound to treat the configured trust island as insecure. This happens because the DNSSEC validator in Unbound accepts and caches the NSEC insecure response bypassing the RRSIG validation that the validator was supposed to do. The acceptance of the NSEC record at the trust anchor apex was allowed due to and old fix for type DS and NSEC queries from clients when trust anchor for that name existed.

Weakness: CWE-354 (Improper Validation of Integrity Check Value)

## Which versions are affected by CVE-2026-35501?


## How do I fix CVE-2026-35501?

Upgrade to a fixed version.

## Is CVE-2026-35501 being exploited?

There is no known exploitation of CVE-2026-35501 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-35501 and fix it if it is affected.

What it is: CVE-2026-35501 is a low-severity vulnerability in NLnet Labs Unbound, published on 21 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-35501 in the wild and no public exploit code.

Affected product: NLnet Labs 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-2026-35501/ (Markdown: https://insights.sbomify.com/CVE-2026-35501/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-35501
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-35501
- OSV: https://osv.dev/vulnerability/CVE-2026-35501

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-35501/
Summaries are generated and may contain mistakes; the official description above is the record.