# CVE-2026-81634: NLnet Labs Unbound heap buffer overflow in RRSet canonicalisation

Source: https://junglewise.ai/threats/cve-2026-81634-nlnet-labs-unbound-heap-buffer-overflow-in-rrset-canonicalisation  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-16. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-81634 · Severity: high · CVSS 7.5 · Published 2026-09-16

Unbound is a DNS resolver that many organizations use to translate domain names into IP addresses. A flaw in how it processes DNS responses from name servers can cause a memory corruption issue that allows an attacker running a malicious name server to crash the resolver or potentially execute code. An attacker would need to control or intercept DNS responses, making this a risk for organizations that query untrusted or compromised upstream DNS servers.

Technologies: [NLnet Labs Unbound](https://junglewise.ai/threats/technologies/unbound). Vendors: [NLnet Labs](https://junglewise.ai/threats/vendors/nlnet-labs).

## Quick answers

**Is CVE-2026-81634 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-81634 being exploited in the wild.

**How severe is CVE-2026-81634?** High severity, with a CVSS score of 7.5, published 16 September 2026.

**Which products and versions are affected?** NLnet Labs Unbound up to and including 1.26.0.

**Is there a fix for CVE-2026-81634?** A patch was published on 2026-09-16: Unbound 1.26.1 released with fix Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-81634 attacked?** It is attacked over the network or from the adjacent network.

**Am I affected by CVE-2026-81634?** Check whether you run NLnet Labs Unbound at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

This is a heap buffer overflow vulnerability in the RRSet canonicalisation routine of Unbound. The root cause is a missing length check for the first owner name when validating buffer boundaries during canonicalisation. An attacker can craft a 255-byte query name with a large TCP response to overflow the heap buffer. The attack requires either a compromised upstream name server or the ability to intercept and modify DNS responses in transit (before DNSSEC validation occurs). An attacker can trigger a denial of service or potentially achieve code execution. The fix is available in Unbound 1.26.1, which adds the missing owner name to the buffer length check.

## Affected products

- NLnet Labs Unbound up to and including 1.26.0

## Timeline

- 2026-09-16: disclosed
- 2026-09-16: patched: Unbound 1.26.1 released with fix

## References

- [https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-81634.txt](https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-81634.txt)

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