---
id: CVE-2026-81634
title: >-
  In NLnet Labs Unbound up to and including 1.26.0, a 255 length query name with
  a large TCP response can lead to a heap buffer overflow during the RRSet
  canonicalisation routine
summary: >-
  In NLnet Labs Unbound up to and including 1.26.0, a 255 length query name with
  a large TCP response can lead to a heap buffer overflow during the RRSet
  canonicalisation routine. This is caused by missing to add the first owner
  name into …
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-122
  - CWE-120
vendor: NLnet Labs
product: Unbound
affected:
  - Unbound < 1.26.1
published: '2026-09-16'
updated: '2026-09-16'
sourceUpdated: '2026-09-16T19:41:10.423'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-81634'
references:
  - url: 'https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-81634.txt'
    label: sep@nlnetlabs.nl
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-81634.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-81634'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2535054'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-81634'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-81634'
  - url: 'https://access.redhat.com/errata/RHSA-2026:68590'
tags:
  - nvd
  - cve.org
  - csaf
  - vex
  - red-hat
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-09-16T14:32:31.803030Z'
ingestedAt: '2026-09-16T08:52:29.597Z'
epss: 0.00355
epssPercentile: 0.29247
patched:
  - hardened_images
---

## Overview

In NLnet Labs Unbound up to and including 1.26.0, a 255 length query name with a large TCP response can lead to a heap buffer overflow during the RRSet canonicalisation routine. This is caused by missing to add the first owner name into the buffer length check. A malicious actor operating a malicious name server or tampering with an incoming response to Unbound (canonicalisation happens before DNSSEC validation), can trigger the vulnerability.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

## Vendor advisories

- **Red Hat VEX** · Important · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, … · updated 2026-09-21 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-81634.json)
- **RHSA-2026:68590** · Red Hat · fixed in: Red Hat Hardened Images · released 2026-09-17 · [advisory](https://access.redhat.com/errata/RHSA-2026:68590)