CVE-2026-81634: In NLnet Labs Unbound up to and including 1.26.0, a 255 length query name with a large TCP response can lead t

Q: How do I fix CVE-2026-81634?

unbound: upgrade to 1.26.1. Exact commands for each OS are on this page.

Q: Is CVE-2026-81634 being exploited?

CVE-2026-81634 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-81634 affect?

CVE-2026-81634 affects unbound. Severity is high (CVSS 7.5). 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

Q: How urgent is CVE-2026-81634?

Priority 42/100. High severity; Remotely exploitable, no login, no user action

This month

HIGH 7.5REMOTEUPGRADE

CVE-2026-81634 is a high-severity flaw (CVSS 7.5) affecting unbound. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 59%)
CISA exploited list	No
Weakness	CWE-122
Published	2026-09-16
Last modified	2026-09-23

Publisher (CNA): NLnet Labs · Vendor: nlnetlabs

Last updated 2026-09-23 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-16 09:17 UTCPublished CVE record published
2026-09-23 19:51 UTCUpdated Record last modified

nlnetlabs unbound UPGRADE

Affected: < 1.26.1

Fixed in: 1.26.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy unbound || rpm -q unbound || unbound --version

2. UPGRADE to 1.26.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade unbound

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade unbound

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Advisories and coverage

Official advisories

Amazon Linux 2 ALAS: ALAS2UNBOUND-1.17-2026-011 (important): unbound 2026-09-28
Amazon Linux 2 ALAS: ALAS2-2026-3941 (important): unbound 2026-09-28

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-81634?

unbound: upgrade to 1.26.1. Exact commands for each OS are on this page.

Is CVE-2026-81634 being exploited?

CVE-2026-81634 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-81634 affect?

CVE-2026-81634 affects unbound. Severity is high (CVSS 7.5). 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

How urgent is CVE-2026-81634?

Priority 42/100. High severity; Remotely exploitable, no login, no user action

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References

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