CVE-2026-81634 Possible heap buffer overflow during DNSSEC canonicalization

High 7.5 Published 16 Sep 2026 Updated 16 Sep 2026

A heap buffer overflow in Unbound allows remote attackers to execute code or cause denial of service via malicious DNS responses.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-81634?

CVE-2026-81634 is a high-severity memory corruption vulnerability in Nlnetlabs Unbound, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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.

Weakness: CWE-122

Which versions are affected by CVE-2026-81634?

It affects 1 product.

Products include Nlnetlabs Unbound.

How do I fix CVE-2026-81634?

Upgrade to a fixed version.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2026-81634 being exploited?

There is no known exploitation of CVE-2026-81634 in the wild and no public exploit code.

All Nlnetlabs Unbound vulnerabilities

Sources

Also known as AZL-101721, DEBIAN-CVE-2026-81634, ECHO-258d-90d7-a305, GHSA-x9ff-rj4v-x6mq, ROOT-OS-DEBIAN-12-CVE-2026-81634, UBUNTU-CVE-2026-81634.