CVE-2026-81634
HIGHDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:P
Exploit Intelligence
Low risk: more likely to be exploited than 29% of all known CVEs.
References
Related Vulnerabilities
Other CWE-122 (Heap-based Buffer Overflow) vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2015-3113 | Critical | 9.8 | 100% | KEV | Fix |
| CVE-2020-25687 | Medium | 5.9 | 87% | - | Fix |
| CVE-2009-3459 | High | 8.8 | 87% | KEV | Fix |
| CVE-2021-21017 | High | 8.8 | 86% | KEV | - |
| CVE-2020-25683 | Medium | 5.9 | 86% | - | Fix |
| CVE-2023-27997 | Critical | 9.8 | 86% | KEV + Ransom | - |
Common questions
How do I fix CVE-2026-81634?
Upgrade unbound on debian unstable to 1.26.1-1 or later.
Is CVE-2026-81634 being actively exploited?
Not that we know of. CVE-2026-81634 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.36% is the estimated probability that it will be exploited in the next 30 days. That is higher than 29% of all scored CVEs.
How severe is CVE-2026-81634?
CVE-2026-81634 has a CVSS v3 base score of 7.5, rated important. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-81634 affect?
Published advisories record a fix for unbound (Debian). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-09-16.