How severe is CVE-2026-81634?

CVE-2026-81634 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2026-81634 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (39th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-81634?

CVE-2026-81634 affects Nlnetlabs Unbound. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-81634?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2026-81634 published?

CVE-2026-81634 was published on 2026-09-16 and last updated on 2026-09-23.

CVE-2026-81634

CVE-2026-81634 is a high-severity vulnerability in Nlnetlabs Unbound with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-122.

Key facts

Severity: High (CVSS 3.x base score 7.5)
EPSS exploit prediction: 0% (39th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-122
Affected product: Nlnetlabs Unbound
Published: 2026-09-16
Last modified: 2026-09-23

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.

Frequently asked questions

What is 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 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 severe is CVE-2026-81634?: CVE-2026-81634 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2026-81634 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (39th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-81634?: CVE-2026-81634 affects Nlnetlabs Unbound. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-81634?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2026-81634 published?: CVE-2026-81634 was published on 2026-09-16 and last updated on 2026-09-23.

References

https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-81634.txt

Affected products (1)

cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*

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