What is CVE-2026-81634?

CVE-2026-81634 is Possible heap buffer overflow during DNSSEC canonicalization. The supplied enrichment data classifies it as Memory Corruption mapped to CWE-122 Heap-based Buffer Overflow.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Active exploitation is not confirmed from current sources for CVE-2026-81634. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-81634 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

A public PoC is not confirmed from current sources for CVE-2026-81634. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-81634 using CVSS 7.5, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

Workarounds are not confirmed from current sources. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

highMemory Corruption

CVE-2026-81634

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

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.

Published Sep 16, 2026Updated Sep 16, 2026Sources: NVD, NLnet Labs (CNA), GitHub, GitHub Security Advisory, Red Hat (VEX), SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Unreported

no source claims exploitation

EPSS

ahead of 0% of scored CVEs

Affects

NLnet Labs

Unbound

CVSS base

7.5

high

Remediation

The vendor's own words where we have them.

This issue is fixed starting with version 1.26.1

Red Hat VEX · CVE-2026-81634

fixed0affected2not affected0

none_available: Affected

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
NLnet Labs	Unbound	0 to < 1.26.1	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Availability

High

Confidentiality

None

Integrity

None

Scope

Unchanged

Attack Complexity

Low

Attack Vector

Network

Privileges Req

None

User Interaction

None

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
7.5	CVSS 3.1	high	3.9	3.6	nlnetlabs.nl (CNA), NLnet Labs (CNA), GHSA, Red Hat (VEX)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-122 · Heap-based Buffer Overflow

DoS: Crash, Exit, or Restart
DoS: Resource Consumption (CPU)
DoS: Resource Consumption (Memory)
Execute Unauthorized Code or Commands

Mitigation: Pre-design: Use a language or compiler that performs automatic bounds checking.

Weakness & attack patterns

CWE-122Heap-based Buffer Overflow

Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.

CAPEC-92 · Forced Integer Overflow

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Prioritize edge telemetry for network-reachable Unbound.

web/proxy/WAF logsapplication logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 7.5 (AV:N)
Sep 16, 2026 · NVD
Initial · CVE published
Sep 16, 2026 · NVD
Reported · Fixes released with version 1.26.1
Sep 16, 2026 · NLnet Labs
Added · GHSA-x9ff-rj4v-x6mq published (high)
Sep 16, 2026 · GitHub Security Advisory
CVE published by MITRE.
Sep 16, 2026 · SOCRadar CTI
Reported · Vlatko Kosturjak verifies patch
Aug 4, 2026 · NLnet Labs
Reported · NLnet Labs shares patch
Aug 3, 2026 · NLnet Labs
Issue reported by Vlatko Kosturjak
Jul 31, 2026 · NLnet Labs

References

6 on the record

www.nlnetlabs.nl/downloads/unbound/CVE-2026-81634.txt
Vendor Advisory
nvd.nist.gov/vuln/detail/CVE-2026-81634
NVD
github.com/advisories/GHSA-x9ff-rj4v-x6mq
Exploit, Third Party Advisory
access.redhat.com/security/cve/CVE-2026-81634
Vendor Advisory
bugzilla.redhat.com/show_bug.cgi?id=2535054
Vendor Advisory
www.cve.org/CVERecord?id=CVE-2026-81634
Vendor Advisory

Elsewhere on this site

NLnet Labsevery CVE for this vendor
Memory Corruptionsame class
CWE-122other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-81634 is Possible heap buffer overflow during DNSSEC canonicalization, a high vulnerability affecting Unbound from NLnet Labs. The current evidence does not list it in CISA KEV, and the exploit status is: Active exploitation is not confirmed from current sources for CVE-2026-81634. Public exploit evidence is: A public PoC is not confirmed from current sources for CVE-2026-81634. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 7.5 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.