What is CVE-2026-77955?

CVE-2026-77955 is Possible ZONEMD verification bypass window. The supplied enrichment data classifies it as Unclassified mapped to CWE-345 CWE-345.

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-2026-77955. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-77955 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-2026-77955. 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-2026-77955 using CVSS 4.4, 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.

medium

CVE-2026-77955

CVE-2026-77955 — Possible ZONEMD verification bypass window

In NLnet Labs Unbound 1.13.2 up to and including 1.26.1, a vulnerability in ZONEMD configured zones (zonemd-check: yes) which are located below (but not at) a trust anchor allow for an attack window where (tampered with) zone contents are served (or stored to disk) prior to the ZONEMD integrity check. This is caused by the needed DS/DNSKEY asynchronous resolution that needs to happen before the ZONEMD check completes. If a zonefile is written to disk (zonefile: option) while the ZONEMD check failed, the tampered data are reloaded on startup and available until ZONEMD verification concludes again. If verification fails, the data is not served any more but still persists on disk for future reloads.

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

4.4

medium

Remediation

The vendor's own words where we have them.

This issue is fixed starting with version 1.26.1

Red Hat VEX · CVE-2026-77955

fixed0affected2not affected0

none_available: Fix deferred

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	1.13.2 to < 1.26.1	Vulnerable

Attack characteristics

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

Availability

None

Confidentiality

None

Integrity

High

Scope

Unchanged

Attack Complexity

High

Attack Vector

Network

Privileges Req

High

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
4.4	CVSS 3.1	medium	0.7	3.6	nlnetlabs.nl (CNA), NLnet Labs (CNA), GHSA, Red Hat (VEX)

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

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-345 · Insufficient Verification of Data Authenticity

Varies by Context
Unexpected State::

Weakness & attack patterns

CWE-345

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

CAPEC-387 · Navigation Remapping To Propagate Malicious ContentCAPEC-218 · Spoofing of UDDI/ebXML MessagesCAPEC-148 · Content SpoofingCAPEC-142 · DNS Cache PoisoningCAPEC-388 · Application API Button HijackingCAPEC-386 · Application API Navigation RemappingCAPEC-384 · Application API Message Manipulation via Man-in-the-MiddleCAPEC-141 · Cache PoisoningCAPEC-111 · JSON Hijacking (aka JavaScript Hijacking)CAPEC-385 · Transaction or Event Tampering via Application API Manipulation

T1557.002Adversary-in-the-Middle: ARP Cache Poisoning
T1584.002Compromise Infrastructure: DNS Server
T1491Defacement

Detection

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

Search application, proxy, and WAF logs for requests touching /DNSKEY.
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 4.4 (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-w62x-5q5r-fgmp published (medium)
Sep 16, 2026 · GitHub Security Advisory
CVE published by MITRE.
Sep 16, 2026 · SOCRadar CTI
Reported · NLnet Labs shares patch with Qifan Zhang
Sep 1, 2026 · NLnet Labs
Reported · Yuqi Qiu verifies patch
Aug 20, 2026 · NLnet Labs
Reported · NLnet Labs shares patch with Yuqi Qiu
Aug 19, 2026 · NLnet Labs
Issue reported by Qifan Zhang
Aug 16, 2026 · NLnet Labs
Issue reported by Yuqi Qiu
Aug 11, 2026 · NLnet Labs

References

6 on the record

www.nlnetlabs.nl/downloads/unbound/CVE-2026-77955.txt
Vendor Advisory
nvd.nist.gov/vuln/detail/CVE-2026-77955
NVD
github.com/advisories/GHSA-w62x-5q5r-fgmp
Exploit, Third Party Advisory
access.redhat.com/security/cve/CVE-2026-77955
Vendor Advisory
bugzilla.redhat.com/show_bug.cgi?id=2535053
Vendor Advisory
www.cve.org/CVERecord?id=CVE-2026-77955
Vendor Advisory

Elsewhere on this site

NLnet Labsevery CVE for this vendor
Unclassifiedsame class
CWE-345other 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-2026-77955 is Possible ZONEMD verification bypass window, a medium 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-2026-77955. Public exploit evidence is: A public PoC is not confirmed from current sources for CVE-2026-77955. 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 4.4 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.