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Fencer Vulns
medium 4.4

CVE-2026-77955 json

Possible ZONEMD verification bypass window

published 2026-09-16 54 d ago modified 2026-09-23 46 d ago also EUVD-2026-80287

cvss
4.4
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 6%
public exploits
0
none known

Summary

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.

Patch / fix

  • NLnet Labs Unbound 1.26.1
  • unbound Debian 13 1.26.1-0+deb13u1
  • unbound Debian 14 1.26.1-1

debian advisories DEBIAN-CVE-2026-77955

  • unbound hummingbird 1 1.26.1-1.hum1
  • unbound-libs hummingbird 1 1.26.1-1.hum1
  • unbound-munin hummingbird 1 1.26.1-1.hum1
  • unbound-utils hummingbird 1 1.26.1-1.hum1
  • unbound-anchor hummingbird 1 1.26.1-1.hum1
  • unbound-dracut hummingbird 1 1.26.1-1.hum1
  • python3-unbound hummingbird 1 1.26.1-1.hum1

red hat advisories RHSA-2026:68590

  • unbound Linux Micro 6.1 1.26.1-slfo.1.1_1.1
  • unbound SLES for SAP 15 SP5, SLES for SAP 15 SP4, SLES 15 SP5-LTSS +6 more 1.26.1-150100.10.31.1
  • unbound SLES for SAP 15 SP6, SLES 15 SP6-LTSS, SLE Module for Package Hub 15 SP7 +1 more 1.26.1-150600.23.24.1
  • unbound SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 1.26.1-160000.1.1
  • unbound Linux Micro 6.0 1.26.1-159998.1.1

suse advisories SUSE-SU-2026:24376-1SUSE-SU-2026:4624-1SUSE-SU-2026:4620-1SUSE-SU-2026:24128-1SUSE-SU-2026:24226-1SUSE-SU-2026:24135-1

  • unbound openSUSE Leap 16.0 1.26.1-160000.1.1
  • unbound openSUSE Tumbleweed 1.26.1-1.1

opensuse advisories openSUSE-SU-2026:22082-1openSUSE-SU-2026:11930-1

  • unbound Azure Linux 3.0 1.26.1-1

azure linux advisories AZL-101715

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-77955 updated cvss 4.4

unbound: Unbound: ZONEMD verification bypass due to asynchronous DNSSEC resolution

  • Red Hat Enterprise Linux 8 unbound, unbound-libs, python3-unbound Red Hat Enterprise Linux 9 unbound, unbound-libs, unbound-dracut, python3-unbound Red Hat Enterprise Linux 10 unbound Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-9 Red Hat OpenShift Container Platform 5 openshift/ose-rhel-coreos-9
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Hardened Images unbound 1.26.1-1.hum1, unbound-libs 1.26.1-1.hum1, unbound-munin 1.26.1-1.hum1, unbound-utils 1.26.1-1.hum1, unbound-anchor 1.26.1-1.hum1, unbound-dracut 1.26.1-1.hum1, +1 more
    fixed: vendor fix RHSA-2026:68590
  • Red Hat Enterprise Linux 6 unbound, unbound-libs, unbound-python Red Hat Enterprise Linux 7 unbound, unbound-libs, unbound-python Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-8
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
53 d ago

Exploit prediction

0.17% chance of exploitation in the next 30 days, higher than 6% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 4.4 CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:N/I:H/A:N

Affected products

  • NLnet Labs Unbound

Classification

Weakness
  • CWE-345 Insufficient Verification of Data Authenticity

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.