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Fencer Vulns
high 7.5

CVE-2026-55973 json

'dns-error-reporting: yes' leads to stack buffer overflow

published 2026-07-22 3 mo ago modified 2026-10-24 15 d ago also EUVD-2026-47681

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 20%
public exploits
0
none known

Summary

In NLnet Labs Unbound 1.23.0 up to and including 1.25.1, when 'dns-error-reporting: yes' is set, the EDNS Report-Channel option (code 18) from the last upstream response is read and uses the option's length as the length of the agent domain. When a domain name check is performed on the agent domain, the returned lenght is not used and if the agent domain is followed by garbage, those bytes are moved onto the tail of the synthetic '_er.' report query name. That query name is later used in the iterator via a subquery to send out the DNS Error Report and when Unbound tries to walk that query name during 'find_closest_of_type()', it strips labels using the query name length rather than stopping at the embedded root, walks one byte past it, and feeds the first garbage byte to 'dname_query_hash()' as a label length writing over the stack variable 'labuf'. One ordinary upstream response from a delegated zone the attacker controls is sufficient to terminate the daemon.

Patch / fix

  • NLnet Labs Unbound 1.25.2
  • unbound Debian 13 1.26.1-0+deb13u1
  • unbound Debian 14 1.25.2-1

debian advisories DEBIAN-CVE-2026-55973

  • unbound Ubuntu 26.04 LTS 1.24.2-1ubuntu2.4

ubuntu advisories UBUNTU-CVE-2026-55973

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • unbound RHEL 10.2 1.24.2-7.el10_2.4
  • unbound-libs RHEL 10.2 1.24.2-7.el10_2.4
  • unbound-utils RHEL 10.2 1.24.2-7.el10_2.4
  • unbound-anchor RHEL 10.2 1.24.2-7.el10_2.4
  • unbound-dracut RHEL 10.2 1.24.2-7.el10_2.4
  • python3-unbound RHEL 10.2 1.24.2-7.el10_2.4
  • unbound RHEL 9 1.24.2-3.el9_8.4
  • unbound-libs RHEL 9 1.24.2-3.el9_8.4

red hat advisories RHSA-2026:55892RHSA-2026:55841RHSA-2026:43588

  • unbound SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 1.25.2-160000.1.1

suse advisories SUSE-SU-2026:23050-1SUSE-SU-2026:23215-1SUSE-SU-2026:23226-1

  • unbound openSUSE Leap 16.0 1.25.2-160000.1.1
  • unbound openSUSE Tumbleweed 1.25.2-1.1

opensuse advisories openSUSE-SU-2026:21550-1openSUSE-SU-2026:11380-1

  • unbound AlmaLinux 9 1.24.2-3.el9_8.4
  • unbound-libs AlmaLinux 9 1.24.2-3.el9_8.4
  • unbound-dracut AlmaLinux 9 1.24.2-3.el9_8.4
  • python3-unbound AlmaLinux 9 1.24.2-3.el9_8.4
  • unbound AlmaLinux 10 1.24.2-7.el10_2.4
  • unbound-libs AlmaLinux 10 1.24.2-7.el10_2.4
  • unbound-utils AlmaLinux 10 1.24.2-7.el10_2.4
  • unbound-anchor AlmaLinux 10 1.24.2-7.el10_2.4

almalinux advisories ALSA-2026:55841ALSA-2026:55892

  • unbound Rocky Linux 10 1.24.2-7.el10_2.4
  • unbound Rocky Linux 9 1.24.2-3.el9_8.4

rocky linux advisories RLSA-2026:55892RLSA-2026:55841

  • unbound Alpine 3.24, Alpine 3.23, Alpine 3.22 1.25.2-r0

alpine advisories ALPINE-CVE-2026-55973

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • unbound Azure Linux 3.0 1.25.2-1

azure linux advisories AZL-92919

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-2026-55973 updated cvss 7.5

unbound: Unbound: Denial of Service via malformed EDNS Report-Channel option

  • Red Hat OpenShift Container Platform 5 openshift/ose-rhel-coreos-9
    affected: no fix available
  • Red Hat Enterprise Linux AppStream (v. 9) unbound 1.24.2-3.el9_8.4, unbound-libs 1.24.2-3.el9_8.4, unbound-dracut 1.24.2-3.el9_8.4, python3-unbound 1.24.2-3.el9_8.4
    fixed: vendor fix RHSA-2026:55841
  • Red Hat Enterprise Linux AppStream (v. 10) unbound 1.24.2-7.el10_2.4, unbound-libs 1.24.2-7.el10_2.4, unbound-utils 1.24.2-7.el10_2.4, unbound-anchor 1.24.2-7.el10_2.4, unbound-dracut 1.24.2-7.el10_2.4, python3-unbound 1.24.2-7.el10_2.4
    fixed: vendor fix RHSA-2026:55892
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 9) unbound-devel 1.24.2-3.el9_8.4, unbound-debuginfo 1.24.2-3.el9_8.4, unbound-debugsource 1.24.2-3.el9_8.4, unbound-libs-debuginfo 1.24.2-3.el9_8.4, python3-unbound-debuginfo 1.24.2-3.el9_8.4
    fixed: vendor fix RHSA-2026:55841
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) unbound-devel 1.24.2-7.el10_2.4, unbound-debuginfo 1.24.2-7.el10_2.4, unbound-debugsource 1.24.2-7.el10_2.4, unbound-libs-debuginfo 1.24.2-7.el10_2.4, unbound-utils-debuginfo 1.24.2-7.el10_2.4, unbound-anchor-debuginfo 1.24.2-7.el10_2.4, +1 more
    fixed: vendor fix RHSA-2026:55892
  • Red Hat Hardened Images unbound 1.25.2-0.1.hum1, unbound-libs 1.25.2-0.1.hum1, unbound-munin 1.25.2-0.1.hum1, unbound-utils 1.25.2-0.1.hum1, unbound-anchor 1.25.2-0.1.hum1, unbound-dracut 1.25.2-0.1.hum1, +1 more
    fixed: vendor fix RHSA-2026:43588
  • Red Hat OpenShift Container Platform 4.22 rhcos 4.22.9.8.202608251819, rhcos 4.22.9.8.202609081748
    fixed: vendor fix RHSA-2026:60440
  • Red Hat Enterprise Linux 6 unbound, unbound-libs, unbound-python Red Hat Enterprise Linux 7 unbound, unbound-libs, unbound-python Red Hat Enterprise Linux 8 unbound, unbound-libs, python3-unbound 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
yes
Technical impact
partial
Assessed
3 mo ago

Exploit prediction

0.28% chance of exploitation in the next 30 days, higher than 20% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

  • NLnet Labs Unbound

Classification

Weakness
  • CWE-20 Improper Input Validation

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.