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Fencer Vulns
high 7.5

CVE-2026-42534 json

Jostle logic bypass degrades resolution performance

published 2026-05-20 5 mo ago modified 2026-09-01 2 mo ago also EUVD-2026-31082

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.6%
EPSS 0.6% chance of exploitation in 30 days, percentile 49%
public exploits
0
none known

Summary

NLnet Labs Unbound up to and including version 1.25.0 has a vulnerability in the jostle logic that could defeat its purpose and degrade resolution performance. Retransmits of the same query could renew the age of slow running queries and not allow the jostle logic to see them as aged and potential targets for replacement with new queries. An adversary who can query a vulnerable Unbound and who can control a domain name server that replies slowly and/or maliciously to Unbound's queries can exploit the vulnerability and degrade the resolution performance of Unbound. When Unbound's 'num-queries-per-thread' reaches its limit, the jostle logic kicks in. When a new query comes in, half of the available queries that are also slow to resolve are candidates for replacement. The vulnerability then happens because duplicate queries that need resolution would skew the aging result by using the timestamp of the latest duplicate query instead of the original one that started the resolution effort. Cache and local data response performance remains unaffected. Coordinated attacks could raise this to a denial of resolution service. Unbound 1.25.1 contains a patch with a fix to attach an initial, non-updatable start time for incoming queries that allow the jostle logic to work as intended.

Patch / fix

  • NLnet Labs Unbound 1.25.1
  • Red Hat Red Hat Enterprise Linux 10 0:1.24.2-7.el10_2.2
  • Red Hat Red Hat Enterprise Linux 8 0:1.16.2-5.12.el8_10
  • Red Hat Red Hat Enterprise Linux 9 0:1.24.2-3.el9_8.2
  • Red Hat Red Hat Hardened Images 1.25.1-2.hum1
  • unbound Debian 13 1.22.0-2+deb13u3
  • unbound Debian 14 1.25.1-1

debian advisories DEBIAN-CVE-2026-42534

  • unbound Ubuntu 22.04 LTS 1.13.1-1ubuntu5.15
  • unbound Ubuntu 24.04 LTS 1.19.2-1ubuntu3.8
  • unbound Ubuntu 25.10 1.22.0-2ubuntu2.3
  • unbound Ubuntu 26.04 LTS 1.24.2-1ubuntu2.1

ubuntu advisories UBUNTU-CVE-2026-42534

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 8 1.16.2-5.12.el8_10
  • unbound-libs RHEL 8 1.16.2-5.12.el8_10
  • python3-unbound RHEL 8 1.16.2-5.12.el8_10
  • unbound RHEL 9 1.24.2-3.el9_8.2
  • unbound-libs RHEL 9 1.24.2-3.el9_8.2
  • unbound-dracut RHEL 9 1.24.2-3.el9_8.2
  • python3-unbound RHEL 9 1.24.2-3.el9_8.2
  • unbound RHEL 10.2 1.24.2-7.el10_2.2

red hat advisories RHSA-2026:37282RHSA-2026:36777RHSA-2026:36320RHSA-2026:24013

  • unbound SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 1.24.1-160000.2.1
  • unbound SLES for SAP 15 SP5, SLES for SAP 15 SP4, SLES 15 SP5-LTSS +6 more 1.20.0-150100.10.25.1

suse advisories SUSE-SU-2026:22160-1SUSE-SU-2026:22213-1SUSE-SU-2026:2369-1

  • unbound openSUSE Leap 16.0 1.24.1-160000.2.1
  • unbound openSUSE Tumbleweed 1.25.1-1.1

opensuse advisories openSUSE-SU-2026:21083-1openSUSE-SU-2026:10903-1

  • unbound AlmaLinux 8 1.16.2-5.12.el8_10
  • unbound-libs AlmaLinux 8 1.16.2-5.12.el8_10
  • python3-unbound AlmaLinux 8 1.16.2-5.12.el8_10
  • unbound AlmaLinux 9 1.24.2-3.el9_8.2
  • unbound-libs AlmaLinux 9 1.24.2-3.el9_8.2
  • unbound-dracut AlmaLinux 9 1.24.2-3.el9_8.2
  • python3-unbound AlmaLinux 9 1.24.2-3.el9_8.2
  • unbound AlmaLinux 10 1.24.2-7.el10_2.2

almalinux advisories ALSA-2026:37282ALSA-2026:36777ALSA-2026:36320

  • unbound Rocky Linux 10 1.24.2-7.el10_2.2
  • unbound Rocky Linux 9 1.24.2-3.el9_8.2
  • unbound Rocky Linux 8 1.16.2-5.12.el8_10

rocky linux advisories RLSA-2026:36320RLSA-2026:36777RLSA-2026:37282

  • unbound Alpine 3.23, Alpine 3.22 1.25.2-r0
  • unbound Alpine 3.24 1.25.1-r0

alpine advisories ALPINE-CVE-2026-42534

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 1.25.1-1

azure linux advisories AZL-87033

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-2026-42534 updated cvss 7.5

unbound: Unbound: Denial of Service due to degraded resolution performance in jostle logic

  • 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, openshift/ose-rhel-coreos-9
    affected: workaround, no fix available
  • Red Hat Enterprise Linux AppStream (v. 8) unbound 1.16.2-5.12.el8_10, unbound-libs 1.16.2-5.12.el8_10, python3-unbound 1.16.2-5.12.el8_10
    fixed: vendor fix RHSA-2026:37282
  • Red Hat Enterprise Linux AppStream (v. 9) unbound 1.24.2-3.el9_8.2, unbound-libs 1.24.2-3.el9_8.2, unbound-dracut 1.24.2-3.el9_8.2, python3-unbound 1.24.2-3.el9_8.2
    fixed: vendor fix RHSA-2026:36777
  • Red Hat Enterprise Linux AppStream (v. 10) unbound 1.24.2-7.el10_2.2, unbound-libs 1.24.2-7.el10_2.2, unbound-utils 1.24.2-7.el10_2.2, unbound-anchor 1.24.2-7.el10_2.2, unbound-dracut 1.24.2-7.el10_2.2, python3-unbound 1.24.2-7.el10_2.2
    fixed: vendor fix RHSA-2026:36320
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 9) unbound-devel 1.24.2-3.el9_8.2, unbound-debuginfo 1.24.2-3.el9_8.2, unbound-debugsource 1.24.2-3.el9_8.2, unbound-libs-debuginfo 1.24.2-3.el9_8.2, python3-unbound-debuginfo 1.24.2-3.el9_8.2
    fixed: vendor fix RHSA-2026:36777
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) unbound-devel 1.24.2-7.el10_2.2, unbound-debuginfo 1.24.2-7.el10_2.2, unbound-debugsource 1.24.2-7.el10_2.2, unbound-libs-debuginfo 1.24.2-7.el10_2.2, unbound-utils-debuginfo 1.24.2-7.el10_2.2, unbound-anchor-debuginfo 1.24.2-7.el10_2.2, +1 more
    fixed: vendor fix RHSA-2026:36320
  • Red Hat Hardened Images unbound 1.25.1-2.hum1, unbound-libs 1.25.1-2.hum1, unbound-munin 1.25.1-2.hum1, unbound-utils 1.25.1-2.hum1, unbound-anchor 1.25.1-2.hum1, unbound-dracut 1.25.1-2.hum1, +1 more
    fixed: vendor fix RHSA-2026:24013

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
5 mo ago

Exploit prediction

0.60% chance of exploitation in the next 30 days, higher than 49% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 6.9 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/U:Amber
CISA ADP 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA 4.0 6.9 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber
NVD 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
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
  • Red Hat Red Hat Enterprise Linux 10
  • Red Hat Red Hat Enterprise Linux 8
  • Red Hat Red Hat Enterprise Linux 9
  • Red Hat Red Hat Hardened Images
  • Red Hat Red Hat Enterprise Linux 6
  • Red Hat Red Hat Enterprise Linux 7
  • Red Hat Red Hat OpenShift Container Platform 4

Classification

Weakness
  • CWE-440 Expected Behavior Violation
  • CWE-911 Improper Update of Reference Count

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.