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Vulnerability · Published May 20, 2026 · Assigned by NLnet Labs

CVE-2026-42534Jostle logic bypass degrades resolution performance

High 7.5

In plain English

A high-severity security flaw in Unbound from NLnet Labs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 49% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.6%top 50.9% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
AlmaLinux:10python3-unbound1.24.2-7.el10_2.2ALSA-2026:36320
AlmaLinux:8python3-unbound1.16.2-5.12.el8_10ALSA-2026:37282
AlmaLinux:9python3-unbound1.24.2-3.el9_8.2ALSA-2026:36777
AlmaLinux:10unbound1.24.2-7.el10_2.2ALSA-2026:36320
AlmaLinux:8unbound1.16.2-5.12.el8_10ALSA-2026:37282
AlmaLinux:9unbound1.24.2-3.el9_8.2ALSA-2026:36777
AlmaLinux:10unbound-anchor1.24.2-7.el10_2.2ALSA-2026:36320
AlmaLinux:10unbound-devel1.24.2-7.el10_2.2ALSA-2026:36320
AlmaLinux:8unbound-devel1.16.2-5.12.el8_10ALSA-2026:37282
AlmaLinux:9unbound-devel1.24.2-3.el9_8.2ALSA-2026:36777
AlmaLinux:10unbound-dracut1.24.2-7.el10_2.2ALSA-2026:36320
AlmaLinux:9unbound-dracut1.24.2-3.el9_8.2ALSA-2026:36777
AlmaLinux:10unbound-libs1.24.2-7.el10_2.2ALSA-2026:36320
AlmaLinux:8unbound-libs1.16.2-5.12.el8_10ALSA-2026:37282
AlmaLinux:9unbound-libs1.24.2-3.el9_8.2ALSA-2026:36777
AlmaLinux:10unbound-utils1.24.2-7.el10_2.2ALSA-2026:36320
Alpine:v3.22unbound1.25.2-r0ALPINE-CVE-2026-42534
Alpine:v3.23unbound1.25.2-r0ALPINE-CVE-2026-42534
Alpine:v3.24unbound1.25.1-r0ALPINE-CVE-2026-42534
Debian:12unboundno fix listedDEBIAN-CVE-2026-42534
Debian:13unbound1.22.0-2+deb13u3DEBIAN-CVE-2026-42534
Debian:14unbound1.25.1-1DEBIAN-CVE-2026-42534
Rocky Linux:10unbound0:1.24.2-7.el10_2.2RLSA-2026:36320
Rocky Linux:8unbound0:1.16.2-5.12.el8_10RLSA-2026:37282
Rocky Linux:9unbound0:1.24.2-3.el9_8.2RLSA-2026:36777
Ubuntu:22.04:LTSunbound1.13.1-1ubuntu5.15UBUNTU-CVE-2026-42534
Ubuntu:24.04:LTSunbound1.19.2-1ubuntu3.8UBUNTU-CVE-2026-42534
Ubuntu:25.10unbound1.22.0-2ubuntu2.3UBUNTU-CVE-2026-42534
Ubuntu:26.04:LTSunbound1.24.2-1ubuntu2.1UBUNTU-CVE-2026-42534

Affected products (as reported)

VendorProductVersions
NLnet Labs Unbound 0 – before 1.25.1

Weakness: CWE-440, CWE-911 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References