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

CVE-2026-42959Crash during DNSSEC validation of malicious content

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 0.8% (higher than 57% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.8%top 43.0% 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 denial of service vulnerability in the DNSSEC validator that can lead to a crash given malicious upstream replies. When Unbound constructs chase-reply messages for validation, the code uses the wrong counter to calculate write offsets for ADDITIONAL section rrsets. DNAME duplication could increase the ANSWER section count and authority filtering could decrease the AUTHORITY section count and create an uninitialized array slot. Combining these two, the validator later dereferences this uninitialized pointer, causing an immediate process crash. An adversary controlling a DNSSEC-signed domain can trigger this bug with a single query by configuring a DNAME chain with unsigned CNAMEs and a response containing unsigned AUTHORITY records alongside signed ADDITIONAL glue records. Unbound 1.25.1 contains a patch with a fix to use the proper counters to calculate the write offsets.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
AlmaLinux:10python3-unbound1.24.2-7.el10_2.1ALSA-2026:23231
AlmaLinux:8python3-unbound1.16.2-5.11.el8_10ALSA-2026:24365
AlmaLinux:9python3-unbound1.24.2-3.el9_8.1ALSA-2026:24369
AlmaLinux:10unbound1.24.2-7.el10_2.1ALSA-2026:23231
AlmaLinux:8unbound1.16.2-5.11.el8_10ALSA-2026:24365
AlmaLinux:9unbound1.24.2-3.el9_8.1ALSA-2026:24369
AlmaLinux:10unbound-anchor1.24.2-7.el10_2.1ALSA-2026:23231
AlmaLinux:10unbound-devel1.24.2-7.el10_2.1ALSA-2026:23231
AlmaLinux:8unbound-devel1.16.2-5.11.el8_10ALSA-2026:24365
AlmaLinux:9unbound-devel1.24.2-3.el9_8.1ALSA-2026:24369
AlmaLinux:10unbound-dracut1.24.2-7.el10_2.1ALSA-2026:23231
AlmaLinux:9unbound-dracut1.24.2-3.el9_8.1ALSA-2026:24369
AlmaLinux:10unbound-libs1.24.2-7.el10_2.1ALSA-2026:23231
AlmaLinux:8unbound-libs1.16.2-5.11.el8_10ALSA-2026:24365
AlmaLinux:9unbound-libs1.24.2-3.el9_8.1ALSA-2026:24369
AlmaLinux:10unbound-utils1.24.2-7.el10_2.1ALSA-2026:23231
Alpine:v3.22unbound1.25.2-r0ALPINE-CVE-2026-42959
Alpine:v3.23unbound1.25.2-r0ALPINE-CVE-2026-42959
Alpine:v3.24unbound1.25.1-r0ALPINE-CVE-2026-42959
Debian:12unboundno fix listedDEBIAN-CVE-2026-42959
Debian:13unbound1.22.0-2+deb13u3DEBIAN-CVE-2026-42959
Debian:14unbound1.25.1-1DEBIAN-CVE-2026-42959
Rocky Linux:10unbound0:1.24.2-7.el10_2.1RLSA-2026:23231
Rocky Linux:8unbound0:1.16.2-5.11.el8_10RLSA-2026:24365
Rocky Linux:9unbound0:1.24.2-3.el9_8.1RLSA-2026:24369
Ubuntu:22.04:LTSunbound1.13.1-1ubuntu5.15UBUNTU-CVE-2026-42959
Ubuntu:24.04:LTSunbound1.19.2-1ubuntu3.8UBUNTU-CVE-2026-42959
Ubuntu:25.10unbound1.22.0-2ubuntu2.3UBUNTU-CVE-2026-42959
Ubuntu:26.04:LTSunbound1.24.2-1ubuntu2.1UBUNTU-CVE-2026-42959

Affected products (as reported)

VendorProductVersions
NLnet Labs Unbound 0 – before 1.25.1

Weakness: CWE-824 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References