NLnet Labs Unbound up to 1.25.0 expected behavior violation

Summaryinfo

A vulnerability, which was classified as problematic, has been found in NLnet Labs Unbound up to 1.25.0. The impacted element is an unknown function. This manipulation causes expected behavior violation. This vulnerability is registered as CVE-2026-42534. Remote exploitation of the attack is possible. No exploit is available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability has been found in NLnet Labs Unbound up to 1.25.0 and classified as problematic. Affected by this vulnerability is an unknown functionality. The manipulation with an unknown input leads to a expected behavior violation vulnerability. The CWE definition for the vulnerability is CWE-440. A feature, API, or function does not perform according to its specification. As an impact it is known to affect availability. The summary by CVE is:

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.

The weakness was published by Qifan Zhang. It is possible to read the advisory at nlnetlabs.nl. This vulnerability is known as CVE-2026-42534 since 05/07/2026. The exploitation appears to be easy. The attack can be launched remotely. The technical details are unknown and an exploit is not publicly available.

The vulnerability scanner Nessus provides a plugin with the ID 315755 (Linux Distros Unpatched Vulnerability : CVE-2026-42534), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 1.25.1 eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (315755) and CERT Bund (WID-SEC-2026-1599). Be aware that VulDB is the high quality source for vulnerability data.

Affected

• Debian Linux
• Red Hat Enterprise Linux
• Fedora Linux
• Ubuntu Linux
• Open Source Unbound
• cPanel cPanel/WHM

Productinfo

Vendor

• NLnet Labs

Name

• Unbound

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5
• 1.6
• 1.7
• 1.8
• 1.9
• 1.10
• 1.11
• 1.12
• 1.13
• 1.14
• 1.15
• 1.16
• 1.17
• 1.18
• 1.19
• 1.20
• 1.21
• 1.22
• 1.23
• 1.24
• 1.25.0

CPE 2.3info

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CPE 2.2info

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Entryinfo

Be aware that VulDB is the high quality source for vulnerability data.

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