CVE-2026-42534: nlnet labs unbound Expected Behavior Violation

Vulnerability summary

CVE-2026-42534 is a high (CVSS 7.5) Expected Behavior Violation vulnerability in nlnet labs unbound. EPSS estimates a 0.72% chance of exploitation in the next 30 days, higher than 52.9% of scored vulnerabilities.

Timeline

1. 20-05-2026
   PublishedVulnerability first disclosed.
2. 01-09-2026
   Last modifiedVulnerability information updated.

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.

• CWE-440Expected Behavior ViolationA feature, API, or function does not perform according to its specification.
• CWE-911Improper Update of Reference CountThe product uses a reference count to manage a resource, but it does not update or incorrectly updates the reference count.