CVE-2026-42534: CWE-440: Expected Behavior Violation in NLnet Labs Unbound
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.
AI Analysis
Technical Summary
CVE-2026-42534 is a medium severity vulnerability in NLnet Labs Unbound DNS resolver up to version 1.25.0. The vulnerability arises from the jostle logic used to manage slow DNS queries. Specifically, retransmitted duplicate queries update the timestamp used for aging, which prevents the system from recognizing these queries as aged and eligible for replacement. An attacker who can send queries to a vulnerable Unbound instance and control a slow or malicious authoritative DNS server can exploit this to degrade resolution performance or cause denial of resolution service. The vulnerability does not affect cache or local data responses. The issue is addressed in Unbound version 1.25.1 by introducing an immutable start time for each query, allowing the jostle logic to function correctly.
Potential Impact
Exploitation of this vulnerability can degrade the DNS resolution performance of Unbound by preventing the replacement of slow-running queries. Coordinated attacks could escalate this degradation to a denial of resolution service, impacting availability of DNS resolution through the affected Unbound instance. Cache and local data responses remain unaffected, limiting the scope of impact to queries requiring external resolution.
Mitigation Recommendations
A patch is available in Unbound version 1.25.1 that fixes this vulnerability by attaching an initial, non-updatable start time to incoming queries. Users of Unbound versions up to 1.25.0 should upgrade to version 1.25.1 or later to remediate this issue. No other mitigation steps are indicated by the vendor advisory.
CVE-2026-42534: CWE-440: Expected Behavior Violation in NLnet Labs Unbound
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.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
CVE-2026-42534 is a medium severity vulnerability in NLnet Labs Unbound DNS resolver up to version 1.25.0. The vulnerability arises from the jostle logic used to manage slow DNS queries. Specifically, retransmitted duplicate queries update the timestamp used for aging, which prevents the system from recognizing these queries as aged and eligible for replacement. An attacker who can send queries to a vulnerable Unbound instance and control a slow or malicious authoritative DNS server can exploit this to degrade resolution performance or cause denial of resolution service. The vulnerability does not affect cache or local data responses. The issue is addressed in Unbound version 1.25.1 by introducing an immutable start time for each query, allowing the jostle logic to function correctly.
Potential Impact
Exploitation of this vulnerability can degrade the DNS resolution performance of Unbound by preventing the replacement of slow-running queries. Coordinated attacks could escalate this degradation to a denial of resolution service, impacting availability of DNS resolution through the affected Unbound instance. Cache and local data responses remain unaffected, limiting the scope of impact to queries requiring external resolution.
Mitigation Recommendations
A patch is available in Unbound version 1.25.1 that fixes this vulnerability by attaching an initial, non-updatable start time to incoming queries. Users of Unbound versions up to 1.25.0 should upgrade to version 1.25.1 or later to remediate this issue. No other mitigation steps are indicated by the vendor advisory.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- NLnet Labs
- Date Reserved
- 2026-05-07T10:07:51.811Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a0d86fdba1db4736270ee63
Added to database: 5/20/2026, 10:03:41 AM
Last enriched: 5/20/2026, 10:19:51 AM
Last updated: 5/21/2026, 5:55:11 AM
Views: 4
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