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
The vulnerability in NLnet Labs Unbound (CVE-2026-42534) affects versions up to 1.25.0 and involves the jostle logic responsible for managing slow DNS queries. Duplicate retransmitted queries renew the age timestamp of slow queries, which prevents the jostle logic from identifying them as aged and eligible for replacement by new queries. This behavior can be exploited by an adversary who can query a vulnerable Unbound instance and control a slow or malicious authoritative DNS server, resulting in degraded resolution performance or denial of resolution service when the 'num-queries-per-thread' limit is reached. The issue is fixed in Unbound 1.25.1 by assigning an initial, non-updatable start time to incoming queries, restoring intended jostle logic functionality.
Potential Impact
Exploitation of this vulnerability can degrade DNS resolution performance by preventing the replacement of slow-running queries, potentially leading to denial of resolution service under high query load. Cache and local data responses remain unaffected. The impact is limited to resolution performance degradation and denial of service conditions, not data integrity or confidentiality.
Mitigation Recommendations
A patch fixing this vulnerability is available in Unbound version 1.25.1. Users should upgrade to version 1.25.1 or later to remediate the issue. The vendor advisory from Red Hat confirms the availability of this fix in their updated packages. No additional mitigation steps are indicated beyond applying the official update.
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.
CVSS v4.0
Score 6.9medium
Affected software
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability in NLnet Labs Unbound (CVE-2026-42534) affects versions up to 1.25.0 and involves the jostle logic responsible for managing slow DNS queries. Duplicate retransmitted queries renew the age timestamp of slow queries, which prevents the jostle logic from identifying them as aged and eligible for replacement by new queries. This behavior can be exploited by an adversary who can query a vulnerable Unbound instance and control a slow or malicious authoritative DNS server, resulting in degraded resolution performance or denial of resolution service when the 'num-queries-per-thread' limit is reached. The issue is fixed in Unbound 1.25.1 by assigning an initial, non-updatable start time to incoming queries, restoring intended jostle logic functionality.
Potential Impact
Exploitation of this vulnerability can degrade DNS resolution performance by preventing the replacement of slow-running queries, potentially leading to denial of resolution service under high query load. Cache and local data responses remain unaffected. The impact is limited to resolution performance degradation and denial of service conditions, not data integrity or confidentiality.
Mitigation Recommendations
A patch fixing this vulnerability is available in Unbound version 1.25.1. Users should upgrade to version 1.25.1 or later to remediate the issue. The vendor advisory from Red Hat confirms the availability of this fix in their updated packages. No additional mitigation steps are indicated beyond applying the official update.
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
- Vendor Advisory Urls
- [{"url":"https://access.redhat.com/security/cve/CVE-2026-42534","vendor":"Red Hat"}]
Threat ID: 6a0d86fdba1db4736270ee63
Added to database: 05/20/2026, 10:03:41 UTC
Last enriched: 07/30/2026, 21:57:35 UTC
Last updated: 07/31/2026, 21:26:44 UTC
Views: 62
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