Jostle logic bypass degrades resolution performance (CVE-2026-42534)
Published Last modified Sources: Ubuntu, Debian, BSI, Red Hat, EUVD, Msrc Update Guide, CVE.org
What is affected, what to do
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.
Update NLnet Labs Unbound to version 1.25.1.
- Affected
- NLnet Labs Unbound
lt *..1.25.1
- NLnet Labs Unbound
- Fixed in
- NLnet Labs Unbound
1.25.1
- NLnet Labs Unbound
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Metrics
6.9
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v4
5.3
CVSS v3
Severity Medium on the scale from 0 to 10.
Source: Ubuntu, Debian, BSI, Red Hat, EUVD, Msrc Update Guide, CVE.org
0.6 %
EPSS score
Low — model estimates < 1% exploitation likelihood.
Source: FIRST EPSS, percentile 48.9
Show all metrics
- Severity
- Medium
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 71.13 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- no public PoC known
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 6.9 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 5.3
CVSS v4
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 6.9
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 0.6 %
- Low — model estimates < 1% exploitation likelihood.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 48.9
- High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- —
- Published
- CISA assessment (SSVC)
- Exploitation: none known · automatable · Technical impact: partial
- Source: CISA Vulnrichment, as of 20 May 2026
CWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-440
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
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.
Source: Ubuntu Security Notices, Debian Security Advisories, BSI WID Portal, BSI CSAF, redhat_csaf, ENISA EUVD, Microsoft Security Update Guide, cvelistv5, NVD (NIST), CVE.org (MITRE)
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| NLnet Labs | Unbound |
|
|
| nlnetlabs | unbound |
|
|
| Oracle | Solaris |
|
|
Vendors and products
Affected operating systems
| Linux | debian | unbound | trixie |
Weakness classes (CWE)
CWE-440Expected Behavior Violation
Base
A feature, API, or function does not perform according to its specification.
Entry at MITRE CWE-440
References & sources
- nlnetlabs.nldownloads/unbound/CVE-2026-42534.txteuvd, Vendor advisory
- access.redhat.comsecurity/cve/CVE-2026-42534Vulnerability database, x_refsource_REDHAT
- bugzilla.redhat.comRHBZ#2480131Issue tracking, x_refsource_REDHAT
- security.access.redhat.comdata/csaf/v2/vex/2026/cve-2026-42534.jsonx_sadp-csaf-vex
- access.redhat.comerrata/RHSA-2026:36320Vendor advisory, x_refsource_REDHAT
- access.redhat.comerrata/RHSA-2026:37282Vendor advisory, x_refsource_REDHAT
- access.redhat.comerrata/RHSA-2026:36777Vendor advisory, x_refsource_REDHAT
- access.redhat.comerrata/RHSA-2026:24013Vendor advisory, x_refsource_REDHAT
Changes to this CVE
Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).
Record modifiednlnetlabs.nl
Show 3 original changes
Individual changes of this event as published by NVD Type Action Before After Reference added —https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42534.txtReference removed https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42534.txt—Reference type removed https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42534.txt Types: Mitigation, Vendor Advisory—Source identifier: sep@nlnetlabs.nl
Record modifiedRed Hat ADP
Show 15 original changes
Individual changes of this event as published by NVD Type Action Before After Affected versions (publisher) added —Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/42xxx/CVE-2026-42534.json">CVE-2026-42534</a>Reference added —https://access.redhat.com/errata/RHSA-2026:24013Reference added —https://access.redhat.com/errata/RHSA-2026:36320Reference added —https://access.redhat.com/errata/RHSA-2026:36777Reference added —https://access.redhat.com/errata/RHSA-2026:37282Reference added —https://access.redhat.com/security/cve/CVE-2026-42534Reference added —https://bugzilla.redhat.com/show_bug.cgi?id=2480131Reference added —https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42534.jsonReference removed https://access.redhat.com/errata/RHSA-2026:24013—Reference removed https://access.redhat.com/errata/RHSA-2026:36320—Reference removed https://access.redhat.com/errata/RHSA-2026:36777—Reference removed https://access.redhat.com/errata/RHSA-2026:37282—Reference removed https://access.redhat.com/security/cve/CVE-2026-42534—Reference removed https://bugzilla.redhat.com/show_bug.cgi?id=2480131—Reference removed https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42534.json—Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
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<a href="https://intel.neosec.eu/en/cve/CVE-2026-42534"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-42534.svg?lang=en" alt="CVE-2026-42534 – NEOSEC Intel" height="20"></a>