Jostle logic bypass degrades resolution performance

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Summary

CVECVE-2026-42534
StatePUBLISHED
AssignerNLnet Labs
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-05-20 10:16:27 UTC
Updated2026-05-20 22:50:00 UTC
DescriptionNLnet 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.

Risk And Classification

Primary CVSS: v4.0 6.9 MEDIUM from [email protected]

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber

EPSS: 0.000460000 probability, percentile 0.144890000 (date 2026-05-27)

Problem Types: CWE-440 | CWE-440 CWE-440: Expected Behavior Violation

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary6.9MEDIUMCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/C...
4.0CNACVSS6.9MEDIUMCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/U:Amber
3.1[email protected]Primary5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS v4.0 Breakdown

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
None
Privileges Required
None
User Interaction
None
Confidentiality
None
Integrity
None
Availability
Low
Sub Conf.
None
Sub Integrity
None
Sub Availability
None

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Nlnetlabs Unbound All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA NLnet Labs Unbound affected 1.25.1 semver Not specified

References

ReferenceSourceLinkTags
www.nlnetlabs.nl/downloads/unbound/CVE-2026-42534.txt [email protected] www.nlnetlabs.nl Mitigation, Vendor Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Qifan Zhang (Palo Alto Networks) (en)

Additional Advisory Data

SourceTimeEvent
CNA2026-04-28T00:00:00.000ZIssue reported by Qifan Zhang
CNA2026-05-07T00:00:00.000ZNLnet Labs shares patch
CNA2026-05-08T00:00:00.000ZQifan Zhang verifies patch
CNA2026-05-20T00:00:00.000ZFixes released with version 1.25.1

Solutions

CNA: This issue is fixed starting with version 1.25.1

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