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CVE Record

CVE-2026-42534: Jostle logic bypass degrades resolution performance

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.

HighCVSS 7.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

Unbound DNS resolvers through version 1.25.0 can be forced to spend capacity on deliberately slow lookups. Sustained, coordinated abuse may delay or prevent new DNS resolutions, disrupting applications and users that depend on the resolver. Cached and locally stored DNS answers remain unaffected.

Executive priority

Prioritize exposed or business-critical recursive resolvers for prompt remediation. DNS resolution degradation can interrupt dependent services even though data confidentiality and integrity are not affected. Internal, tightly restricted resolvers warrant scheduled urgent patching after exposure verification.

Technical view

Duplicate queries can replace the original query timestamp used by Unbound’s jostle logic. Retransmissions therefore keep slow-running work appearing recent, preventing its intended replacement when num-queries-per-thread is exhausted. Exploitation requires query access plus control of a domain name server that responds slowly or maliciously. Version 1.25.1 uses a non-updatable initial start time.

Likely exposure

Exposure exists where Unbound 1.25.0 or earlier accepts queries from potential attackers. Internet-accessible resolvers face broader risk; restricted internal resolvers may still be exposed through untrusted clients. Impact concerns uncached resolution capacity, while cached and local-data responses remain available.

Exploitation context

The supplied sources do not report active exploitation, and the CVE is not listed as KEV in the bundle. The attack is remotely reachable without privileges or user interaction but requires both resolver query access and attacker-controlled authoritative DNS behavior. Coordinated traffic could cause denial of resolution service.

Researcher notes

The defect maps to improper handling of duplicate-query age under capacity pressure. Assessment should distinguish cached/local responses from recursive misses and examine num-queries-per-thread saturation. The bundle provides no proof-of-concept or observed-attack evidence. Its affected-version array is inconsistent, but the narrative and vendor fix statement specify versions through 1.25.0.

Mitigation direction

  • Upgrade Unbound to version 1.25.1 or a vendor-supported package containing the fix.
  • Review applicable operating-system vendor advisories and install their security update where Unbound is distribution-managed.
  • Restrict resolver query access to authorized networks and clients where operationally feasible.
  • Monitor resolution latency, outstanding-query saturation, and unusual repeated requests for uncached attacker-controlled domains.

Validation and detection

  • Inventory Unbound instances and confirm their installed versions and distribution package revisions.
  • Verify each instance is upgraded to 1.25.1 or covered by an applicable vendor security update.
  • Confirm resolver access controls match intended client networks and exclude unnecessary public access.
  • Review telemetry for sustained query saturation, increased uncached lookup latency, or abnormal duplicate-query patterns.
Prepared
Confidence
high
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-440: Exact CWE lookup

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cwe · low confidence lookup

CWE-911: Exact CWE lookup

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cve · low confidence lookup

CVE-2026-42534 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.5 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
9Timeline events
2ADP providers
9Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: yesTechnical Impact: partial

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6redhat-SADP
6.9CVSS 4.0MediumCVSS: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——NLnet Labs

Vulnerability scoring details

Base CVSS 4.0 score

6.9Medium
CVSS 4.0 vector shape for CVE-2026-42534Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: 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/U:Amber

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. Source timelineNLnet Labs

    Issue reported by Qifan Zhang

  2. Source timelineNLnet Labs

    NLnet Labs shares patch

  3. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  4. Source timelineNLnet Labs

    Qifan Zhang verifies patch

  5. Source timelineNLnet Labs

    Fixes released with version 1.25.1

  6. ADP timelineredhat-SADP

    Made public.

  7. CVE publishedCVE Program

    The CVE record was published.

  8. ADP timelineredhat-SADP

    Reported to Red Hat.

  9. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPunbound: Unbound: Denial of Service due to degraded resolution performance in jostle logic
other:Red Hat severity ratingcvssV3_1
  • 2026-05-20T10:01:15.833Z: Reported to Red Hat.
  • 2026-05-20T09:19:37.920Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
NLnet LabsUnbound0unaffected
Weakness

CWE details

CWE-440CWE-911

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-440 · source CWE mapping

Expected Behavior Violation

Expected Behavior Violation represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs
CWE-911 · source CWE mapping

Improper Update of Reference Count

Improper Update of Reference Count represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs