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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

This flaw can make vulnerable Unbound DNS resolvers spend capacity on slow unresolved lookups instead of replacing them efficiently. If abused at scale, users may see DNS resolution delays or failures for uncached domains. Cached and local answers are reported unaffected.

Executive priority

Treat this as a high-priority availability risk for DNS infrastructure. Prioritize exposed recursive resolvers first, because DNS degradation can affect many dependent services even without data theft or system compromise.

Technical view

Unbound through 1.25.0 mishandles jostle aging when duplicate retransmitted queries update the effective timestamp. Slow queries may avoid replacement after num-queries-per-thread is exhausted. An attacker needs query access to Unbound and control of a slow or malicious authoritative server. Unbound 1.25.1 fixes this with a non-updatable initial start time.

Likely exposure

Internet-facing or broadly reachable recursive Unbound resolvers are the primary concern, especially where untrusted clients can send queries. Internal-only resolvers are lower risk but still exposed if compromised or untrusted network segments can query them.

Exploitation context

The bundle does not report active exploitation, and KEV is false. Exploitation requires both access to query the vulnerable resolver and control of a domain name server that responds slowly or maliciously. Coordinated activity could become denial of resolution service.

Researcher notes

Focus validation on duplicate query handling and jostle candidate aging, not cache correctness. The described failure is availability-only: confidentiality and integrity are not impacted by the CVSS vector. Evidence for affected scope and fix comes mainly from NLnet Labs and CVE metadata.

Mitigation direction

  • Upgrade NLnet Labs Unbound to 1.25.1 or later.
  • Apply distribution vendor packages, including relevant Red Hat advisories where applicable.
  • Restrict recursive resolver access to trusted clients and networks.
  • Monitor resolver saturation, slow uncached lookups, and query queue pressure.
  • Check vendor guidance for any environment-specific mitigations.

Validation and detection

  • Inventory Unbound instances and confirm whether versions are 1.25.0 or earlier.
  • Confirm recursive query access is limited to intended clients.
  • Review resolver metrics for num-queries-per-thread exhaustion or unusual slow resolutions.
  • Verify upgraded systems report Unbound 1.25.1 or patched vendor builds.
  • Confirm cached and local response paths remain normal after remediation.
Prepared
Confidence
high
Sources
7

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

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-440: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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cwe · low confidence lookup

CWE-911: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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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
6Source 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