CVE-2026-42534 (GCVE-0-2026-42534)

Vulnerability from cvelistv5 – Published: 2026-05-20 09:19 – Updated: 2026-06-30 12:08
VLAI
Title
Jostle logic bypass degrades resolution performance
Summary
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.
Severity
6.9 (Medium)
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
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
SSVC
Exploitation: none Automatable: yes Technical Impact: partial
CISA Coordinator (v2.0.3)
CWE
  • CWE-440 - Expected Behavior Violation
  • CWE-911 - Improper Update of Reference Count
Assigner
References
Impacted products
Date Public
2026-05-20 00:00
Credits
Qifan Zhang (Palo Alto Networks)
Show details on NVD website
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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.\"}], \"problemTypes\": [{\"descriptions\": [{\"lang\": \"en\", \"type\": \"CWE\", \"cweId\": \"CWE-440\", \"description\": \"CWE-440: Expected Behavior Violation\"}]}], \"providerMetadata\": {\"orgId\": \"206fc3a0-e175-490b-9eaa-a5738056c9f6\", \"shortName\": \"NLnet Labs\", \"dateUpdated\": \"2026-05-20T09:19:37.920Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2026-42534\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-06-30T12:08:40.350Z\", \"dateReserved\": \"2026-05-07T10:07:51.811Z\", \"assignerOrgId\": \"206fc3a0-e175-490b-9eaa-a5738056c9f6\", \"datePublished\": \"2026-05-20T09:19:37.920Z\", \"assignerShortName\": \"NLnet Labs\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}


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Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.