{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-42534","assignerOrgId":"206fc3a0-e175-490b-9eaa-a5738056c9f6","state":"PUBLISHED","assignerShortName":"NLnet Labs","dateReserved":"2026-05-07T10:07:51.811Z","datePublished":"2026-05-20T09:19:37.920Z","dateUpdated":"2026-06-30T12:08:40.350Z"},"containers":{"cna":{"title":"Jostle logic bypass degrades resolution performance","datePublic":"2026-05-20T00:00:00.000Z","affected":[{"vendor":"NLnet Labs","product":"Unbound","versions":[{"version":"0","status":"affected","lessThan":"1.25.1","versionType":"semver"}],"defaultStatus":"unaffected"}],"descriptions":[{"lang":"en","value":"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."}],"metrics":[{"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}],"cvssV4_0":{"version":"4.0","baseScore":6.9,"baseSeverity":"MEDIUM","vectorString":"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"}}],"problemTypes":[{"descriptions":[{"lang":"en","cweId":"CWE-440","description":"CWE-440: Expected Behavior Violation","type":"CWE"}]}],"solutions":[{"lang":"en","value":"This issue is fixed starting with version 1.25.1"}],"timeline":[{"time":"2026-04-28T00:00:00.000Z","lang":"en","value":"Issue reported by Qifan Zhang"},{"time":"2026-05-07T00:00:00.000Z","lang":"en","value":"NLnet Labs shares patch"},{"time":"2026-05-08T00:00:00.000Z","lang":"en","value":"Qifan Zhang verifies patch"},{"time":"2026-05-20T00:00:00.000Z","lang":"en","value":"Fixes released with version 1.25.1"}],"credits":[{"lang":"en","value":"Qifan Zhang (Palo Alto Networks)","type":"finder"}],"references":[{"url":"https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42534.txt","tags":["vendor-advisory"]}],"providerMetadata":{"orgId":"206fc3a0-e175-490b-9eaa-a5738056c9f6","shortName":"NLnet Labs","dateUpdated":"2026-05-20T09:19:37.920Z"},"x_generator":{"engine":"cvelib 1.8.0"}},"adp":[{"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-05-20T12:10:33.391042Z","id":"CVE-2026-42534","options":[{"Exploitation":"none"},{"Automatable":"yes"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"title":"CISA ADP Vulnrichment","providerMetadata":{"orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","shortName":"CISA-ADP","dateUpdated":"2026-05-20T12:10:40.700Z"}},{"affected":[{"cpes":["cpe:/o:redhat:enterprise_linux:10"],"defaultStatus":"affected","product":"Red Hat Enterprise Linux 10","vendor":"Red Hat"},{"cpes":["cpe:/o:redhat:enterprise_linux:6"],"defaultStatus":"affected","product":"Red Hat Enterprise Linux 6","vendor":"Red Hat"},{"cpes":["cpe:/o:redhat:enterprise_linux:7"],"defaultStatus":"affected","product":"Red Hat Enterprise Linux 7","vendor":"Red Hat"},{"cpes":["cpe:/o:redhat:enterprise_linux:8"],"defaultStatus":"affected","product":"Red Hat Enterprise Linux 8","vendor":"Red Hat"},{"cpes":["cpe:/o:redhat:enterprise_linux:9"],"defaultStatus":"affected","product":"Red Hat Enterprise Linux 9","vendor":"Red Hat"},{"cpes":["cpe:/a:redhat:hummingbird:1"],"defaultStatus":"affected","product":"Red Hat Hardened Images","vendor":"Red Hat"},{"cpes":["cpe:/a:redhat:openshift:4"],"defaultStatus":"affected","product":"Red Hat OpenShift Container Platform 4","vendor":"Red Hat"}],"datePublic":"2026-05-20T09:19:37.920Z","descriptions":[{"lang":"en","value":"A flaw was found in Unbound. An adversary who can query a vulnerable Unbound instance and control a slow or malicious domain name server can exploit a vulnerability in the jostle logic. This flaw allows retransmitted queries to renew the age of slow-running queries, preventing them from being identified as aged and replaced. This can degrade Unbound's resolution performance, potentially leading to a denial of resolution service through coordinated attacks."}],"metrics":[{"other":{"content":{"namespace":"https://access.redhat.com/security/updates/classification/","value":"Important"},"type":"Red Hat severity rating"}},{"cvssV3_1":{"attackComplexity":"LOW","attackVector":"NETWORK","availabilityImpact":"HIGH","baseScore":7.5,"baseSeverity":"HIGH","confidentialityImpact":"NONE","integrityImpact":"NONE","privilegesRequired":"NONE","scope":"UNCHANGED","userInteraction":"NONE","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","version":"3.1"},"format":"CVSS"}],"problemTypes":[{"descriptions":[{"cweId":"CWE-911","description":"Improper Update of Reference Count","lang":"en","type":"CWE"}]}],"references":[{"tags":["vdb-entry","x_refsource_REDHAT"],"url":"https://access.redhat.com/security/cve/CVE-2026-42534"},{"name":"RHBZ#2480131","tags":["issue-tracking","x_refsource_REDHAT"],"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2480131"},{"tags":["x_sadp-csaf-vex"],"url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42534.json"}],"timeline":[{"lang":"en","time":"2026-05-20T10:01:15.833Z","value":"Reported to Red Hat."},{"lang":"en","time":"2026-05-20T09:19:37.920Z","value":"Made public."}],"title":"unbound: Unbound: Denial of Service due to degraded resolution performance in jostle logic","workarounds":[{"lang":"en","value":"To mitigate the risk of denial of resolution service, restrict Unbound's network exposure. Configure Unbound to only accept queries from trusted internal networks and to forward its own queries exclusively to trusted, well-maintained upstream DNS servers. This reduces the opportunity for an adversary to interact with or act as a malicious upstream for the vulnerable Unbound instance. After modifying Unbound's configuration, a service restart is required for changes to take effect. This may temporarily interrupt DNS resolution services."}],"x_adpType":"supplier","x_generator":{"engine":"sadp-cli 1.0.0"},"providerMetadata":{"orgId":"0b0ca135-0b70-47e7-9f44-1890c2a1c46c","shortName":"redhat-SADP","dateUpdated":"2026-06-30T12:08:40.350Z"}}]}}