CVE-2026-42923

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An adversary that controls a DNSSEC signed zone can exploit this by signing NSEC3 records with acceptably high iterations for child delegations and querying a vulnerable Unbound. Unbound will keep performing the allowed hash calculations on the NSEC3 records and will not limit the work by the mitigation introduced in 1.19.1. As a side effect, a global lock for the negative cache will be held for the duration of the hashing, blocking other threads that need to consult the negative cache. Coordinated attacks could raise the vulnerability to denial of service. Unbound 1.25.1 contains a patch with a fix to bound the vulnerable code path with the existing limit for NSEC3 hash calculations."}, {"lang": "es", "value": "NLnet Labs Unbound hasta la versi\u00f3n 1.25.0 inclusive tiene una vulnerabilidad en el validador DNSSEC donde la ruta de c\u00f3digo para consultar la cach\u00e9 negativa de registros DS no tiene en cuenta el l\u00edmite de c\u00e1lculos de hash NSEC3 introducido en la versi\u00f3n 1.19.1. Esto conduce a una degradaci\u00f3n del servicio durante el ataque. Un adversario que controla una zona firmada con DNSSEC puede explotar esto firmando registros NSEC3 con iteraciones aceptablemente altas para delegaciones secundarias y consultando un Unbound vulnerable. Unbound seguir\u00e1 realizando los c\u00e1lculos de hash permitidos en los registros NSEC3 y no limitar\u00e1 el trabajo mediante la mitigaci\u00f3n introducida en la versi\u00f3n 1.19.1. Como efecto secundario, se mantendr\u00e1 un bloqueo global para la cach\u00e9 negativa durante la duraci\u00f3n del hashing, bloqueando otros hilos que necesitan consultar la cach\u00e9 negativa. Ataques coordinados podr\u00edan elevar la vulnerabilidad a denegaci\u00f3n de servicio. Unbound 1.25.1 contiene un parche con una correcci\u00f3n para limitar la ruta de c\u00f3digo vulnerable con el l\u00edmite existente para los c\u00e1lculos de hash NSEC3."}], "id": "CVE-2026-42923", "lastModified": "2026-07-24T10:10:00.197", "metrics": {"cvssMetricV31": [{"cvssData": {"attackComplexity": "LOW", "attackVector": "NETWORK", "availabilityImpact": "LOW", "baseScore": 5.3, "baseSeverity": "MEDIUM", "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:L", "version": "3.1"}, "exploitabilityScore": 3.9, "impactScore": 1.4, "source": "nvd@nist.gov", "type": "Primary"}], "cvssMetricV40": [{"cvssData": {"Automatable": "NOT_DEFINED", "Recovery": "NOT_DEFINED", "Safety": "NOT_DEFINED", "attackComplexity": "LOW", "attackRequirements": "NONE", "attackVector": "NETWORK", "availabilityRequirement": "NOT_DEFINED", "baseScore": 6.9, "baseSeverity": "MEDIUM", "confidentialityRequirement": "NOT_DEFINED", "exploitMaturity": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "privilegesRequired": "NONE", "providerUrgency": "AMBER", "subAvailabilityImpact": "NONE", "subConfidentialityImpact": "NONE", "subIntegrityImpact": "NONE", "userInteraction": "NONE", "valueDensity": "NOT_DEFINED", "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/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", "version": "4.0", "vulnAvailabilityImpact": "LOW", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "NONE", "vulnerabilityResponseEffort": "NOT_DEFINED"}, "source": "sep@nlnetlabs.nl", "type": "Secondary"}], "ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"id": "CVE-2026-42923", "options": [{"exploitation": "none"}, {"automatable": "yes"}, {"technicalImpact": "partial"}], "role": "CISA Coordinator", "timestamp": "2026-05-20T12:10:04.210764Z", "version": "2.0.3"}}]}, "published": "2026-05-20T10:16:27.630", "references": [{"source": "sep@nlnetlabs.nl", "tags": ["Mitigation", "Vendor Advisory"], "url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42923.txt"}], "sourceIdentifier": "sep@nlnetlabs.nl", "vulnStatus": "Analyzed", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-407"}], "source": "sep@nlnetlabs.nl", "type": "Secondary"}]}

{"bugzilla": {"description": "unbound: Unbound DNSSEC Validator NSEC3 Hash Calculation Limit Bypass via Negative Cache Code Path Leading to DoS", "id": "2479825", "url": "https://bugzilla.redhat.com/show_bug.cgi?id=2479825"}, "csaw": false, "cvss3": {"cvss3_base_score": "5.3", "cvss3_scoring_vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L", "status": "draft"}, "cwe": "CWE-400", "details": ["A flaw was found in Unbound's DNSSEC validator where the code path for consulting the negative cache for DS records does not honor the limit on NSEC3 hash calculations introduced in version 1.19.1. An adversary who controls a DNSSEC-signed zone can sign NSEC3 records with high iteration counts for child delegations, causing Unbound to perform excessive hash computations while holding a global lock on the negative cache. This temporarily blocks other resolver threads from accessing the negative cache, leading to degraded DNS resolution performance for the duration of the attack."], "mitigation": {"lang": "en:us", "value": "Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability."}, "name": "CVE-2026-42923", "package_state": [{"cpe": "cpe:/o:redhat:enterprise_linux:10", "fix_state": "Fix deferred", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 10"}, {"cpe": "cpe:/o:redhat:enterprise_linux:6", "fix_state": "Fix deferred", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 6"}, {"cpe": "cpe:/o:redhat:enterprise_linux:7", "fix_state": "Fix deferred", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 7"}, {"cpe": "cpe:/o:redhat:enterprise_linux:8", "fix_state": "Fix deferred", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 8"}, {"cpe": "cpe:/o:redhat:enterprise_linux:9", "fix_state": "Fix deferred", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 9"}, {"cpe": "cpe:/a:redhat:hummingbird:1", "fix_state": "Affected", "package_name": "unbound", "product_name": "Red Hat Hardened Images"}, {"cpe": "cpe:/a:redhat:openshift:4", "fix_state": "Fix deferred", "package_name": "rhcos", "product_name": "Red Hat OpenShift Container Platform 4"}], "public_date": "2026-05-20T00:00:00Z", "references": ["https://www.cve.org/CVERecord?id=CVE-2026-42923\nhttps://nvd.nist.gov/vuln/detail/CVE-2026-42923"], "statement": "The Red Hat Product Security team has assessed the severity of this vulnerability as Moderate. The flaw can be remotely triggered without authentication, but the impact is limited to service degradation rather than a full denial-of-service.The Unbound process does not crash,a global negative cache lock is held during excessive NSEC3 hash computations, temporarily blocking other resolver threads. The root cause is a negative cache code path that bypasses the existing NSEC3 hash computation limits.", "threat_severity": "Moderate"}

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