CVE-2026-20957
📋 TL;DR
An integer underflow vulnerability in Microsoft Office Excel allows attackers to execute arbitrary code on affected systems by opening specially crafted Excel files. This affects users running vulnerable versions of Microsoft Excel. The vulnerability requires user interaction to open a malicious file.
💻 Affected Systems
- Microsoft Excel
- Microsoft Office suites containing Excel
📦 What is this software?
365 Apps by Microsoft
365 Apps by Microsoft
Excel by Microsoft
Office by Microsoft
Office Long Term Servicing Channel by Microsoft
View all CVEs affecting Office Long Term Servicing Channel →
Office Long Term Servicing Channel by Microsoft
View all CVEs affecting Office Long Term Servicing Channel →
Office Long Term Servicing Channel by Microsoft
View all CVEs affecting Office Long Term Servicing Channel →
Office Long Term Servicing Channel by Microsoft
View all CVEs affecting Office Long Term Servicing Channel →
Office Long Term Servicing Channel by Microsoft
View all CVEs affecting Office Long Term Servicing Channel →
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining full control of the victim's computer, enabling data theft, ransomware deployment, or lateral movement within the network.
Likely Case
Local code execution leading to malware installation, credential theft, or data exfiltration from the compromised system.
If Mitigated
Limited impact with proper application sandboxing and user privilege restrictions preventing system-wide compromise.
🎯 Exploit Status
Requires user interaction to open malicious Excel file. Integer underflow vulnerabilities typically require precise memory manipulation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: To be specified in Microsoft's security update
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20957
Restart Required: Yes
Instructions:
1. Open any Office application
2. Go to File > Account > Update Options > Update Now
3. Alternatively, use Windows Update for Office updates
4. Restart computer after update installation
🔧 Temporary Workarounds
Disable Excel file opening
windowsTemporarily block Excel file execution via Group Policy or registry modification
reg add "HKLM\SOFTWARE\Microsoft\Office\ClickToRun\REGISTRY\MACHINE\Software\Microsoft\Office\16.0\Common\FileIO" /v "DisableFileOpen" /t REG_DWORD /d 1 /f
Use Protected View
windowsForce all Excel files to open in Protected View to prevent code execution
reg add "HKLM\SOFTWARE\Microsoft\Office\16.0\Excel\Security" /v "ProtectedView" /t REG_DWORD /d 1 /f
🧯 If You Can't Patch
- Implement application whitelisting to block unauthorized Excel execution
- Deploy email filtering to block Excel attachments and educate users about phishing risks
🔍 How to Verify
Check if Vulnerable:
Check Excel version against Microsoft's advisory. Vulnerable if running affected version without patch.Check Version:
In Excel: File > Account > About Excel (Windows) or Excel > About Excel (macOS)Verify Fix Applied:
Verify Excel version matches patched version in Microsoft advisory and test with known safe files.📡 Detection & Monitoring
Log Indicators:
- Excel crash logs with memory access violations
- Windows Event Logs showing Excel process spawning unexpected child processes
Network Indicators:
- Unusual outbound connections from Excel process
- DNS requests to suspicious domains after Excel file opening
SIEM Query:
source="*excel*" AND (event_id=1000 OR process_creation_parent="excel.exe")