CVE-2026-20955
📋 TL;DR
This vulnerability allows an attacker to execute arbitrary code on a victim's system by exploiting an untrusted pointer dereference in Microsoft Excel. Attackers can achieve this by tricking users into opening a malicious Excel file. All users running vulnerable versions of Microsoft Excel are affected.
💻 Affected Systems
- Microsoft Excel
📦 What is this software?
365 Apps by Microsoft
365 Apps by Microsoft
Office 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
Full system compromise with attacker gaining complete control over the victim's computer, enabling data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Local privilege escalation leading to unauthorized access to sensitive files, credential harvesting, or installation of additional malware.
If Mitigated
Limited impact with only application crash or denial of service if proper sandboxing and security controls prevent code execution.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file). No public exploit code available at this time.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check Microsoft Security Update Guide for specific version
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20955
Restart Required: Yes
Instructions:
1. Open Microsoft Excel. 2. Go to File > Account > Update Options > Update Now. 3. Restart Excel after update completes. 4. Alternatively, apply latest Microsoft Office security updates through Windows Update.
🔧 Temporary Workarounds
Block Excel file execution via Group Policy
windowsPrevent execution of Excel files from untrusted sources using application control policies.
Configure via Windows Group Policy: Computer Configuration > Windows Settings > Security Settings > Application Control Policies > AppLocker
Disable macros and ActiveX controls
windowsConfigure Excel to disable potentially dangerous content execution.
File > Options > Trust Center > Trust Center Settings > Macro Settings > Disable all macros without notification
🧯 If You Can't Patch
- Implement application whitelisting to only allow trusted Excel executables
- Use Microsoft Office Viewer or web-based Excel to open untrusted files instead of desktop application
🔍 How to Verify
Check if Vulnerable:
Check Excel version against Microsoft's security bulletin for affected versions.Check Version:
In Excel: File > Account > About ExcelVerify Fix Applied:
Verify Excel has been updated to the patched version specified in Microsoft's security advisory.📡 Detection & Monitoring
Log Indicators:
- Excel crash logs with memory access violations
- Windows Event Logs showing unexpected Excel process termination
Network Indicators:
- Unusual outbound connections from Excel process after opening files
SIEM Query:
Process:excel.exe AND (EventID:1000 OR EventID:1001) AND Keywords:AccessViolation