CVE-2026-21261
📋 TL;DR
This vulnerability allows an unauthorized attacker to read memory outside the intended buffer in Microsoft Excel, potentially exposing sensitive information. It affects users who open malicious Excel files locally. The attacker must trick a user into opening a specially crafted file.
💻 Affected Systems
- Microsoft Office Excel
📦 What is this software?
365 Apps by Microsoft
365 Apps by Microsoft
Excel by Microsoft
Excel 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
Disclosure of sensitive data from Excel process memory, potentially including credentials, document contents, or system information.
Likely Case
Limited information disclosure from Excel's memory space, possibly revealing fragments of other documents or application data.
If Mitigated
No impact if proper file validation and least privilege principles are followed.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file). No authentication bypass needed beyond file access.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check Microsoft Security Update for specific version
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21261
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, use Windows Update for system-wide Office updates
🔧 Temporary Workarounds
Disable automatic Excel file opening
windowsPrevent Excel from automatically opening files from untrusted sources
Set Excel Trust Center settings: File > Options > Trust Center > Trust Center Settings > File Block Settings
Use Protected View
windowsForce all files from internet to open in Protected View
File > Options > Trust Center > Trust Center Settings > Protected View > Enable all Protected View options
🧯 If You Can't Patch
- Implement application whitelisting to block unauthorized Excel execution
- Educate users to never open Excel files from untrusted sources
🔍 How to Verify
Check if Vulnerable:
Check Excel version against Microsoft's patched version list in advisoryCheck Version:
In Excel: File > Account > About Excel (Windows) or Excel > About Excel (macOS)Verify Fix Applied:
Verify Excel version matches or exceeds patched version from Microsoft advisory📡 Detection & Monitoring
Log Indicators:
- Excel crash logs with memory access violations
- Event Viewer logs showing Excel process exceptions
Network Indicators:
- File downloads of Excel files from untrusted sources
SIEM Query:
source="*excel*" AND (event_id="1000" OR "Application Error") AND memory_access_violation