CVE-2026-20956
📋 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 Office Excel
📦 What is this software?
365 Apps by Microsoft
365 Apps 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 →
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise leading to data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Local privilege escalation and execution of malicious payloads within the context of the user opening the file.
If Mitigated
Limited impact if file execution is blocked or user runs with minimal privileges, though data exposure may still occur.
🎯 Exploit Status
Exploitation requires user interaction (opening a file). No public exploit code is known at this time.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: To be determined from Microsoft's monthly security updates; check the advisory URL.
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20956
Restart Required: Yes
Instructions:
1. Open any Office application. 2. Go to File > Account > Update Options > Update Now. 3. Alternatively, use Windows Update for Microsoft 365 apps. 4. Restart the system if prompted.
🔧 Temporary Workarounds
Block Excel file execution via Group Policy
windowsPrevents opening of Excel files from untrusted sources.
Use Group Policy Editor to configure software restriction policies or AppLocker rules blocking .xls/.xlsx files.
Disable macros and ActiveX controls
windowsReduces attack surface by disabling potentially malicious content.
In Excel: File > Options > Trust Center > Trust Center Settings > Macro Settings > Disable all macros without notification.
🧯 If You Can't Patch
- Apply Microsoft's recommended workarounds from the advisory, such as using the Microsoft Office File Block policy.
- Restrict user permissions to prevent execution of untrusted files and use application whitelisting.
🔍 How to Verify
Check if Vulnerable:
Check Excel version against patched version listed in Microsoft advisory. Unpatched versions are vulnerable.Check Version:
In Excel: File > Account > About Excel. On command line: wmic product where name='Microsoft Office Excel' get versionVerify Fix Applied:
Verify Excel version matches or exceeds the patched version from Microsoft's update guide.📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs: Application crashes (Event ID 1000) for Excel.exe, suspicious child processes spawned from Excel.
Network Indicators:
- Unusual outbound connections from Excel process to external IPs post-file open.
SIEM Query:
source='windows' event_id=1000 process_name='EXCEL.EXE' | stats count by host