CVE-2021-40442
📋 TL;DR
CVE-2021-40442 is a remote code execution vulnerability in Microsoft Excel that allows attackers to execute arbitrary code by tricking users into opening specially crafted Excel files. This affects users of Microsoft Excel on Windows systems who open malicious documents.
💻 Affected Systems
- Microsoft Excel
📦 What is this software?
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 →
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining the same privileges as the logged-in user, potentially leading to data theft, ransomware deployment, or lateral movement within the network.
Likely Case
Malware installation, credential theft, or data exfiltration when users open malicious Excel files from untrusted sources.
If Mitigated
Limited impact with proper email filtering, user education, and application sandboxing preventing successful exploitation.
🎯 Exploit Status
Exploitation requires user interaction to open malicious Excel files. Proof-of-concept code has been publicly released.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: October 2021 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40442
Restart Required: Yes
Instructions:
1. Open Microsoft Excel. 2. Go to File > Account > Update Options > Update Now. 3. Alternatively, use Windows Update to install the October 2021 security updates. 4. Restart the system after installation.
🔧 Temporary Workarounds
Block Office file types from email
allConfigure email gateways to block .xls, .xlsx, .xlsm files or scan them for malicious content
Enable Protected View
windowsEnsure Excel's Protected View is enabled for files from the internet
File > Options > Trust Center > Trust Center Settings > Protected View > Enable all options
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized Excel execution
- Use Microsoft Office Viewer or web-based Excel to open untrusted files instead of desktop Excel
🔍 How to Verify
Check if Vulnerable:
Check Excel version: Open Excel > File > Account > About Excel. If version is before October 2021 updates, system is vulnerable.Check Version:
In Excel: File > Account > About ExcelVerify Fix Applied:
Verify Excel version includes October 2021 updates (e.g., Version 2109 Build 14430.20298 or later for Microsoft 365).📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs showing Excel crashes with unusual parameters
- Process creation logs showing unexpected child processes from Excel.exe
Network Indicators:
- Outbound connections from Excel process to suspicious IPs
- DNS queries for known malicious domains from Excel
SIEM Query:
Process Creation where (Image contains 'excel.exe' AND CommandLine contains unusual file extensions OR CommandLine contains suspicious URLs)