CVE-2021-43256
📋 TL;DR
CVE-2021-43256 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 vulnerable Microsoft Excel versions who open malicious files, potentially leading to full system compromise.
💻 Affected Systems
- Microsoft Excel
📦 What is this software?
365 Apps by Microsoft
365 Apps by Microsoft
Excel by Microsoft
Excel by Microsoft
Excel by Microsoft
Excel by Microsoft
Excel Rt by Microsoft
Office by Microsoft
Office by Microsoft
Office by Microsoft
Office by Microsoft
Office Web Apps by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete system takeover with attacker gaining the same privileges as the logged-in user, enabling data theft, ransomware deployment, or lateral movement within networks.
Likely Case
Malware installation or credential theft when users open malicious Excel files from phishing emails or untrusted sources.
If Mitigated
Limited impact with proper email filtering, user training, and application sandboxing preventing successful exploitation.
🎯 Exploit Status
Exploitation requires user interaction to open malicious file. No public exploit code was available at disclosure time.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Security updates released in December 2021. For Microsoft 365 Apps, version 2111 or later.
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43256
Restart Required: Yes
Instructions:
1. Open Excel and go to File > Account > Update Options > Update Now. 2. For enterprise deployments, deploy security updates via Microsoft Update, WSUS, or Configuration Manager. 3. Restart systems after update installation.
🔧 Temporary Workarounds
Block Office file types via email filtering
allConfigure email gateways to block or quarantine Excel files (.xlsx, .xlsm, .xlsb) from untrusted sources.
Enable Protected View for Internet files
windowsEnsure Excel's Protected View is enabled for files from the Internet to prevent automatic macro execution.
Excel Options > Trust Center > Trust Center Settings > Protected View > Enable all Protected View options
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized Excel execution
- Deploy Microsoft Attack Surface Reduction rules to block Office macro threats
🔍 How to Verify
Check if Vulnerable:
Check Excel version via File > Account > About Excel. Vulnerable if version is prior to security updates from December 2021.Check Version:
In Excel: File > Account > About Excel. In PowerShell: Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\*" | Where-Object {$_.DisplayName -like "*Excel*"} | Select-Object DisplayName, DisplayVersionVerify Fix Applied:
Verify Excel version is 2111 or later for Microsoft 365 Apps, or check Windows Update history for KB5002124 or later security updates.📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs: Excel crash events (Event ID 1000), suspicious child process creation from Excel.exe
Network Indicators:
- Outbound connections from Excel process to suspicious IPs, DNS queries for command and control domains
SIEM Query:
Process creation where parent_process_name contains "excel.exe" and (process_name contains "powershell.exe" or process_name contains "cmd.exe")