CVE-2021-27053
📋 TL;DR
CVE-2021-27053 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 documents, potentially leading to full system compromise.
💻 Affected Systems
- Microsoft Excel
📦 What is this software?
365 Apps by Microsoft
Excel by Microsoft
Excel by Microsoft
Excel by Microsoft
Excel 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 the network.
Likely Case
Malware installation leading to data exfiltration, credential theft, or persistence establishment on the victim's system.
If Mitigated
Limited impact with proper application whitelisting and macro restrictions preventing malicious code execution.
🎯 Exploit Status
Exploitation requires user interaction to open malicious Excel file; no authentication bypass needed but requires social engineering.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Security updates released in March 2021
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-27053
Restart Required: Yes
Instructions:
1. Open Microsoft Excel. 2. Go to File > Account > Update Options > Update Now. 3. Install available updates. 4. Restart Excel and system if prompted.
🔧 Temporary Workarounds
Disable automatic opening of Excel files
windowsConfigure Excel to open files in Protected View by default
Set registry key: HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView to 1
Block external Excel files
windowsUse application control policies to block Excel files from untrusted sources
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized Excel execution
- Educate users about phishing risks and safe file handling practices
🔍 How to Verify
Check if Vulnerable:
Check Excel version: Open Excel > File > Account > About Excel. If version is before March 2021 updates, likely vulnerable.Check Version:
wmic product where "name like 'Microsoft Office%'" get versionVerify Fix Applied:
Verify Excel version includes March 2021 security updates and check Windows Update history for KB5000802 or later.📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs showing Excel crashes with unusual parameters
- Process creation events for unexpected executables from Excel
Network Indicators:
- Outbound connections from Excel process to suspicious IPs
- DNS queries for known malicious domains from Excel
SIEM Query:
source="windows" event_id=1 process_name="excel.exe" parent_process="explorer.exe" | stats count by process_command_line