CVE-2021-27057
📋 TL;DR
This vulnerability allows remote code execution through specially crafted Microsoft Office files. Attackers can exploit this by tricking users into opening malicious documents, potentially gaining control of affected systems. All users running vulnerable versions of Microsoft Office are affected.
💻 Affected Systems
- Microsoft Office
- Microsoft 365 Apps
- Microsoft Office LTSC
📦 What is this software?
365 Apps by Microsoft
Excel by Microsoft
Excel by Microsoft
Excel by Microsoft
Excel by Microsoft
Office by Microsoft
Office 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
Full system compromise with attacker gaining administrative privileges, data exfiltration, ransomware deployment, and lateral movement through the network.
Likely Case
Local privilege escalation leading to malware installation, credential theft, and persistence mechanisms on the compromised workstation.
If Mitigated
Limited impact with proper application whitelisting, macro restrictions, and user training preventing malicious file execution.
🎯 Exploit Status
Exploitation requires user interaction to open malicious documents. Proof-of-concept code has been published, making exploitation more accessible to attackers.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: March 2021 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-27057
Restart Required: Yes
Instructions:
1. Apply March 2021 Microsoft Office security updates through Windows Update or Microsoft Update. 2. For enterprise deployments, deploy updates through WSUS or Microsoft Endpoint Configuration Manager. 3. Restart affected systems after patch installation.
🔧 Temporary Workarounds
Block Office file types via Group Policy
windowsPrevent opening of potentially malicious Office file attachments
Use Group Policy to block .doc, .docx, .xls, .xlsx, .ppt, .pptx files from email attachments
Enable Office Protected View
windowsForce all documents from the internet to open in Protected View
Set registry key: HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView to 1
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized Office file execution
- Deploy email filtering to block malicious Office attachments and enable macro restrictions
🔍 How to Verify
Check if Vulnerable:
Check Office version against March 2021 updates. Vulnerable if running versions prior to the March 2021 security updates.Check Version:
In Office application: File > Account > About [Application Name]Verify Fix Applied:
Verify Office version is updated to March 2021 or later security updates. Check Windows Update history for KB5000808 or similar Office security updates.📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs showing Office application crashes, suspicious child processes spawned from Office applications, unexpected PowerShell or cmd.exe execution from Office
Network Indicators:
- Outbound connections from Office applications to suspicious IPs, DNS queries for command and control domains
SIEM Query:
source="Windows Security" EventCode=4688 AND (ParentImage="*\WINWORD.EXE" OR ParentImage="*\EXCEL.EXE" OR ParentImage="*\POWERPNT.EXE") AND NewProcessName="*\powershell.exe" OR NewProcessName="*\cmd.exe"