CVE-2021-31175
📋 TL;DR
This vulnerability allows remote code execution through specially crafted Microsoft Office documents. Attackers can exploit it by tricking users into opening malicious files, potentially taking control of affected systems. All users running vulnerable Microsoft Office versions are at risk.
💻 Affected Systems
- Microsoft Office
- Microsoft 365 Apps
📦 What is this software?
365 Apps by Microsoft
Excel by Microsoft
Excel by Microsoft
Excel by Microsoft
Office by Microsoft
Office by Microsoft
Office by Microsoft
Office by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining full control, data theft, ransomware deployment, and lateral movement across networks.
Likely Case
Malware installation, credential theft, and data exfiltration from individual workstations.
If Mitigated
Limited impact with proper security controls like application whitelisting, macro restrictions, and user training preventing successful exploitation.
🎯 Exploit Status
Exploitation requires user interaction. Proof-of-concept code has been published. Attack vectors typically involve phishing emails with malicious attachments.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: May 2021 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31175
Restart Required: Yes
Instructions:
1. Apply May 2021 Microsoft Office security updates via Windows Update or Microsoft Update Catalog. 2. For Microsoft 365 Apps, ensure automatic updates are enabled. 3. Restart affected systems after patch installation.
🔧 Temporary Workarounds
Block Office file types via email filtering
allConfigure email gateways to block or quarantine Office documents from untrusted sources
Enable Protected View
windowsConfigure Office to open files from internet in Protected View by default
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized Office execution
- Disable Office macro execution and restrict document sources to trusted locations only
🔍 How to Verify
Check if Vulnerable:
Check Office version and compare against patched versions from Microsoft advisory. Vulnerable if running pre-May 2021 updates.Check Version:
In Office: File > Account > About [Application]. In Windows: wmic product where "name like 'Microsoft Office%'" get name, versionVerify Fix Applied:
Verify Office version shows May 2021 or later security updates installed. Check Windows Update history for KB5003167 or similar Office security updates.📡 Detection & Monitoring
Log Indicators:
- Unusual Office process spawning child processes
- Office applications loading unexpected DLLs
- Multiple failed document openings from same source
Network Indicators:
- Outbound connections from Office processes to suspicious IPs
- DNS requests for known malicious domains from Office context
SIEM Query:
source="windows" (process_name="winword.exe" OR process_name="excel.exe" OR process_name="powerpnt.exe") AND (event_id=4688 OR event_id=1) AND parent_process_name IN ("winword.exe", "excel.exe", "powerpnt.exe")