CVE-2021-31179
📋 TL;DR
CVE-2021-31179 is a remote code execution vulnerability in Microsoft Office that allows attackers to execute arbitrary code on a victim's system by tricking them into opening a specially crafted Office document. This affects users of Microsoft Office on Windows systems. Successful exploitation requires user interaction but can lead to full system compromise.
💻 Affected Systems
- Microsoft Office
- Microsoft 365 Apps for Enterprise
📦 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 takeover with administrative privileges, data theft, ransomware deployment, and lateral movement within the network.
Likely Case
Malware installation leading to data exfiltration, credential theft, and persistence mechanisms being established on the compromised system.
If Mitigated
Limited impact with proper email filtering, application whitelisting, and user training preventing malicious documents from reaching users or executing.
🎯 Exploit Status
Exploitation requires social engineering to get users to open malicious documents. No authentication bypass is needed once the document is opened.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Security updates released in May 2021
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31179
Restart Required: Yes
Instructions:
1. Open any Office application. 2. Go to File > Account > Update Options > Update Now. 3. Alternatively, use Windows Update: Settings > Update & Security > Windows Update > Check for updates. 4. Install all available Office security updates. 5. Restart the system if prompted.
🔧 Temporary Workarounds
Block Office documents from untrusted sources
windowsConfigure Office to open documents from the internet in Protected View or Application Guard
Set-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Office\ClickToRun\REGISTRY\MACHINE\Software\Microsoft\Office\16.0\Common\Security" -Name "ProtectedView" -Value 1 -Type DWord
Disable automatic opening of embedded content
windowsPrevent Office from automatically loading embedded objects that could trigger the vulnerability
Set-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Outlook\Security" -Name "Level1Remove" -Value "*.rtf" -Type String
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized Office document execution
- Deploy email filtering to block Office documents from untrusted sources
- Enable Windows Defender Application Control (WDAC) to restrict Office macro execution
🔍 How to Verify
Check if Vulnerable:
Check Office version: Open any Office app > File > Account > About [Application]. Look for version numbers before May 2021 updates.Check Version:
powershell Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\*" | Where-Object {$_.DisplayName -like "*Office*"} | Select-Object DisplayName, DisplayVersionVerify Fix Applied:
Verify Office has May 2021 security updates installed: Control Panel > Programs > Programs and Features > Microsoft Office > Check update history.📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs: Application crashes from Office applications (Event ID 1000)
- Security logs: Process creation from Office applications spawning unusual child processes
- Office telemetry: Documents with unusual embedded objects or macros
Network Indicators:
- Outbound connections from Office processes to unknown external IPs
- DNS queries for suspicious domains from Office applications
- Unusual SMB or RPC traffic originating from Office processes
SIEM Query:
source="windows" event_id=4688 process_name="WINWORD.EXE" OR process_name="EXCEL.EXE" OR process_name="POWERPNT.EXE" | stats count by process_name, parent_process_name