CVE-2021-24067
📋 TL;DR
CVE-2021-24067 is a use-after-free vulnerability in Microsoft Excel that allows remote code execution when a user opens a specially crafted malicious Excel file. Attackers can exploit this to execute arbitrary code with the privileges of the current user. This affects all users who open untrusted Excel files, particularly in organizations where Excel is widely used.
💻 Affected Systems
- Microsoft Excel
- Microsoft Office
- Microsoft 365 Apps
📦 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 Web Apps by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining full control of the victim's computer, enabling data theft, ransomware deployment, or lateral movement within networks.
Likely Case
Local privilege escalation leading to malware installation, credential theft, or data exfiltration from the compromised system.
If Mitigated
Limited impact with proper application whitelisting, macro security settings, and user training preventing malicious file execution.
🎯 Exploit Status
Exploitation requires user interaction to open malicious Excel files. Proof-of-concept code has been publicly released, increasing likelihood of weaponization.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Security updates released in February 2021 (KB4493172 for Office 2016, KB4493173 for Office 2019, etc.)
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-24067
Restart Required: Yes
Instructions:
1. Open any Office application. 2. Go to File > Account > Update Options > Update Now. 3. For managed environments, deploy through Microsoft Update, WSUS, or Configuration Manager. 4. Restart systems after update installation.
🔧 Temporary Workarounds
Block Office file types via Group Policy
windowsPrevent opening of Excel files from untrusted sources using Attachment Manager
Configure via Group Policy: Computer Configuration > Administrative Templates > Windows Components > Attachment Manager > 'Do not preserve zone information' and 'Hide mechanisms to remove zone information'
Enable Protected View for Internet files
windowsForce Excel files from Internet to open in Protected View
Excel Options > Trust Center > Trust Center Settings > Protected View > Check 'Enable Protected View for files originating from the Internet'
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized Excel execution
- Deploy email filtering to block malicious attachments and train users not to open untrusted Excel files
🔍 How to Verify
Check if Vulnerable:
Check Office version in Excel via File > Account > About Excel. Compare against patched versions (Office 2016: 16.0.5071.1000+, Office 2019: 16.0.10386.20000+)Check Version:
wmic product where "name like 'Microsoft Office%'" get name, versionVerify Fix Applied:
Verify security update KB4493172 (Office 2016) or KB4493173 (Office 2019) is installed via Control Panel > Programs > View installed updates📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs: Application crashes (Event ID 1000), suspicious child processes spawned from EXCEL.EXE
- Office telemetry logs showing abnormal file openings
Network Indicators:
- Outbound connections from Excel to suspicious IPs, DNS requests for command-and-control domains
SIEM Query:
source="*windows*" event_id=1000 process_name="EXCEL.EXE" | stats count by host, user