CVE-2021-40485
📋 TL;DR
CVE-2021-40485 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 running vulnerable versions of Microsoft Excel on Windows systems. Successful exploitation requires user interaction but can lead 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
Office by Microsoft
Office by Microsoft
Office Long Term Servicing Channel by Microsoft
View all CVEs affecting Office Long Term Servicing Channel →
⚠️ Risk & Real-World Impact
Worst Case
Attacker gains full control of the victim's system, enabling data theft, ransomware deployment, lateral movement, and persistent backdoor installation.
Likely Case
Targeted attacks against specific organizations or individuals using malicious Excel attachments in phishing emails, leading to initial access for further compromise.
If Mitigated
With proper email filtering, user training, and application control policies, the risk is reduced to isolated incidents with limited impact.
🎯 Exploit Status
Exploitation requires user interaction to open a malicious file. Proof-of-concept code has been publicly released.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: October 2021 security updates
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40485
Restart Required: Yes
Instructions:
1. Open Microsoft Excel. 2. Go to File > Account > Update Options > Update Now. 3. Alternatively, use Windows Update or Microsoft Update to install the October 2021 security updates. 4. Restart the system if prompted.
🔧 Temporary Workarounds
Block Office file types from email
allConfigure email gateways to block .xls, .xlsx, and other Office file attachments from untrusted sources.
Enable Protected View
windowsEnsure Protected View is enabled for files from the internet to prevent automatic execution of macros and embedded content.
File > Options > Trust Center > Trust Center Settings > Protected View > Enable all options
🧯 If You Can't Patch
- Implement application control policies to restrict execution of Excel to trusted locations only.
- Educate users to never open Excel files from untrusted sources and to verify sender authenticity.
🔍 How to Verify
Check if Vulnerable:
Check Excel version: Open Excel > File > Account > About Excel. If version is before October 2021 updates, it's vulnerable.Check Version:
In Excel: File > Account > About ExcelVerify Fix Applied:
Verify Excel version is 16.0.14430.20234 or later for Microsoft 365, or check that October 2021 security updates are installed via Windows Update history.📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs: Application crashes of EXCEL.EXE with unusual error codes
- Security logs: Process creation from Excel spawning unexpected child processes like cmd.exe or powershell.exe
Network Indicators:
- Outbound connections from Excel process to external IPs
- DNS queries for suspicious domains initiated by Excel
SIEM Query:
Process Creation where (ParentImage contains "excel.exe" AND (Image contains "cmd.exe" OR Image contains "powershell.exe" OR Image contains "wscript.exe"))