CVE-2021-28451
📋 TL;DR
CVE-2021-28451 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 of Microsoft Excel on Windows systems who open untrusted documents. The vulnerability 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
⚠️ Risk & Real-World Impact
Worst Case
Complete system takeover with attacker gaining the same privileges as the logged-in user, potentially leading to data theft, ransomware deployment, or lateral movement within networks.
Likely Case
Malware installation or data exfiltration from the compromised system, often delivered via phishing emails with malicious Excel attachments.
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 a malicious file. Multiple proof-of-concept examples exist in security research communities.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: May 2021 security updates for Microsoft Office
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-28451
Restart Required: Yes
Instructions:
1. Open Excel and go to File > Account > Update Options > Update Now. 2. Install all available updates. 3. Restart Excel and verify updates are applied.
🔧 Temporary Workarounds
Block Office macros from the internet
windowsPrevent Excel from running macros in files downloaded from the internet
Set GPO: Computer Configuration > Administrative Templates > Microsoft Office 2016 > Security Settings > Block macros from running in Office files from the Internet
Use Protected View
windowsConfigure Excel to open files from untrusted sources in Protected View
File > Options > Trust Center > Trust Center Settings > Protected View > Enable all Protected View options
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized Excel execution
- Deploy email filtering to block Excel attachments from untrusted sources
🔍 How to Verify
Check if Vulnerable:
Check Excel version: File > Account > About Excel. If version is older than May 2021 updates, system is vulnerable.Check Version:
In Excel: File > Account > About ExcelVerify Fix Applied:
Verify Excel version shows May 2021 or later updates installed. Check Windows Update history for KB5001342 or later Office security updates.📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs showing Excel crashes with unusual memory addresses
- Process creation events showing unexpected child processes from Excel.exe
Network Indicators:
- Outbound connections from Excel process to unknown external IPs
- DNS queries for suspicious domains from Excel
SIEM Query:
source="windows" process_name="excel.exe" AND (event_id=1000 OR event_id=1001) AND message="*exception*"