Hayden

Did you know that the Excel files you use daily at work could become a security threat? CVE-2025-21354, announced by Microsoft in early 2025, is a severe security vulnerability that allows complete system compromise simply by opening a spreadsheet file.

In this article, we’ll walk through everything from the root cause of this critical vulnerability to the solution methods, explained in terms that any IT professional can easily understand.

 

 

1. Understanding CVE-2025-21354

CVE-2025-21354 is a Remote Code Execution (RCE) vulnerability discovered in Microsoft Excel. This vulnerability was patched through Microsoft’s official security update on January 14, 2025, and is classified as Critical level alongside CVE-2025-21362, posing a serious security risk where attackers can execute code when users open malicious files.

Vulnerability Severity

  • Rating: Critical (highest risk level)
  • Scope: All Excel versions (Microsoft 365, Office 2019, Office 2021, Office LTSC, etc.)
  • Attack Vector: Remote code execution through malicious Excel files
  • Patch Date: January 14, 2025

 

 

2. How the Attack Works

This vulnerability operates in a manner very similar to typical phishing attacks, but its destructive power is beyond imagination.

Attack Scenario

  1. Email Lure: Attacker sends a malicious Excel file disguised as a legitimate business document via email
  2. File Execution: Victim downloads the attachment and opens it with Excel
  3. Automatic Execution: Malicious code executes automatically the moment the file is opened
  4. System Compromise: Attacker gains complete control of the victim’s computer

Real-World Attack Example

Scenario: Accounting team member receives "Monthly Budget Report.xlsx"
File Content: Appears to be a normal spreadsheet but contains hidden malicious code
Result: Opening the file instantly installs ransomware, encrypting the entire corporate system

 

 

3. Root Cause of the Vulnerability

CVE-2025-21354 occurs due to missing security validation in Excel’s process of handling OLE (Object Linking and Embedding) objects.

Technical Analysis

  • Memory Handling Error: Memory corruption occurs when Excel parses specially crafted files
  • Input Validation Deficiency: Insufficient security checks for externally sourced data
  • Privilege Escalation: Code execution affecting the entire system even with standard user privileges

 

 

4. Affected Microsoft Office Products

The following products are impacted by this vulnerability:

Product Impact Level Update Status
Microsoft 365 Apps for Enterprise High Patched
Microsoft Excel 2019 High Patched
Microsoft Excel 2021 High Patched
Microsoft Office LTSC 2021 High Patched
Microsoft Office LTSC 2024 High Patched
Microsoft Office Online Server Medium Patched

 

 

5. Immediate Security Measures (Step-by-Step Guide)

5-1. Automatic Patching via Windows Update

Step 1: Run Windows Update

  • Press Windows key + I to open Settings
  • Click “Update & Security”
  • Go to Windows Update → Click “Check for updates”

Step 2: Verify Office Updates

  • Launch Excel → File → Account menu
  • Navigate to Product Information → Office Updates
  • Click “Update Now” button

5-2. Manual Patch Download and Installation

You can also directly install official updates provided by Microsoft:

For Office 2019/2021/365 Users:

For Office Online Server Administrators:

 

 

6. Additional Security Hardening Methods

6-1. Strengthening Excel Security Settings

Modifying Macro Security Settings:

  1. Launch Excel → File → Options
  2. Click Trust Center → Trust Center Settings
  3. In Macro Settings, select “Disable all macros without notification”

Enabling Protected View:

  1. Trust Center → Protected View
  2. Check all of the following options:
    • Enable Protected View for files originating from the Internet
    • Enable Protected View for Outlook attachments
    • Enable Protected View for files located in potentially unsafe locations

6-2. Enterprise-Level Security Policies

Central Management via Group Policy:

  • Administrative Templates → Microsoft Office 2016 → Security Settings
  • Enable “Require that application add-ins are signed by Trusted Publisher” policy

 

 

7. Emergency Response if Compromised

If you’ve opened a suspicious Excel file or detected system anomalies:

7-1. Immediate Actions

  1. Disconnect Network: Immediately disconnect from Internet and internal networks
  2. Full Antivirus Scan: Run complete system scan with Windows Defender or installed antivirus
  3. System Restore: Restore system to a point before infection
  4. Password Changes: Immediately change all critical account passwords

7-2. Enterprise Environment Response

  1. Contact IT Security Team immediately
  2. Isolate infected systems
  3. Execute incident response procedures
  4. Verify data backup integrity

 

 

8. Future Prevention and Security Awareness

8-1. Daily Security Habits

  • Avoid suspicious attachments: Never execute Excel files from unverified senders
  • Regular backups: Regularly backup important data to external storage
  • Security training: Actively participate in corporate security training programs

8-2. Staying Current with Security Trends

  • Regular monitoring of Microsoft Security Response Center
  • Stay informed about IT department security notifications
  • Continuous monitoring of security-related news and information

 

 

CVE-2025-21354 demonstrates how everyday-use Excel can become a significant security risk. However, with proper patching and security awareness, this is a threat that can be adequately prevented.

The most important steps are to update your system immediately and follow basic security practices like not opening suspicious files. Rather than complex technical knowledge, these fundamental precautions will protect your valuable data and systems.

If you’re reading this and haven’t updated your system yet, take a moment to run a system update. Five minutes of updating can prevent months of recovery work.

 

 

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