CVE-2021-40479: Microsoft Excel RCE Vulnerability

CVE-2021-40479 Overview

CVE-2021-40479 is a Remote Code Execution (RCE) vulnerability affecting Microsoft Excel across multiple versions of Microsoft Office products. This vulnerability allows an attacker to execute arbitrary code on a victim's system when a user opens a specially crafted Excel file. The attack requires user interaction, making social engineering a likely attack vector for exploitation.

Critical Impact

Successful exploitation allows attackers to execute arbitrary code with the privileges of the current user, potentially leading to complete system compromise, data theft, or lateral movement within enterprise networks.

Affected Products

• Microsoft 365 Apps Enterprise
• Microsoft Office 2013 SP1, 2016, and 2019
• Microsoft Office Long Term Servicing Channel 2021

Discovery Timeline

• 2021-10-13 - CVE CVE-2021-40479 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-40479

Vulnerability Analysis

This Microsoft Excel Remote Code Execution vulnerability exists in how Excel processes certain file components. The vulnerability requires local access, meaning an attacker must convince a user to open a malicious file locally on their system. No authentication is required to exploit this vulnerability, but user interaction is mandatory—specifically, the victim must open the malicious Excel document.

When successfully exploited, this vulnerability enables an attacker to achieve full compromise of confidentiality, integrity, and availability on the affected system. The attacker could install programs, view, change, or delete data, or create new accounts with full user rights. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative privileges.

Root Cause

While Microsoft has not disclosed specific technical details about the root cause (classified as NVD-CWE-noinfo), the vulnerability is categorized as a Remote Code Execution flaw in Microsoft Excel's file parsing and rendering components. These types of vulnerabilities typically stem from memory corruption issues, improper input validation, or unsafe handling of file format structures within the application.

Attack Vector

The attack vector is local, requiring the attacker to deliver a maliciously crafted Excel file to the victim. Common delivery methods include:

• Phishing emails with malicious Excel attachments
• Links to compromised websites hosting malicious Excel files
• USB drives or shared network locations containing the weaponized document

Once the victim opens the malicious file, the exploit triggers without requiring any additional user interaction beyond the initial file open action. The vulnerability has low attack complexity, meaning no special conditions or configurations are needed for successful exploitation.

For detailed technical information about this vulnerability, refer to the Microsoft Security Advisory CVE-2021-40479.

Detection Methods for CVE-2021-40479

Indicators of Compromise

• Unusual Excel processes spawning child processes (e.g., cmd.exe, powershell.exe, wscript.exe)
• Excel attempting to access unusual system resources or network locations
• Unexpected Office application crashes followed by suspicious process activity
• Malformed or obfuscated Excel files received via email or downloaded from unknown sources

Detection Strategies

• Monitor for Excel spawning unexpected child processes using endpoint detection tools
• Implement email gateway scanning for suspicious Excel attachments
• Deploy behavioral analysis rules to detect anomalous Excel process behavior
• Enable Windows Defender Attack Surface Reduction (ASR) rules for Office applications

Monitoring Recommendations

• Enable detailed logging for Microsoft Office applications via Windows Event Log
• Monitor process creation events (Sysmon Event ID 1) for Excel parent-child relationships
• Configure alerts for Excel processes making network connections to unusual destinations
• Review file access patterns for Excel opening files from temporary or download directories

How to Mitigate CVE-2021-40479

Immediate Actions Required

• Apply the latest Microsoft security updates for all affected Office products immediately
• Enable Protected View in Microsoft Office to open files from untrusted sources in a sandbox
• Implement application whitelisting to prevent unauthorized code execution
• Train users to avoid opening Excel files from unknown or suspicious sources

Patch Information

Microsoft has released security updates to address this vulnerability as part of the October 2021 Patch Tuesday release cycle. Organizations should apply the appropriate updates for their Office installations:

• Microsoft 365 Apps: Update to the latest version via automatic updates or the Microsoft 365 Admin Center
• Microsoft Office 2019/2016/2013: Apply the security updates from Windows Update or WSUS
• Office LTSC 2021: Install the corresponding security update

For detailed patch information and download links, visit the Microsoft Security Advisory CVE-2021-40479.

Workarounds

• Enable Protected View for files originating from the Internet in Excel Trust Center settings
• Block Excel files with macros at the email gateway level
• Configure Office to block OLE object linking for untrusted documents
• Use Microsoft Office Application Guard to isolate potentially malicious documents

# Enable Protected View via Registry (for deployment via GPO or scripts)
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f