CVE-2024-20677: Microsoft 365 Apps RCE Vulnerability

CVE-2024-20677 Overview

CVE-2024-20677 is a Remote Code Execution vulnerability affecting Microsoft Office products through the handling of FBX (Filmbox) 3D model files. This vulnerability allows attackers to execute arbitrary code on victim systems when users open maliciously crafted documents containing FBX files. Microsoft has addressed this vulnerability by completely disabling the ability to insert FBX files in affected Office applications.

Critical Impact

Successful exploitation could allow an attacker 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 for Enterprise
• Microsoft Office 2019
• Microsoft Office LTSC 2021 (Windows and Mac)
• Microsoft Office Long Term Servicing Channel 2021 for macOS
• Microsoft 3D Viewer (as of February 13, 2024)

Discovery Timeline

• January 9, 2024 - CVE-2024-20677 published to NVD
• January 9, 2024 - Microsoft releases security patch disabling FBX file insertion
• February 13, 2024 - FBX file insertion disabled in 3D Viewer
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2024-20677

Vulnerability Analysis

This vulnerability stems from insecure processing of FBX (Filmbox) 3D model files within Microsoft Office applications. FBX is a proprietary file format developed by Autodesk used for 3D asset exchange between digital content creation applications. The vulnerability is classified under CWE-122 (Heap-based Buffer Overflow), indicating that the flaw involves memory corruption during the parsing or rendering of FBX file content.

The attack requires local access and user interaction—specifically, the victim must open a malicious document containing a specially crafted FBX file. When processed, the malformed FBX content triggers a heap-based buffer overflow condition, allowing the attacker to corrupt memory and potentially redirect execution flow to achieve arbitrary code execution.

Root Cause

The root cause is a heap-based buffer overflow (CWE-122) in the FBX file parsing functionality within Microsoft Office applications. This memory corruption vulnerability occurs when the application fails to properly validate or handle certain data structures within FBX files, leading to writes beyond allocated heap buffer boundaries.

Attack Vector

The attack vector is local, requiring user interaction. An attacker must craft a malicious Office document (Word, Excel, PowerPoint, or Outlook) containing a weaponized FBX 3D model file. The attack flow typically involves:

1. Attacker creates a malicious FBX file designed to trigger the heap overflow
2. The FBX file is embedded into an Office document
3. The document is distributed to victims via email, file sharing, or other delivery methods
4. When the victim opens the document, the vulnerable FBX parsing code processes the malicious content
5. The heap overflow allows the attacker to execute arbitrary code with the victim's privileges

Note that 3D models previously inserted from FBX files will continue to work unless the "Link to File" option was selected during insertion, which maintains a live connection to the external FBX file.

Detection Methods for CVE-2024-20677

Indicators of Compromise

• Office documents (.docx, .xlsx, .pptx) containing embedded FBX 3D model references
• Unusual memory allocation patterns or crashes in Office applications related to 3D content rendering
• Presence of FBX files in unexpected locations on systems or as email attachments
• Office application crashes with heap corruption indicators in crash dumps

Detection Strategies

• Monitor for Office applications attempting to load or process FBX file content, particularly in environments where the January 2024 patch has not been applied
• Implement email gateway rules to quarantine Office documents containing embedded 3D models or FBX references
• Deploy endpoint detection and response (EDR) solutions to identify heap corruption attempts in Office processes
• Review application crash reports for signs of exploitation attempts targeting 3D model processing

Monitoring Recommendations

• Enable detailed logging for Microsoft Office applications and monitor for abnormal behavior during document processing
• Track patch compliance across the organization to ensure all systems have received the January 9, 2024 security update
• Monitor network traffic for unusual document sharing patterns that may indicate attempted delivery of malicious files
• Implement file integrity monitoring on systems handling sensitive data to detect post-exploitation activities

How to Mitigate CVE-2024-20677

Immediate Actions Required

• Apply the January 9, 2024 Microsoft security update to all affected Office installations immediately
• Verify that FBX file insertion functionality is disabled in Word, Excel, PowerPoint, and Outlook across all endpoints
• Update 3D Viewer application to the version released after February 13, 2024
• Review and audit any documents that may have been created with the "Link to File" option for FBX models

Patch Information

Microsoft addressed this vulnerability through a security update released on January 9, 2024, which disables the ability to insert FBX files in Word, Excel, PowerPoint, and Outlook for Windows and Mac. This mitigation approach effectively removes the attack surface by eliminating the vulnerable functionality entirely.

Affected versions include:

• Office 2019
• Office 2021
• Office LTSC for Mac 2021
• Microsoft 365 Apps

As of February 13, 2024, the same mitigation was extended to the 3D Viewer application. Organizations should ensure all Office installations are updated through Windows Update, WSUS, or Microsoft Endpoint Configuration Manager.

For detailed patch information, refer to the Microsoft Security Response Center Advisory.

Workarounds

• Block FBX file attachments at email gateways and web proxies as a defense-in-depth measure
• Implement application control policies to prevent execution of untrusted Office documents containing 3D content
• Educate users about the risks of opening Office documents from untrusted sources, particularly those containing 3D models
• Consider disabling 3D content rendering capabilities in Office through Group Policy if the feature is not required for business operations

# PowerShell: Verify Office patch status for CVE-2024-20677
Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\Office\ClickToRun\Configuration" | Select-Object -Property VersionToReport

# Check if security update is applied (version should be January 2024 or later)
# Microsoft 365 Apps: Version 2312 (Build 17126.20132) or later