CVE-2023-21737: Microsoft 365 Apps RCE Vulnerability

CVE-2023-21737 Overview

CVE-2023-21737 is a Remote Code Execution vulnerability affecting Microsoft Office Visio. This vulnerability allows an attacker to execute arbitrary code on the target system when a user opens a specially crafted Visio file. The attack requires local access and user interaction, making it a target for phishing campaigns or malicious file distribution.

Critical Impact

Successful exploitation of this vulnerability could allow an attacker to execute arbitrary code in the context of the current user, potentially leading to complete system compromise, data theft, or lateral movement within a network.

Affected Products

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

Discovery Timeline

• 2023-01-10 - CVE-2023-21737 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-21737

Vulnerability Analysis

This Remote Code Execution vulnerability in Microsoft Office Visio is classified under CWE-122 (Heap-based Buffer Overflow). The vulnerability exists in the way Visio processes certain file elements. When a user opens a maliciously crafted Visio document, the application fails to properly validate input data, leading to a heap-based buffer overflow condition.

The attack requires local access to the target system and user interaction—specifically, the victim must open a malicious Visio file. This interaction requirement makes the vulnerability particularly suited for social engineering attacks, where attackers might distribute malicious files via email attachments or file-sharing platforms.

Root Cause

The root cause of this vulnerability is improper bounds checking when processing Visio document elements. The heap-based buffer overflow (CWE-122) occurs when the application writes data beyond the allocated buffer boundaries in heap memory. This memory corruption can be leveraged by an attacker to overwrite critical data structures or function pointers, ultimately gaining control of program execution.

Attack Vector

The attack vector for CVE-2023-21737 is local, meaning an attacker needs to convince a user to open a malicious file on their system. The typical attack scenario involves:

1. An attacker crafts a malicious Visio file (.vsd, .vsdx, or similar formats) containing specially structured data designed to trigger the heap overflow
2. The malicious file is delivered to the victim through phishing emails, compromised websites, or file-sharing services
3. When the victim opens the file with a vulnerable version of Microsoft Visio, the heap overflow is triggered
4. The attacker's code executes with the privileges of the current user

No verified exploit code is publicly available for this vulnerability. The vulnerability mechanism involves heap memory corruption through malformed Visio document structures. For detailed technical information, refer to the Microsoft Security Update CVE-2023-21737.

Detection Methods for CVE-2023-21737

Indicators of Compromise

• Unusual Visio document files with malformed internal structures or suspicious embedded objects
• Unexpected child processes spawned by VISIO.EXE such as cmd.exe, powershell.exe, or scripting engines
• Application crashes or abnormal termination of Microsoft Visio when opening specific files
• Memory corruption artifacts in crash dumps or application event logs

Detection Strategies

• Deploy endpoint detection rules to monitor for abnormal process behavior from VISIO.EXE, including unexpected child process creation or network connections
• Implement file inspection capabilities to analyze Visio documents for malformed structures before they reach end users
• Configure application whitelisting to prevent unauthorized executables from being spawned by Office applications
• Enable Windows Defender Exploit Guard with Attack Surface Reduction rules targeting Office applications

Monitoring Recommendations

• Monitor Windows Event Logs for Application Crash events (Event ID 1000) involving VISIO.EXE
• Enable advanced audit logging for process creation to detect suspicious child processes of Microsoft Visio
• Implement email gateway scanning for malicious Visio file attachments
• Deploy network traffic analysis to detect potential post-exploitation command and control communications

How to Mitigate CVE-2023-21737

Immediate Actions Required

• Apply the latest security updates from Microsoft for all affected Visio and Office products immediately
• Enable Microsoft Defender for Endpoint or equivalent endpoint protection with up-to-date signatures
• Educate users about the risks of opening Visio files from untrusted sources
• Consider temporarily blocking Visio file attachments at the email gateway until patches are deployed

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should apply the appropriate patches for their installed versions of Microsoft Visio and Office products. The patches are available through Windows Update, Microsoft Update Catalog, and WSUS. For detailed patch information and download links, refer to the Microsoft Security Update CVE-2023-21737.

Workarounds

• Do not open Visio files from untrusted or unknown sources until patches are applied
• Enable Protected View in Microsoft Office to open documents in a sandboxed environment by default
• Configure Microsoft Office File Block settings to prevent opening of older Visio file formats
• Use application sandboxing solutions to isolate Microsoft Visio from critical system resources

# Enable Protected View via Registry for Microsoft Office
# Run in elevated PowerShell
New-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Visio\Security\ProtectedView" -Name "DisableInternetFilesInPV" -Value 0 -PropertyType DWORD -Force
New-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Visio\Security\ProtectedView" -Name "DisableAttachmentsInPV" -Value 0 -PropertyType DWORD -Force
New-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Visio\Security\ProtectedView" -Name "DisableUnsafeLocationsInPV" -Value 0 -PropertyType DWORD -Force