CVE-2023-21738: Microsoft Office Visio RCE Vulnerability

CVE-2023-21738 Overview

CVE-2023-21738 is a remote code execution vulnerability affecting Microsoft Office Visio. This heap-based buffer overflow vulnerability allows attackers to execute arbitrary code on affected systems when a user opens a specially crafted Visio file. The vulnerability requires local access and user interaction, typically through social engineering tactics to convince users to open malicious files.

Critical Impact

Successful exploitation of this vulnerability could allow an attacker to execute arbitrary code with the privileges of the current user, potentially leading to complete system compromise, data theft, or deployment of additional malware.

Affected Products

• Microsoft 365 Apps for Enterprise
• Microsoft Office 2019
• Microsoft Office Long Term Servicing Channel 2021

Discovery Timeline

• 2023-01-10 - CVE-2023-21738 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-21738

Vulnerability Analysis

This vulnerability is classified as CWE-122 (Heap-based Buffer Overflow), a memory corruption issue that occurs when data is written beyond the allocated boundaries of a buffer in heap memory. In the context of Microsoft Visio, the vulnerability is triggered during the parsing of specially crafted Visio diagram files.

When Visio processes certain malformed file structures, insufficient bounds checking allows an attacker to overwrite adjacent memory regions. This memory corruption can be leveraged to hijack program execution flow and execute attacker-controlled code. The local attack vector means the malicious file must be delivered to and opened by the target user, typically through email attachments, file shares, or compromised downloads.

Root Cause

The root cause of CVE-2023-21738 is a heap-based buffer overflow condition in Microsoft Visio's file parsing routines. The application fails to properly validate the size or boundaries of data elements within Visio files before copying them to heap-allocated buffers. This lack of proper input validation allows crafted file content to overflow the intended buffer boundaries, corrupting adjacent heap memory and potentially allowing code execution.

Attack Vector

The attack vector for this vulnerability requires local access with user interaction. An attacker must deliver a maliciously crafted Visio file (.vsd, .vsdx, or related formats) to the victim and convince them to open it. Common delivery methods include:

• Spear-phishing emails with malicious Visio attachments
• Compromised network file shares containing weaponized diagrams
• Drive-by downloads from malicious or compromised websites
• Social engineering via messaging platforms or collaboration tools

When the victim opens the malicious file, the heap overflow is triggered during file parsing, allowing the attacker to execute code with the victim's privileges. For technical details, see the Microsoft Security Update for CVE-2023-21738.

Detection Methods for CVE-2023-21738

Indicators of Compromise

• Unusual Visio file attachments received via email from unknown or spoofed senders
• Visio process (visio.exe) exhibiting abnormal memory consumption or crash behavior
• Unexpected child processes spawned by visio.exe
• Suspicious network connections initiated by Visio or its child processes

Detection Strategies

• Monitor for anomalous process creation events where visio.exe spawns unexpected child processes such as cmd.exe, powershell.exe, or rundll32.exe
• Implement file analysis solutions to scan Visio files for malformed structures before delivery to end users
• Deploy endpoint detection rules that alert on heap corruption indicators or abnormal memory access patterns in Office applications
• Correlate email gateway logs with endpoint telemetry to identify delivery of suspicious Visio attachments

Monitoring Recommendations

• Enable enhanced logging for Microsoft Office applications to capture file open events and crash telemetry
• Configure SIEM rules to alert on process tree anomalies involving Visio executables
• Monitor for Windows Error Reporting events related to Visio crashes that may indicate exploitation attempts
• Review endpoint protection alerts for memory-based attack detection triggers in Office suite applications

How to Mitigate CVE-2023-21738

Immediate Actions Required

• Apply the latest Microsoft security updates for affected Office products immediately
• Educate users about the risks of opening Visio files from untrusted sources
• Consider blocking external Visio file attachments at the email gateway until patches are deployed
• Enable Protected View and Application Guard features where available to sandbox potentially malicious documents

Patch Information

Microsoft has released security updates to address CVE-2023-21738. Administrators should apply the January 2023 security updates for affected Microsoft Office products. Detailed patch information and update guidance are available in the Microsoft Security Update for CVE-2023-21738.

Organizations using Microsoft 365 Apps should ensure automatic updates are enabled and verify deployment through the Microsoft 365 Admin Center. For LTSC installations, apply the corresponding cumulative updates through Windows Update or WSUS.

Workarounds

• Enable Protected View for files originating from the internet or untrusted locations to provide a sandboxed environment
• Implement file type restrictions at email gateways to quarantine Visio attachments pending manual review
• Deploy Application Guard for Office to isolate potentially malicious documents in a container-based sandbox
• Restrict Visio file associations to prevent automatic opening, requiring manual application launch

# Example: Registry modification to enable Protected View for Visio files
# Apply via Group Policy or manual registry edit
reg add "HKCU\Software\Microsoft\Office\16.0\Visio\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Visio\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f