CVE-2025-21366: Microsoft 365 Apps RCE Vulnerability

CVE-2025-21366 Overview

CVE-2025-21366 is a Use After Free (CWE-416) vulnerability affecting Microsoft Access and related Microsoft Office products. This remote code execution flaw allows attackers to execute arbitrary code on vulnerable systems when a user opens a specially crafted Microsoft Access file. The vulnerability requires local access and user interaction to exploit, making social engineering or phishing attacks a likely delivery mechanism.

Critical Impact

Successful exploitation enables 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 environments.

Affected Products

• Microsoft 365 Apps (Enterprise x64 and x86)
• Microsoft Access 2016
• Microsoft Office 2019 (x64 and x86)
• Microsoft Office Long Term Servicing Channel 2021 (x64 and x86)
• Microsoft Office Long Term Servicing Channel 2024 (x64 and x86)

Discovery Timeline

• 2025-01-14 - CVE-2025-21366 published to NVD
• 2025-07-01 - Last updated in NVD database

Technical Details for CVE-2025-21366

Vulnerability Analysis

This vulnerability stems from a Use After Free (UAF) condition in Microsoft Access. UAF vulnerabilities occur when a program continues to reference memory after it has been freed, allowing attackers to manipulate the freed memory and potentially execute arbitrary code. In this case, the vulnerability can be triggered when processing malicious Access database files.

The attack requires local access to the target system and user interaction—specifically, the victim must open a maliciously crafted file. Despite requiring user interaction, the vulnerability poses significant risk in enterprise environments where users regularly interact with Access database files as part of their workflow.

Root Cause

The vulnerability is classified under CWE-416 (Use After Free). This class of memory corruption vulnerability occurs when application code references a memory location after it has been deallocated. In Microsoft Access, this manifests during the processing of certain database structures where memory management fails to properly track object lifecycles. When the freed memory is subsequently reallocated and manipulated by attacker-controlled data, code execution becomes possible.

Attack Vector

The attack vector for CVE-2025-21366 is local, requiring an attacker to convince a user to open a specially crafted Microsoft Access file. Common delivery mechanisms include:

• Phishing emails with malicious Access database attachments (.accdb, .mdb files)
• File sharing platforms where victims may download compromised database files
• Compromised internal file shares in enterprise environments
• Social engineering attacks targeting users who regularly work with Access databases

Once the victim opens the malicious file, the Use After Free condition is triggered, allowing the attacker's payload to execute with the privileges of the current user. This could lead to installation of malware, credential theft, or further network compromise.

Detection Methods for CVE-2025-21366

Indicators of Compromise

• Unusual Microsoft Access process behavior including unexpected child process spawning
• Access database files (.accdb, .mdb) from untrusted sources or received via email
• Suspicious memory access patterns or crashes in MSACCESS.EXE
• Unexpected network connections initiated by Microsoft Access processes

Detection Strategies

• Monitor for unusual process creation events where MSACCESS.EXE spawns unexpected child processes such as cmd.exe, powershell.exe, or wscript.exe
• Implement email attachment filtering to quarantine Access database files from external senders
• Deploy endpoint detection rules to identify memory corruption exploitation attempts in Office applications
• Enable Windows Defender Application Guard for Office to isolate potentially malicious documents

Monitoring Recommendations

• Configure SIEM alerts for Microsoft Access executing with network activity or spawning shell processes
• Monitor file system activity for Access database files being created or modified in unusual locations (temp directories, user downloads)
• Enable Enhanced Process Auditing (Event ID 4688) to track process creation chains involving Office applications
• Review endpoint telemetry for exploitation patterns consistent with Use After Free attacks

How to Mitigate CVE-2025-21366

Immediate Actions Required

• Apply Microsoft security updates as soon as available for all affected products
• Enable Microsoft Defender Antivirus with cloud-delivered protection for real-time threat detection
• Configure email security gateways to sandbox or block unsolicited Access database file attachments
• Educate users about the risks of opening Access files from untrusted sources

Patch Information

Microsoft has released security updates addressing CVE-2025-21366. Detailed patch information and download links are available in the Microsoft Security Update Guide. Organizations should prioritize patching for systems where Microsoft Access is installed and actively used.

For Microsoft 365 Apps, updates are delivered through the standard update channels. Organizations using Click-to-Run deployments should ensure automatic updates are enabled. For volume-licensed Office installations (LTSC 2021 and 2024), administrators should deploy the appropriate security updates through Windows Server Update Services (WSUS) or System Center Configuration Manager.

Workarounds

• Restrict Access database file associations to prevent automatic opening of potentially malicious files
• Implement application whitelisting to control which applications can execute on endpoint systems
• Use Protected View settings in Microsoft Office to open untrusted files in a restricted environment
• Consider disabling Microsoft Access on systems where it is not required for business operations

# Registry configuration to enable Protected View for files from the Internet
reg add "HKCU\Software\Microsoft\Office\16.0\Access\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f

# Enable Application Guard for Office (requires Windows 10/11 Enterprise)
Enable-WindowsOptionalFeature -Online -FeatureName "Windows-Defender-ApplicationGuard" -NoRestart