CVE-2025-21395: Microsoft 365 Apps RCE Vulnerability

CVE-2025-21395 Overview

CVE-2025-21395 is a heap-based buffer overflow vulnerability affecting Microsoft Access and related Microsoft Office products. This remote code execution vulnerability allows attackers to execute arbitrary code on affected systems when a user opens a specially crafted Microsoft Access file. The vulnerability exists due to improper handling of memory operations within Microsoft Access, enabling potential attackers to leverage malicious files to compromise system security.

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 installation of persistent malware.

Affected Products

• Microsoft 365 Apps for 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

• January 14, 2025 - CVE-2025-21395 published to NVD
• July 1, 2025 - Last updated in NVD database

Technical Details for CVE-2025-21395

Vulnerability Analysis

This vulnerability is classified under CWE-122 (Heap-based Buffer Overflow), indicating a memory corruption flaw that occurs when data is written beyond the allocated boundaries of a heap buffer. The vulnerability requires local access and user interaction to exploit, meaning an attacker must convince a user to open a malicious file. Once triggered, the vulnerability allows attackers to achieve full compromise of the affected system, including the ability to read, modify, and delete sensitive data, as well as install malicious software.

The attack surface centers on Microsoft Access file parsing functionality. When a specially crafted Access database file is opened, insufficient bounds checking during memory operations allows an attacker to overwrite critical heap structures, potentially redirecting program execution to attacker-controlled code.

Root Cause

The root cause of CVE-2025-21395 is improper memory management within Microsoft Access file handling routines. Specifically, heap-based buffer overflow conditions arise when the application fails to properly validate the size of input data before copying it to a fixed-size heap buffer. This allows malicious input embedded within Access database files to overflow the buffer boundaries and corrupt adjacent memory regions.

Attack Vector

The attack vector for this vulnerability is local, requiring user interaction. An attacker would typically deliver the malicious Access file through social engineering techniques such as:

• Phishing emails with malicious .accdb or .mdb file attachments
• Hosting malicious files on compromised or attacker-controlled websites
• Distributing files through file-sharing platforms or removable media

Once the victim opens the malicious file with a vulnerable version of Microsoft Access, the heap buffer overflow is triggered, allowing the attacker to execute arbitrary code in the context of the current user. If the user has administrative privileges, the attacker could gain full control over the system.

The vulnerability mechanism involves crafting Access database file structures that trigger the overflow condition during file parsing. The attacker-controlled data overwrites heap metadata or function pointers, which can be leveraged to redirect execution flow to shellcode embedded within the malicious file. For detailed technical information, refer to the Microsoft Security Advisory.

Detection Methods for CVE-2025-21395

Indicators of Compromise

• Unexpected Microsoft Access files (.accdb, .mdb) received via email from unknown or suspicious sources
• Microsoft Access (MSACCESS.EXE) spawning unexpected child processes such as cmd.exe, powershell.exe, or wscript.exe
• Unusual memory allocation patterns or crashes in Microsoft Access processes
• Network connections initiated by Microsoft Access to external IP addresses

Detection Strategies

• Deploy endpoint detection and response (EDR) solutions such as SentinelOne Singularity to monitor for suspicious process behavior and memory exploitation attempts
• Configure email security gateways to scan and quarantine suspicious Microsoft Access file attachments
• Enable Microsoft Defender for Office 365 or similar email protection to detect malicious file attachments
• Monitor for unusual Access file activity in user directories and temporary folders

Monitoring Recommendations

• Implement application whitelisting to restrict execution of unauthorized code spawned from Office applications
• Configure SIEM rules to alert on Microsoft Access processes initiating network connections or spawning command-line interpreters
• Enable detailed logging for Microsoft Office applications and monitor for signs of exploitation attempts
• Conduct regular vulnerability scans to identify unpatched Microsoft Office installations

How to Mitigate CVE-2025-21395

Immediate Actions Required

• Apply Microsoft security updates for affected Office products immediately
• Educate users about the risks of opening unsolicited Microsoft Access files from unknown sources
• Implement email filtering rules to block or quarantine Access database file attachments from external sources
• Consider temporarily restricting the use of Microsoft Access if immediate patching is not possible

Patch Information

Microsoft has released security updates to address CVE-2025-21395. Organizations should apply the appropriate patches for their Microsoft Office installations as documented in the Microsoft Security Response Center advisory. Ensure that automatic updates are enabled for Microsoft 365 Apps, or manually deploy patches through Windows Update, Microsoft Update Catalog, or enterprise patch management systems such as WSUS or Microsoft Endpoint Configuration Manager.

Workarounds

• Block Microsoft Access file attachments (.accdb, .mdb, .accde, .accdt) at the email gateway level
• Configure Microsoft Office Protected View settings to open files from untrusted sources in a sandboxed read-only mode
• Implement application control policies to restrict which file types can be opened by Microsoft Access
• Use network segmentation to limit the impact of potential compromise from workstations running Microsoft Office

# Example: Configure Outlook to block Access file attachments via registry
# Add registry key to block potentially dangerous file extensions
reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Security" /v Level1Remove /t REG_SZ /d "" /f
reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Security" /v Level1Add /t REG_SZ /d ".accdb;.mdb;.accde;.accdt" /f