CVE-2025-21387: Microsoft Excel RCE Vulnerability

CVE-2025-21387 Overview

CVE-2025-21387 is a remote code execution vulnerability affecting Microsoft Excel and related Microsoft Office products. This Use-After-Free (CWE-416) vulnerability allows attackers to execute arbitrary code on affected systems when a user opens a specially crafted Excel file. The vulnerability requires user interaction, as the target must open a malicious document for exploitation to succeed.

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 full system compromise, data theft, or lateral movement within an organization's network.

Affected Products

• Microsoft 365 Apps (Enterprise, x64 and x86)
• Microsoft Excel 2016 (x64 and x86)
• Microsoft Office 2019 (x64 and x86)
• Microsoft Office Long Term Servicing Channel 2021 and 2024 (Windows and macOS)
• Microsoft Office Online Server

Discovery Timeline

• February 11, 2025 - CVE-2025-21387 published to NVD
• July 1, 2025 - Last updated in NVD database

Technical Details for CVE-2025-21387

Vulnerability Analysis

This vulnerability is classified as a Use-After-Free (UAF) memory corruption issue within Microsoft Excel. Use-After-Free vulnerabilities occur when a program continues to use a pointer after the memory it references has been freed. In the context of Excel, this flaw likely exists in the document parsing or rendering engine, where memory management for complex spreadsheet objects can be exploited.

The local attack vector indicates that exploitation requires the attacker to deliver a malicious file to the victim, typically via email attachment, file share, or download. Once the victim opens the crafted Excel file, the vulnerability is triggered during document processing. The lack of privilege requirements combined with the need for user interaction creates a classic social engineering attack scenario.

Root Cause

The root cause of CVE-2025-21387 is a Use-After-Free (CWE-416) condition in Microsoft Excel's memory management routines. This type of vulnerability typically arises when:

1. Memory is allocated for an object during spreadsheet processing
2. The memory is subsequently freed but the pointer to that memory is not nullified
3. The application later attempts to use the dangling pointer
4. An attacker can control the contents of the reallocated memory, redirecting execution flow

The vulnerability allows attackers to corrupt memory in a controlled manner, potentially overwriting function pointers or vtable entries to achieve code execution.

Attack Vector

The attack vector for CVE-2025-21387 is local, requiring user interaction to exploit. A typical attack scenario involves:

1. Delivery Phase: The attacker crafts a malicious Excel file (.xlsx, .xlsm, .xlsb, or similar) containing specially constructed data that triggers the UAF condition
2. Social Engineering: The malicious file is delivered to the target via phishing email, compromised website, or shared network drive
3. Execution Phase: When the victim opens the file in Microsoft Excel, the vulnerability is triggered during document parsing
4. Code Execution: The attacker achieves arbitrary code execution in the context of the user's session

The vulnerability affects both Windows and macOS versions of affected Office products, expanding the potential attack surface across enterprise environments.

Detection Methods for CVE-2025-21387

Indicators of Compromise

• Suspicious Excel file attachments from unknown or spoofed senders with unusual file characteristics
• Excel processes (EXCEL.EXE) spawning unexpected child processes such as cmd.exe, powershell.exe, or wscript.exe
• Abnormal memory access patterns or crash dumps in Excel indicating memory corruption
• Unusual network connections originating from Excel processes
• Windows Event Log entries showing application crashes in Excel with memory-related errors

Detection Strategies

• Deploy endpoint detection and response (EDR) solutions to monitor for exploitation attempts and suspicious process behavior from Office applications
• Implement email security solutions to scan and sandbox Excel attachments before delivery to end users
• Configure application-level logging to capture document open events and monitor for unusual behavior patterns
• Utilize YARA rules or similar pattern matching to identify potentially malicious Excel files based on structural anomalies

Monitoring Recommendations

• Monitor for Excel processes spawning child processes, particularly command shells or scripting interpreters
• Track file system activity for Excel creating or modifying files in unusual locations such as %TEMP% or %APPDATA%
• Review Windows Defender Application Guard logs if enabled for Office applications
• Implement network monitoring to detect unusual outbound connections from Excel processes

How to Mitigate CVE-2025-21387

Immediate Actions Required

• Apply Microsoft security updates immediately to all affected Microsoft Office installations across the organization
• Enable Protected View for files originating from the Internet in Microsoft Office Trust Center settings
• Educate users about the risks of opening Excel files from untrusted sources
• Consider blocking Excel file attachments at the email gateway for files from external senders pending patch deployment
• Review and restrict macro execution policies to prevent potential follow-on attacks

Patch Information

Microsoft has released security updates to address this vulnerability. Detailed patch information and download links are available in the Microsoft Security Response Center (MSRC) Advisory. Organizations should apply the appropriate updates for their Office version:

• Microsoft 365 Apps: Updates available through Microsoft Update or Configuration Manager
• Microsoft Excel 2016: Security update available via Windows Update
• Microsoft Office 2019: Updates available through Click-to-Run or Microsoft Update
• Office LTSC 2021/2024: Updates available through Windows Update or volume licensing servicing

Workarounds

• Enable Protected View for all Office documents to prevent automatic execution of potentially malicious content
• Implement Application Guard for Office to isolate document rendering in a sandboxed environment
• Configure file type blocking at email gateways to quarantine Excel files from untrusted sources for manual review
• Restrict Office macro execution through Group Policy to reduce post-exploitation risk

# PowerShell: Enable Protected View via Registry for all users
Set-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Office\16.0\Excel\Security\ProtectedView" -Name "DisableInternetFilesInPV" -Value 0 -Type DWord
Set-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Office\16.0\Excel\Security\ProtectedView" -Name "DisableAttachementsInPV" -Value 0 -Type DWord
Set-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Office\16.0\Excel\Security\ProtectedView" -Name "DisableUnsafeLocationsInPV" -Value 0 -Type DWord