CVE-2025-27751: Microsoft 365 Apps Use-After-Free Flaw

CVE-2025-27751 Overview

CVE-2025-27751 is a use-after-free vulnerability in Microsoft Office Excel that allows an unauthorized attacker to execute arbitrary code locally. This memory corruption flaw occurs when Excel improperly handles objects in memory, enabling attackers to craft malicious spreadsheet files that, when opened by a victim, can execute code in the context of the current user.

Critical Impact

Successful exploitation of this vulnerability could allow attackers to gain complete control over the affected system with the privileges of the logged-in user, potentially leading to data theft, malware installation, or lateral movement within an organization's network.

Affected Products

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

Discovery Timeline

• April 8, 2025 - CVE-2025-27751 published to NVD
• July 9, 2025 - Last updated in NVD database

Technical Details for CVE-2025-27751

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) exists within the memory management routines of Microsoft Excel. When processing specially crafted Excel documents, the application may continue to reference memory that has already been freed. An attacker who can convince a user to open a malicious Excel file can leverage this dangling pointer to corrupt memory and ultimately achieve arbitrary code execution.

The attack requires user interaction, specifically opening a malicious Excel document. The attack is local in nature, meaning the attacker must deliver the malicious file to the victim through social engineering tactics such as phishing emails, malicious downloads, or compromised file shares. Once opened, the exploit can execute code with the same privilege level as the current user.

Root Cause

The vulnerability stems from improper memory management within Excel's document parsing engine. When certain Excel objects are deallocated, the application fails to properly invalidate references to the freed memory. Subsequent operations that attempt to access these stale references can be manipulated by an attacker to redirect execution flow or corrupt critical data structures.

This type of use-after-free condition occurs when:

1. Memory is allocated for an Excel object
2. The memory is freed during document processing
3. The application continues to hold and use the now-invalid pointer
4. An attacker can control what data occupies the freed memory region

Attack Vector

The attack vector for CVE-2025-27751 requires local access and user interaction. An attacker would typically:

1. Craft a malicious Excel document containing triggers for the use-after-free condition
2. Deliver the malicious file to the victim via email attachment, web download, or shared network location
3. Social engineer the victim into opening the document
4. Upon opening, the malicious document exploits the memory corruption to execute attacker-controlled code

The vulnerability does not require elevated privileges to exploit, but successful exploitation grants the attacker the same access level as the victim user. This could include access to sensitive documents, email, and network resources.

Detection Methods for CVE-2025-27751

Indicators of Compromise

• Unexpected Excel crashes or error messages when opening spreadsheet files from untrusted sources
• Suspicious child processes spawned by EXCEL.EXE such as cmd.exe, powershell.exe, or other scripting interpreters
• Unusual network connections originating from Excel processes
• Presence of suspicious Excel files with unusual embedded objects or macros in temporary directories

Detection Strategies

• Monitor for Excel processes exhibiting abnormal behavior such as spawning unexpected child processes or making network connections
• Implement endpoint detection rules to identify memory corruption indicators in Office applications
• Deploy YARA rules to scan for known malicious Excel document patterns targeting this vulnerability
• Use application control policies to restrict execution of untrusted Office documents

Monitoring Recommendations

• Enable Windows Defender Application Guard for Office to isolate potentially malicious documents
• Configure Enhanced Mitigation Experience Toolkit (EMET) or Windows Defender Exploit Guard with Address Space Layout Randomization (ASLR) and Data Execution Prevention (DEP) protections
• Review and correlate logs from email security gateways for suspicious Excel attachments
• Monitor file system activity for Excel files being written to unusual locations

How to Mitigate CVE-2025-27751

Immediate Actions Required

• Apply the latest Microsoft security updates for all affected Office products immediately
• Enable Protected View in Microsoft Excel to prevent automatic execution of potentially malicious content
• Block Excel files from untrusted sources at email and web gateways
• Educate users about the risks of opening Excel documents from unknown or suspicious sources

Patch Information

Microsoft has released security updates to address this vulnerability. Administrators should consult the Microsoft Security Update Guide for detailed patch information and download links for all affected products.

Organizations should prioritize patching the following products:

• Microsoft 365 Apps for Enterprise
• Microsoft Excel 2016
• Microsoft Office 2019
• Microsoft Office LTSC 2021 and 2024

Workarounds

• Enable Protected View for files originating from the Internet in Excel Trust Center settings
• Configure Office to block files with embedded OLE objects from untrusted locations
• Use Microsoft Defender Application Guard for Office to open untrusted documents in an isolated container
• Implement strict email attachment policies to quarantine or scan all incoming Excel files

# PowerShell: Enable Protected View for Excel documents from the Internet
Set-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" -Name "DisableInternetFilesInPV" -Value 0
Set-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" -Name "DisableUnsafeLocationsInPV" -Value 0
Set-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" -Name "DisableAttachmentsInPV" -Value 0