CVE-2025-21364: Microsoft 365 Apps Auth Bypass Flaw

CVE-2025-21364 Overview

CVE-2025-21364 is a security feature bypass vulnerability affecting Microsoft Excel in Microsoft 365 Apps and Office Long Term Servicing Channel. This vulnerability allows attackers to circumvent security controls designed to protect users from malicious content, potentially enabling the execution of harmful operations that would normally be blocked by Excel's built-in security mechanisms.

Critical Impact

Successful exploitation of this vulnerability allows attackers to bypass Excel's security features, potentially leading to high impact on confidentiality, integrity, and availability of affected systems. User interaction is required for exploitation.

Affected Products

• Microsoft 365 Apps for Enterprise (x64 and x86)
• Microsoft Office Long Term Servicing Channel 2024 (x64 and x86)

Discovery Timeline

• 2025-01-14 - CVE-2025-21364 published to NVD
• 2025-07-01 - Last updated in NVD database

Technical Details for CVE-2025-21364

Vulnerability Analysis

This vulnerability stems from improper handling of serialized data within Microsoft Excel, classified under CWE-502 (Deserialization of Untrusted Data). The flaw enables attackers to craft malicious Excel documents that bypass security features intended to prevent dangerous operations. When a user opens a specially crafted file, the deserialization process fails to properly validate the incoming data, allowing the security bypass to occur.

The attack requires local access and user interaction—specifically, a victim must be persuaded to open a malicious Excel file. Once opened, the attacker can achieve high impact across confidentiality, integrity, and availability of the affected system, potentially gaining unauthorized access to sensitive data or executing malicious operations within the user's security context.

Root Cause

The root cause of CVE-2025-21364 lies in insecure deserialization practices within Microsoft Excel's file parsing mechanisms. When Excel processes certain embedded objects or data structures within a spreadsheet file, it fails to adequately validate the serialized content before processing. This allows attackers to inject malicious serialized objects that bypass normal security checks, such as macro warnings, Protected View, or other Office security features designed to sandbox potentially harmful content.

Attack Vector

The attack vector for CVE-2025-21364 requires local access with user interaction. An attacker would typically deliver a malicious Excel file through social engineering methods such as:

• Email attachments containing crafted .xlsx, .xlsm, or other Excel file formats
• Downloads from compromised or malicious websites
• Shared network drives or cloud storage links
• Instant messaging or collaboration platform file sharing

When the victim opens the malicious file, the security feature bypass occurs during the deserialization of embedded content, allowing the attacker's payload to execute operations that would normally be blocked by Excel's security controls.

Detection Methods for CVE-2025-21364

Indicators of Compromise

• Unusual Excel processes spawning child processes or making unexpected network connections
• Excel files with suspicious embedded objects or unusual OLE/ActiveX content
• User reports of Excel files that behave unexpectedly or bypass normal security prompts
• Windows Event Log entries indicating security feature exceptions in Office applications

Detection Strategies

• Monitor for Excel processes exhibiting abnormal behavior such as spawning command interpreters (cmd.exe, powershell.exe) or making network connections
• Implement email gateway scanning for Excel attachments with suspicious embedded content or anomalous file structures
• Deploy endpoint detection rules to identify attempts to bypass Protected View or macro security settings
• Utilize application control policies to restrict Excel from executing unexpected operations

Monitoring Recommendations

• Enable enhanced logging for Microsoft Office applications to capture detailed execution events
• Configure SIEM rules to correlate Excel file access with subsequent suspicious process creation or network activity
• Monitor for modifications to Office security-related registry keys that may indicate tampering
• Track file downloads and email attachments containing Excel documents for post-incident analysis

How to Mitigate CVE-2025-21364

Immediate Actions Required

• Apply the latest security updates from Microsoft for affected Microsoft 365 Apps and Office LTSC 2024 installations
• Educate users about the risks of opening Excel files from untrusted sources
• Enable Protected View for files originating from the internet, email, and potentially unsafe locations
• Consider blocking Excel file attachments from external email sources until patches are applied

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should immediately apply the appropriate patches for their installed Office products. Detailed patch information and download links are available in the Microsoft Security Response Center advisory.

For Microsoft 365 Apps, ensure automatic updates are enabled or manually trigger an update through the Office application. For Office LTSC 2024, download and apply the appropriate update from the Microsoft Update Catalog.

Workarounds

• Enable Attack Surface Reduction (ASR) rules to block Office applications from creating executable content or child processes
• Configure Group Policy to enforce Protected View settings and block macros from internet-sourced documents
• Use Application Guard for Office to open untrusted documents in an isolated container environment
• Restrict Office file associations to prevent automatic opening of potentially malicious files

# Enable ASR rule to block Office apps from creating child processes via PowerShell
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled

# Enable ASR rule to block Office apps from creating executable content
Add-MpPreference -AttackSurfaceReductionRules_Ids 3B576869-A4EC-4529-8536-B80A7769E899 -AttackSurfaceReductionRules_Actions Enabled