CVE-2025-29816: Microsoft 365 Apps Auth Bypass Vulnerability

CVE-2025-29816 Overview

CVE-2025-29816 is an improper input validation vulnerability in Microsoft Office Word that allows an unauthorized attacker to bypass a security feature over a network. This vulnerability stems from CWE-349 (Acceptance of Extraneous Untrusted Data With Trusted Data), where the application fails to properly validate input, potentially allowing attackers to circumvent built-in security mechanisms designed to protect users from malicious content.

Critical Impact

An attacker could bypass security features in Microsoft Word by exploiting improper input validation, potentially leading to compromise of confidentiality, integrity, and availability of affected systems.

Affected Products

• Microsoft 365 Apps for Enterprise
• Microsoft Office 2016, 2019
• Microsoft Office LTSC 2021 (Windows and macOS)
• Microsoft Office LTSC 2024 (Windows and macOS)
• Microsoft Word 2016

Discovery Timeline

• 2025-04-08 - CVE-2025-29816 published to NVD
• 2025-07-09 - Last updated in NVD database

Technical Details for CVE-2025-29816

Vulnerability Analysis

This vulnerability exists due to improper input validation within Microsoft Office Word's document processing functionality. The flaw is classified under CWE-349, which involves the acceptance of extraneous untrusted data alongside trusted data. When Word processes specially crafted content, it fails to adequately validate certain inputs, allowing attackers to bypass security features that would normally protect users from malicious operations.

The attack requires user interaction, meaning a victim must open a malicious document or click on a specially crafted link for exploitation to succeed. While the attack complexity is high, successful exploitation could result in significant impact to confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of CVE-2025-29816 lies in insufficient input validation within Microsoft Word's document parsing and processing logic. The vulnerability falls under CWE-349 (Acceptance of Extraneous Untrusted Data With Trusted Data), indicating that the application accepts untrusted data in contexts where it should be rejected or sanitized. This improper validation allows attackers to craft input that bypasses security controls designed to prevent malicious document behavior.

Attack Vector

The attack vector for CVE-2025-29816 is network-based, requiring an attacker to deliver a malicious document or link to the victim. The exploitation scenario typically involves:

1. An attacker crafts a specially formatted Word document containing malicious content designed to exploit the input validation flaw
2. The malicious document is delivered to the victim via email, web download, or other network-based delivery mechanism
3. The victim opens the document in a vulnerable version of Microsoft Word
4. The improper input validation allows the attacker to bypass security features, potentially leading to further compromise

The vulnerability requires user interaction (opening a malicious document) but does not require prior authentication, making it accessible to external attackers through phishing or social engineering campaigns.

Detection Methods for CVE-2025-29816

Indicators of Compromise

• Unusual Word document files with unexpected embedded content or malformed structures
• Word processes (WINWORD.EXE) exhibiting anomalous behavior such as spawning child processes or making unexpected network connections
• Document files received from untrusted sources that trigger security warnings or Protected View bypass
• Registry modifications or file system changes initiated by Word processes outside normal operation

Detection Strategies

• Monitor Microsoft Word process behavior for suspicious child process creation or network activity
• Implement email filtering rules to quarantine suspicious Word document attachments from external sources
• Deploy endpoint detection rules that alert on attempts to bypass Protected View or other Office security features
• Use SentinelOne's behavioral AI to detect anomalous document processing patterns indicative of exploitation attempts

Monitoring Recommendations

• Enable and review Microsoft Office security event logs for evidence of security feature bypass attempts
• Configure SentinelOne to monitor for suspicious Office application behavior and file system modifications
• Implement network monitoring to detect unusual outbound connections from Word processes
• Establish baseline behavior for Office applications to identify deviations that may indicate exploitation

How to Mitigate CVE-2025-29816

Immediate Actions Required

• Apply the latest Microsoft security updates to all affected Microsoft Office and Microsoft 365 installations
• Enable Protected View for all documents from the Internet and untrusted locations
• Educate users about the risks of opening documents from unknown or untrusted sources
• Consider implementing application whitelisting to restrict document sources

Patch Information

Microsoft has released security updates to address CVE-2025-29816. Organizations should consult the Microsoft Security Update Guide for detailed patch information and download links. Apply patches to all affected products including Microsoft 365 Apps, Office 2016, Office 2019, Office LTSC 2021, Office LTSC 2024, and Word 2016.

Workarounds

• Enable Protected View in Microsoft Word by navigating to File > Options > Trust Center > Trust Center Settings > Protected View and enabling all options
• Block Office documents from untrusted sources at the email gateway and web proxy level
• Configure Group Policy to enforce strict macro and document security settings across the organization
• Consider deploying Microsoft Defender Application Guard for Office to isolate potentially malicious documents

# Enable Protected View via Registry (Windows)
# This enforces Protected View for files from the Internet
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f