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. The flaw is categorized under [CWE-349] (Acceptance of Extraneous Untrusted Data With Trusted Data) and affects multiple Microsoft Office product lines, including Microsoft 365 Apps, Office 2016, Office 2019, and Office Long Term Servicing Channel (LTSC) 2021 and 2024 on both Windows and macOS. Successful exploitation requires user interaction, typically through opening a specially crafted document.

Critical Impact
An attacker who successfully exploits this flaw can bypass a Word security feature, leading to high impact on confidentiality, integrity, and availability of the affected host.

Affected Products

Microsoft 365 Apps (Enterprise)
Microsoft Office 2016 and Office 2019
Microsoft Office LTSC 2021 and 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

The vulnerability stems from improper input validation in how Microsoft Word processes specific document content. Word fails to adequately distinguish trusted data from extraneous untrusted data, allowing a crafted file to bypass an internal security control. This control is intended to gate the execution of potentially dangerous content, such as protected view restrictions or trust-boundary checks applied to externally sourced documents.

The attack vector is network-based, but exploitation requires the victim to open a malicious document. Attack complexity is rated high because the attacker must shape the document in a way that defeats validation logic without triggering rejection. When the bypass succeeds, the embedded content is treated as if it originated from a trusted context, enabling further compromise of confidentiality, integrity, and availability.

Root Cause

The root cause is the acceptance of extraneous untrusted data alongside trusted data during document parsing. Word's input validation routine does not fully separate adversary-controlled fields from elements that the security feature treats as safe, allowing the protection to be circumvented.

Attack Vector

An attacker delivers a malicious Word document over the network, commonly through email, web download, or shared file storage. When a user opens the file, the malformed structure causes Word to bypass the security feature, exposing the user to follow-on payloads or trust-boundary violations. Refer to the Microsoft CVE-2025-29816 Advisory for vendor technical details.

// No public proof-of-concept code is available for CVE-2025-29816.
// Exploitation requires a specially crafted Word document that abuses
// input validation in the security feature's trust evaluation logic.

Detection Methods for CVE-2025-29816

Indicators of Compromise

Unexpected Word documents (.doc, .docx, .docm, .rtf) arriving from external senders with embedded objects or unusual structural elements.
WINWORD.EXE spawning unexpected child processes such as cmd.exe, powershell.exe, wscript.exe, or mshta.exe.
Outbound network connections initiated by Word to untrusted hosts shortly after document open.
Creation or modification of files in user profile or startup directories following Word activity.

Detection Strategies

Inspect Office documents at the email gateway for malformed structures, anomalous OLE objects, and known abuse patterns referenced in the Microsoft advisory.
Correlate Word process telemetry with subsequent script interpreter execution, suspicious child processes, and network egress.
Track Microsoft Office version inventory to identify endpoints running unpatched builds of Microsoft 365 Apps, Office 2016/2019, or Office LTSC 2021/2024.

Monitoring Recommendations

Enable Microsoft Office logging and audit Protected View bypass attempts and Mark-of-the-Web (MOTW) handling.
Forward Office and endpoint telemetry to a centralized analytics platform to support hunting for document-borne attack patterns.
Alert on documents with macros or embedded objects that arrive from outside the organization and are opened on multiple endpoints in short succession.

How to Mitigate CVE-2025-29816

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2025-29816 Advisory to all affected Office installations.
Prioritize patching internet-facing knowledge workers and high-value users who routinely process external documents.
Re-validate Office update channels for Microsoft 365 Apps and LTSC editions on both Windows and macOS.

Patch Information

Microsoft has released security updates that address the improper input validation flaw. Administrators should consult the vendor advisory and deploy the appropriate updates for each affected channel: Microsoft 365 Apps, Office 2016, Office 2019, Office LTSC 2021, and Office LTSC 2024. macOS builds of Office LTSC are also in scope.

Workarounds

Block or quarantine inbound Word documents from untrusted senders at the email gateway until patches are deployed.
Enforce Protected View and Office Trust Center settings via Group Policy, and disable automatic processing of external content.
Apply Attack Surface Reduction rules that prevent Office applications from spawning child processes and creating executable content.

bash

# Example: configure Microsoft Defender ASR rules via PowerShell to harden Office
# Block Office applications from creating child processes
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled

# Block Office applications from creating executable content
Add-MpPreference -AttackSurfaceReductionRules_Ids 3B576869-A4EC-4529-8536-B80A7769E899 `
                 -AttackSurfaceReductionRules_Actions Enabled