CVE-2026-69734: Microsoft 365 Apps Information Disclosure

CVE-2026-69734 is an information disclosure vulnerability in Microsoft 365 Apps caused by an integer overflow in Office Word. Attackers can exploit this flaw over a network to access sensitive data. This article covers technical details, affected versions, security impact, and mitigation strategies.

CVE-2026-69734 Overview

CVE-2026-69734 is an integer overflow vulnerability in Microsoft Office Word that allows an unauthenticated attacker to disclose sensitive information over a network. The flaw is classified under [CWE-125] (Out-of-Bounds Read) and requires user interaction, typically by opening a crafted document. Exploitation exposes in-process memory contents, which can leak sensitive data such as document fragments, memory pointers, or other application state. Microsoft has published guidance for affected Office versions, including Microsoft 365 Apps, Office 2016, Office 2019, Office 2021, and Office 2024 across Windows and macOS builds.

Critical Impact
A crafted Word document can trigger an out-of-bounds read caused by an integer overflow, disclosing sensitive process memory to a remote attacker after a single user action.

Affected Products

Microsoft 365 Apps (x64 and x86 enterprise builds)
Microsoft Office 2016, 2019, 2021 LTSC, and 2024 LTSC (Windows and macOS)
Microsoft Word 2016 and Microsoft 365 for macOS

Discovery Timeline

2026-09-08 - CVE-2026-69734 published to NVD
2026-09-08 - Last updated in NVD database

Technical Details for CVE-2026-69734

Vulnerability Analysis

The vulnerability originates in Microsoft Word's parsing logic for document structures. An integer overflow occurs during size or offset calculations, producing a value smaller than intended. Word then trusts this miscalculated value when performing subsequent memory reads. The result is an out-of-bounds read that returns data from adjacent heap regions to the attacker-controlled parsing context.

Because the read happens during document processing, disclosed memory can include portions of previously opened documents, uninitialized buffer contents, or memory layout information useful for chaining with other exploits. Microsoft's advisory categorizes the flaw as information disclosure with no impact on integrity or availability.

Root Cause

The root cause is unchecked arithmetic during length or index computation. When attacker-controlled fields in a Word document push a calculation past the maximum representable value, the result wraps around. The wrapped value bypasses subsequent bounds checks, allowing the parser to read past the end of an allocated buffer.

Attack Vector

Exploitation requires an attacker to deliver a crafted document, typically through email, a web download, or a shared file service. The victim must open the file in an affected version of Word. No privileges or prior authentication are needed on the target system. The disclosed information is returned to the attacker through downstream channels available to the document processing context.

See the Microsoft CVE-2026-69734 Update Guide for vendor-specific technical details.

Detection Methods for CVE-2026-69734

Indicators of Compromise

Inbound .doc, .docx, .rtf, or .docm files from untrusted senders containing malformed structural elements or abnormally large size fields.
winword.exe processes generating unexpected outbound network connections shortly after opening a document.
Word process crashes or exception events in the Windows Application log correlated with document opens.

Detection Strategies

Inspect email gateways and web proxies for Office documents with malformed OLE, OOXML, or RTF structures.
Correlate document-open telemetry with subsequent network egress from winword.exe to non-corporate destinations.
Enable Attack Surface Reduction (ASR) rules to block Office child processes and monitor triggered events for anomalous document behavior.

Monitoring Recommendations

Ingest Office telemetry, EDR process events, and email gateway logs into a central analytics platform for cross-source correlation.
Alert on winword.exe loading unusual DLLs or spawning unexpected script interpreters after document open events.
Track Microsoft Defender for Office 365 detonation verdicts on inbound attachments and flag repeat senders of malformed Office files.

How to Mitigate CVE-2026-69734

Immediate Actions Required

Apply the security updates referenced in the Microsoft advisory to all affected Office channels and platforms.
Prioritize patch deployment on endpoints that regularly process documents from external senders.
Enable Protected View and Office Application Guard for documents originating from the internet or email attachments.

Patch Information

Microsoft has released updates addressing CVE-2026-69734 for Microsoft 365 Apps, Office 2016, 2019, 2021 LTSC, and 2024 LTSC on both Windows and macOS. Refer to the Microsoft CVE-2026-69734 Update Guide for the specific build numbers and KB articles that remediate the issue in each channel.

Workarounds

Block or quarantine inbound Office documents from untrusted senders at the email gateway until patches are deployed.
Enforce Group Policy settings that require Protected View for files from the internet, unsafe locations, and Outlook attachments.
Use ASR rules such as "Block all Office applications from creating child processes" to reduce exploitation impact.

bash

# Enforce Protected View via registry for Microsoft Word
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 DisableUnsafeLocationsInPV /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