CVE-2026-108436: Microsoft 365 Apps Information Disclosure

CVE-2026-108436 Overview

CVE-2026-108436 is an out-of-bounds read vulnerability [CWE-125] in Microsoft Office Word. An unauthorized attacker can exploit this flaw to disclose information over a network when a user opens a specially crafted document. The vulnerability affects Microsoft 365 Apps, Microsoft 365, Office LTSC 2021, and Office LTSC 2024 across Windows and macOS platforms. Exploitation requires user interaction but does not require privileges or authentication. The flaw impacts confidentiality but does not affect integrity or availability of the targeted system.

Critical Impact
Successful exploitation allows an attacker to read memory contents outside the intended buffer, potentially exposing sensitive data processed by Microsoft Word.

Affected Products

Microsoft 365 Apps (Enterprise, x64 and x86)
Microsoft 365 for macOS
Microsoft Office LTSC 2021 (Windows and macOS)
Microsoft Office LTSC 2024 (Windows and macOS)

Discovery Timeline

2026-10-15 - CVE-2026-108436 published to the National Vulnerability Database (NVD)
2026-10-20 - Last updated in NVD database

Technical Details for CVE-2026-108436

Vulnerability Analysis

The vulnerability is an out-of-bounds read condition within Microsoft Office Word's document parsing logic. When Word processes a malformed document structure, the application reads memory outside the bounds of an allocated buffer. The contents of this adjacent memory can then be reflected into application state accessible to a remote attacker. Because the flaw affects confidentiality only, exploitation does not grant code execution or allow modification of files on disk. The attack surface spans all currently supported desktop versions of Microsoft Word, including the macOS builds of Microsoft 365 and Office LTSC.

Root Cause

The root cause is improper validation of boundary conditions during the parsing of Word document data [CWE-125]. The affected parser does not verify that read offsets remain within the allocated buffer before dereferencing memory. This permits adjacent process memory to be read and included in content returned or rendered to the attacker's crafted document structure.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker crafts a malicious Word document and delivers it through email, a shared drive, a chat platform, or a hosted web link. When the target opens the document in a vulnerable version of Word, the parser triggers the out-of-bounds read. The leaked memory may include document fragments, authentication artifacts, or data from other processes mapped into the Word process space. See the Microsoft CVE-2026-108436 Update Guide for vendor technical details.

Detection Methods for CVE-2026-108436

Indicators of Compromise

Inbound email attachments containing .docx, .doc, .rtf, or .docm files from untrusted senders, particularly files with malformed structure or unusual object streams.
Microsoft Word process (WINWORD.EXE on Windows, Microsoft Word on macOS) crashes or exceptions logged shortly after opening an external document.
Outbound network traffic from Word processes to unexpected hosts following document open events.

Detection Strategies

Deploy endpoint detection rules that correlate Word process execution with document opens from internet-origin paths such as the Downloads folder or Outlook temporary attachment directories.
Hunt for Word child processes spawning cmd.exe, powershell.exe, wscript.exe, or network utilities, which may indicate chained exploitation attempts.
Inspect Office telemetry for Protected View bypass attempts and documents that trigger parser exceptions.

Monitoring Recommendations

Enable Microsoft Office audit logging and forward events to a centralized SIEM for correlation with email gateway logs.
Monitor the EPSS trend for CVE-2026-108436 and prioritize response if exploitation probability increases.
Track patch deployment status for Microsoft 365 Apps, Office 2021, and Office 2024 across all managed endpoints including macOS fleets.

How to Mitigate CVE-2026-108436

Immediate Actions Required

Apply the security update referenced in the Microsoft CVE-2026-108436 Update Guide to all affected Office installations.
Verify that Microsoft 365 Apps clients are receiving updates through the Current Channel or Monthly Enterprise Channel and audit any devices that are behind on updates.
Enforce Protected View and Office File Block policies for documents originating from the internet or email attachments.

Patch Information

Microsoft has published guidance and patch details in the Microsoft Security Response Center advisory. Administrators should deploy the appropriate update for each affected product channel, including Microsoft 365 Apps (x64 and x86), Microsoft 365 for macOS, Office LTSC 2021, and Office LTSC 2024 on both Windows and macOS.

Workarounds

Block or quarantine inbound Office documents from untrusted external senders at the email gateway until patches are deployed.
Configure Office to open documents from the internet in Protected View and disable automatic content activation.
Use Attack Surface Reduction (ASR) rules to prevent Office applications from launching child processes and limit the impact of chained exploitation.

bash

# Example: enable Microsoft Defender ASR rule to block Office child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled