CVE-2026-108434: Microsoft 365 Apps Information Disclosure

CVE-2026-108434 Overview

CVE-2026-108434 is an out-of-bounds read vulnerability [CWE-125] in Microsoft Office Word. The flaw allows an unauthenticated remote attacker to disclose sensitive information from the Word process memory when a victim opens a crafted document. Exploitation requires user interaction, typically opening a malicious .docx or .doc file delivered via email, web download, or shared storage. The vulnerability affects multiple Microsoft Office editions across Windows and macOS, including Microsoft 365 Apps, Microsoft 365, Office LTSC 2021, and Office LTSC 2024. No active exploitation has been reported, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
Successful exploitation leaks confidential data from Word process memory, potentially including document contents, authentication artifacts, or memory addresses useful for chaining additional exploits.

Affected Products

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

Discovery Timeline

2026-10-15 - CVE-2026-108434 published to NVD
2026-10-20 - Last updated in NVD database

Technical Details for CVE-2026-108434

Vulnerability Analysis

The vulnerability is an out-of-bounds read [CWE-125] in Microsoft Office Word's document parsing logic. When Word processes a specially crafted file, the parser reads memory outside the bounds of an allocated buffer. The adjacent memory can contain fragments of other open documents, heap metadata, or process state. An attacker who controls the structure of the input document can influence which memory regions are read and exfiltrated through document rendering artifacts or follow-on operations.

The issue carries a confidentiality impact only; integrity and availability are not affected. Exploitation does not require privileges, but user interaction is mandatory because the victim must open the malicious file.

Root Cause

The root cause is missing or insufficient boundary validation when Word parses a specific field, record, or structure within a Word document. The parser trusts a length or offset value supplied in the file without validating it against the allocated buffer size, leading to a read beyond the intended range.

Attack Vector

The attack vector is network-based delivery of a weaponized Word document. Typical delivery mechanisms include phishing emails with attachments, links to malicious files hosted on cloud storage or SharePoint, and watering-hole web pages that trigger document downloads. Once opened, the document is parsed by Word and the out-of-bounds read occurs. See the Microsoft CVE-2026-108434 Update Guide for Microsoft's authoritative description.

No public proof-of-concept code has been published, and no exploit is available in Exploit-DB at this time.

Detection Methods for CVE-2026-108434

Indicators of Compromise

Inbound email attachments containing .docx, .doc, .rtf, or .dotm files from untrusted or spoofed senders.
Word processes (WINWORD.EXE) spawning unexpected child processes, network connections, or script interpreters shortly after opening a document.
Crash telemetry or Windows Error Reporting events referencing access violations in wwlib.dll or related Office parsers.

Detection Strategies

Deploy EDR rules that flag Word opening files from Downloads, Temp, or Outlook attachment cache followed by anomalous memory activity.
Use email security gateways to perform static and dynamic analysis of Office attachments, including sandbox detonation.
Correlate Office telemetry with outbound network connections to catch potential post-exploitation data staging.

Monitoring Recommendations

Monitor Microsoft Defender ASR rules and Office telemetry for blocked or anomalous document parsing events.
Track Office version and patch level across the fleet to identify systems still exposed to CVE-2026-108434.
Alert on repeated Word crashes across multiple endpoints, which can indicate exploitation attempts.

How to Mitigate CVE-2026-108434

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-108434 Update Guide across all affected Office installations.
Prioritize patching for users who routinely open documents from external sources, such as finance, HR, and executive staff.
Verify that macOS clients running Microsoft 365 and Office LTSC 2021/2024 receive the corresponding update through Microsoft AutoUpdate.

Patch Information

Microsoft has issued updates addressing CVE-2026-108434 for Microsoft 365 Apps, Microsoft 365 for macOS, Office LTSC 2021, and Office LTSC 2024. Administrators should consult the vendor advisory for exact build numbers and deploy through Microsoft Update, Click-to-Run, Intune, Configuration Manager, or Microsoft AutoUpdate for macOS. Confirm that Click-to-Run update channels are not paused or deferred beyond the patch release.

Workarounds

Enable Protected View for files originating from the Internet, Outlook attachments, and unsafe locations to limit parser exposure.
Configure Attack Surface Reduction rules to block Office applications from creating child processes and from injecting into other processes.
Use Group Policy to block macros from running in Office files from the Internet and to enforce File Block policies for legacy Word formats where feasible.

bash

# Example: enforce Protected View via Group Policy registry keys (Windows)
reg add "HKCU\Software\Policies\Microsoft\office\16.0\word\security\protectedview" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\office\16.0\word\security\protectedview" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\office\16.0\word\security\protectedview" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f