CVE-2026-108430: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-108430 Overview

CVE-2026-108430 is a heap-based buffer overflow vulnerability in Microsoft Office Word that allows an unauthorized attacker to execute arbitrary code over a network. The flaw is categorized under [CWE-122] (Heap-based Buffer Overflow) and affects multiple supported Microsoft Office releases, including Microsoft 365 Apps, Microsoft 365, Office LTSC 2021, and Office LTSC 2024 across Windows and macOS platforms. Exploitation requires user interaction, typically through opening a crafted document, but no authentication or elevated privileges are needed. Successful exploitation grants attackers code execution in the context of the current user, enabling payload delivery, credential theft, and lateral movement.

Critical Impact
An attacker who successfully exploits CVE-2026-108430 can execute arbitrary code on the target system by delivering a malicious Word document, compromising confidentiality, integrity, and availability.

Affected Products

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

Discovery Timeline

2026-10-15 - CVE-2026-108430 published to the National Vulnerability Database
2026-10-20 - Last updated in NVD database

Technical Details for CVE-2026-108430

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow within the Microsoft Office Word document parsing pipeline. When Word processes a specially crafted document, it writes attacker-controlled data past the bounds of a heap-allocated buffer. The out-of-bounds write corrupts adjacent heap metadata or object pointers, which an attacker can leverage to redirect execution flow. Because the overflow occurs in a user-mode process with network-reachable file ingestion paths (email attachments, cloud shares, web downloads), the vulnerability provides a reliable code execution primitive. The attacker gains execution in the security context of the user running Word, inheriting that user's filesystem, registry, and network access rights.

Root Cause

The root cause is improper validation of length or size fields in a parsed document structure before data is copied into a heap buffer [CWE-122]. The parser trusts attacker-supplied size metadata, allocating a buffer sized incorrectly for the subsequent write operation. This boundary mismatch enables the overflow during document deserialization.

Attack Vector

The attack vector is network-based with required user interaction. An attacker crafts a malicious Word document and delivers it through phishing email, instant messaging, a watering-hole download, or a shared document repository. When the victim opens the document, or in some cases previews it, the parser triggers the heap overflow and executes attacker-supplied shellcode. No prior authentication to the target host is required, and the scope remains unchanged, which means code executes within the current user's context.

No verified public proof-of-concept code is available for CVE-2026-108430 at the time of publication. See the Microsoft Vulnerability Advisory CVE-2026-108430 for vendor technical details.

Detection Methods for CVE-2026-108430

Indicators of Compromise

Unexpected child processes spawned by winword.exe, including cmd.exe, powershell.exe, mshta.exe, rundll32.exe, or wscript.exe.
Crash events or Windows Error Reporting (WER) entries referencing winword.exe with access violation exceptions during document open.
Outbound network connections initiated by winword.exe to untrusted or newly registered domains shortly after a document is opened.
Creation of unsigned executables or scripts in user-writable paths such as %APPDATA%, %LOCALAPPDATA%\Temp, or %PUBLIC% immediately after document interaction.

Detection Strategies

Hunt for Word spawning scripting or living-off-the-land binaries using process-lineage telemetry mapped to MITRE ATT&CK techniques T1566.001 and T1203.
Inspect inbound email attachments and SharePoint/OneDrive uploads for .doc, .docx, .rtf, and .docm files with malformed structures or embedded objects.
Correlate winword.exe crash telemetry with subsequent process creation, file write, or network connection events on the same host.

Monitoring Recommendations

Enable Microsoft Office telemetry and Attack Surface Reduction (ASR) rule auditing, including the rule blocking Office child-process creation.
Forward endpoint process, file, and network events to a centralized data lake for retrospective hunting against new indicators tied to CVE-2026-108430.
Alert on Protected View bypass attempts and on documents loaded directly from the Internet zone without Mark-of-the-Web.

How to Mitigate CVE-2026-108430

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Vulnerability Advisory CVE-2026-108430 to all affected Microsoft 365 Apps, Microsoft 365, Office LTSC 2021, and Office LTSC 2024 installations.
Prioritize patching on hosts used by high-risk users such as executives, finance, HR, and administrators who routinely open externally sourced documents.
Enable Microsoft Defender ASR rules that block Office applications from creating child processes and from injecting code into other processes.

Patch Information

Microsoft has published a security update through the Microsoft Security Response Center. Administrators should deploy the fix via Microsoft Update, Microsoft 365 Apps Click-to-Run channels, or Windows Server Update Services (WSUS) according to their patch management process. Verify the installed Office build after deployment to confirm remediation.

Workarounds

Enforce Protected View and Office File Block policies for documents originating from the internet and email, preventing automatic parsing of untrusted content.
Disable or restrict legacy file formats (.doc, .rtf) at the mail gateway and endpoint where business processes permit.
Deploy Group Policy to require Mark-of-the-Web enforcement so downloaded documents open in a sandboxed Protected View session.

bash

# Example: Enforce Protected View for files originating from the Internet (per-user registry)
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

# Example: Enable the ASR rule blocking Office apps from creating child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled