CVE-2026-108426: Microsoft 365 Apps Buffer Overflow

CVE-2026-108426 Overview

CVE-2026-108426 is a heap-based buffer overflow [CWE-122] in Microsoft Office Word that allows an unauthenticated attacker to execute arbitrary code over a network. Exploitation requires a user to open a crafted document, after which the attacker gains code execution in the context of the current user. The vulnerability affects Microsoft 365 Apps, Microsoft Office LTSC 2021, and Microsoft Office LTSC 2024 on both x86 and x64 platforms. Microsoft published the advisory on October 15, 2026.

Critical Impact
Successful exploitation yields arbitrary code execution with the privileges of the user opening a malicious Word document, enabling full compromise of confidentiality, integrity, and availability on the host.

Affected Products

Microsoft 365 Apps Enterprise (x86 and x64)
Microsoft Office LTSC 2021 (x86 and x64)
Microsoft Office LTSC 2024 (x86 and x64)

Discovery Timeline

2026-10-15 - CVE-2026-108426 published to NVD
2026-10-20 - Last updated in NVD database

Technical Details for CVE-2026-108426

Vulnerability Analysis

The flaw is a heap-based buffer overflow in Microsoft Word's document parsing logic. Word allocates a heap buffer sized from attacker-controlled fields in a crafted document. Subsequent write operations exceed the allocated size, corrupting adjacent heap metadata and object pointers.

An attacker who controls adjacent heap chunks can overwrite function pointers, virtual table entries, or saved return addresses. This grants control of the instruction pointer and allows arbitrary code execution inside the WINWORD.EXE process. Code runs under the opening user's security context, which typically includes access to local files, mapped shares, and cached credentials.

Root Cause

The root cause is improper validation of a length or count field read from a Word document structure before the destination heap buffer is written. The parser trusts the attacker-supplied size, allocating one amount while copying a larger quantity of data. This classic size-mismatch pattern falls under CWE-122, Heap-Based Buffer Overflow.

Attack Vector

The attack vector is network-based with user interaction required. A typical delivery chain involves phishing email attachments, malicious links hosting .docx or .rtf payloads, or documents staged on compromised SharePoint and file-share locations. Protected View may blunt some exploitation attempts, but documents originating from trusted zones or files opened after a user clicks "Enable Editing" execute the vulnerable parsing path. The Preview Pane may also render content and trigger the vulnerable code depending on configuration.

No public proof-of-concept or in-the-wild exploitation has been reported. See the Microsoft Security Update for CVE-2026-108426 for the authoritative technical details.

Detection Methods for CVE-2026-108426

Indicators of Compromise

Unexpected child processes spawned by WINWORD.EXE, especially cmd.exe, powershell.exe, rundll32.exe, mshta.exe, or wscript.exe.
WINWORD.EXE writing executable files, scripts, or DLLs to %APPDATA%, %TEMP%, or %PUBLIC% directories.
Outbound network connections from WINWORD.EXE to untrusted hosts shortly after a document is opened.
Crashes or Application Error events (Event ID 1000) referencing WINWORD.EXE with heap corruption signatures.

Detection Strategies

Hunt for Office applications spawning command interpreters or scripting hosts using process-lineage telemetry.
Flag .docx, .doc, and .rtf files delivered by email that fail Protected View enforcement or have Mark-of-the-Web stripped.
Correlate Word process crashes with subsequent persistence activity such as scheduled tasks, Run key modifications, or new services.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction rules that block Office applications from creating child processes and writing executable content.
Collect Sysmon Event IDs 1, 3, 7, and 11 from endpoints running Office and forward to centralized analytics.
Alert on Office process memory allocations followed by RWX region creation, consistent with shellcode staging.

How to Mitigate CVE-2026-108426

Immediate Actions Required

Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-108426 to all affected Microsoft 365 Apps, Office LTSC 2021, and Office LTSC 2024 installations.
Confirm Microsoft 365 Apps clients are on a supported update channel and have received the current build.
Block inbound email attachments containing legacy Office formats at the mail gateway when business-justified.
Enforce Protected View and Mark-of-the-Web for all documents sourced from the internet.

Patch Information

Microsoft released patches concurrent with the advisory publication on October 15, 2026. Administrators should deploy the latest cumulative updates for Microsoft 365 Apps, Office LTSC 2021, and Office LTSC 2024. Validate deployment through Microsoft Endpoint Configuration Manager, Intune, or Windows Update for Business reporting. Full details are available in the Microsoft Security Update for CVE-2026-108426.

Workarounds

Disable the Outlook Preview Pane to prevent automatic rendering of attached documents.
Enable Attack Surface Reduction rule D4F940AB-401B-4EFC-AADC-AD5F3C50688A to block Office applications from creating child processes.
Open untrusted documents only in Office for the web, which executes parsing in an isolated server-side sandbox.
Restrict macro execution via Group Policy and block macros in files originating from the internet.

bash

# Configuration example: enable ASR rules via PowerShell
Add-MpPreference -AttackSurfaceReductionRules_Ids `
    D4F940AB-401B-4EFC-AADC-AD5F3C50688A, `
    3B576869-A4EC-4529-8536-B80A7769E899, `
    75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84 `
    -AttackSurfaceReductionRules_Actions Enabled, Enabled, Enabled