CVE-2026-108427: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-108427 Overview

CVE-2026-108427 is a heap-based buffer overflow in Microsoft Office Word that enables remote code execution. An unauthenticated attacker can execute arbitrary code on a target system when a user opens a crafted document. The flaw is categorized under [CWE-20] (Improper Input Validation) and affects Microsoft 365 Apps, Office 2021 LTSC, and Office 2024 LTSC on both x64 and x86 architectures. Exploitation requires user interaction, typically by opening a malicious Word document delivered through email or a web download. Microsoft published the advisory on October 15, 2026.

Critical Impact
Successful exploitation grants an attacker the ability to execute code in the context of the current user, enabling data theft, credential compromise, and lateral movement across Office-connected environments.

Affected Products

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

Discovery Timeline

2026-10-15 - CVE-2026-108427 published to NVD
2026-10-20 - Last updated in NVD database

Technical Details for CVE-2026-108427

Vulnerability Analysis

The vulnerability resides in the document parsing logic of Microsoft Word. When Word processes certain structures inside a crafted document, it allocates a heap buffer whose size does not account for all possible input values. Writing past the allocated boundary corrupts adjacent heap metadata and object pointers. An attacker who shapes the heap layout before the overwrite can redirect execution to attacker-controlled code. The resulting process runs with the privileges of the user who opened the file, which in most enterprise environments includes access to mailboxes, SharePoint sites, OneDrive content, and cached authentication tokens.

Root Cause

The root cause is improper input validation ([CWE-20]) during the parsing of a Word document element. Word trusts a length or count field inside the document without verifying it against the actual allocated buffer size. This mismatch produces an out-of-bounds write on the heap. Because Office components share process memory with add-ins and protocol handlers, corruption of a single structure can be leveraged to subvert control flow within the Word process.

Attack Vector

The attack vector is network-based with required user interaction. An attacker delivers a weaponized .docx, .doc, or .rtf file through phishing email, instant messaging, cloud file sharing, or a malicious website. When a user opens the document outside of Protected View, Word parses the malformed structure and triggers the overflow. The Preview Pane in Outlook may also render document fragments and expand the exposure surface. No credentials are required on the target system because the exploit executes within the user's own session.

No verified public proof-of-concept code is available at the time of writing. See the Microsoft Security Update Guide for vendor-authoritative technical details.

Detection Methods for CVE-2026-108427

Indicators of Compromise

Unexpected child processes spawned by winword.exe, such as cmd.exe, powershell.exe, rundll32.exe, or mshta.exe.
Word writing executable content (.exe, .dll, .ps1, .hta) to %TEMP%, %APPDATA%, or user profile directories.
Outbound network connections initiated by winword.exe to previously unseen domains or IP addresses shortly after a document is opened.
Crash events in the Windows Application log referencing winword.exe with heap corruption exception codes such as 0xC0000374 or 0xC0000005.

Detection Strategies

Monitor Office processes for anomalous child process creation and script interpreter launches, aligning rules to MITRE ATT&CK technique T1566.001 (Spearphishing Attachment).
Alert on Office applications performing memory injection, DLL loads from user-writable paths, or writes to startup and Run registry keys.
Inspect inbound email attachments and web downloads for Word documents containing malformed OLE, OOXML, or RTF structures.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction rules that block Office applications from creating child processes and from injecting into other processes.
Forward Office telemetry, Sysmon process and file events, and EDR alerts to a centralized data lake for correlation across mailboxes and endpoints.
Review Protected View bypass events and Mark-of-the-Web stripping activity on file shares and collaboration platforms.

How to Mitigate CVE-2026-108427

Immediate Actions Required

Apply the Microsoft security updates referenced in the Microsoft Security Update Guide for Microsoft 365 Apps, Office 2021 LTSC, and Office 2024 LTSC.
Confirm patch deployment across both x64 and x86 Office installations, including kiosk and shared workstations.
Prioritize patching for users who routinely open documents from external senders, including executives, finance, HR, and legal teams.
Hunt retroactively for the indicators listed above across the last 30 days of endpoint and email telemetry.

Patch Information

Microsoft released fixed builds for all affected Office channels. Administrators should verify the current build number against the fixed version published in the Microsoft Security Update Guide and force an Office update cycle where client-side auto-update is disabled.

Workarounds

Enforce Protected View for files originating from the internet, Outlook attachments, and unsafe locations through Group Policy.
Disable the Outlook Reading Pane for untrusted mailboxes until patches are deployed.
Block or quarantine inbound .doc, .docx, and .rtf attachments from unauthenticated external senders at the mail gateway.
Enable Attack Surface Reduction rules that block Office applications from creating executable content and spawning child processes.

bash

# Enable key Microsoft Defender ASR rules via PowerShell
Set-MpPreference -AttackSurfaceReductionRules_Ids `
  D4F940AB-401B-4EFC-AADC-AD5F3C50688A, `
  3B576869-A4EC-4529-8536-B80A7769E899, `
  75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84 `
  -AttackSurfaceReductionRules_Actions Enabled,Enabled,Enabled