CVE-2026-108451: Microsoft 365 Apps RCE Vulnerability

CVE-2026-108451 Overview

CVE-2026-108451 is a numeric truncation error 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 classified under CWE-122: Heap-based Buffer Overflow and affects multiple supported Office product lines, including Microsoft 365 Apps and the LTSC editions of Office 2021 and Office 2024. Successful exploitation yields high impact to confidentiality, integrity, and availability. Microsoft published the advisory in the Microsoft Security Update Guide.

Critical Impact
Attackers who convince a user to open a malicious Word document can execute code with the user's privileges, leading to full compromise of the account context.

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-2026-108451 published to the National Vulnerability Database
2026-10-19 - Last updated in NVD database

Technical Details for CVE-2026-108451

Vulnerability Analysis

The vulnerability stems from a numeric truncation error during the parsing of Word document structures. When Office processes attacker-controlled size or length fields, a wider integer value is truncated to a smaller type. The truncated value is then used to allocate or index into a heap buffer, producing an undersized allocation relative to the data subsequently written. This mismatch results in a heap-based buffer overflow [CWE-122], which attackers leverage to corrupt adjacent heap metadata or object pointers.

Exploitation requires user interaction. The target must open the malicious document or trigger a parsing path through the Preview Pane, depending on configuration. No prior authentication to the victim system is needed, and the attack can be delivered over a network via email attachments, cloud shares, or web downloads. Successful exploitation runs code in the context of the Word process, inheriting the user's privileges.

Root Cause

The root cause is unsafe conversion of a size or length value within the Word document parser. The truncated integer is treated as authoritative for buffer sizing, allowing out-of-bounds writes when the actual data length exceeds the truncated bound. Office's structured document formats contain many size-prefixed records, which increases the surface area for this class of defect.

Attack Vector

An attacker crafts a malicious .doc, .docx, or related Office document and delivers it through phishing, watering-hole hosting, or collaboration platforms. When the victim opens the file in a vulnerable Word build, the truncation triggers the heap overflow and leads to code execution. See the Microsoft Security Update Guide for additional vendor technical context.

No verified public proof-of-concept code is available at the time of writing. The vulnerability mechanism is described in prose above rather than through synthetic exploit code.

Detection Methods for CVE-2026-108451

Indicators of Compromise

Unexpected child processes spawned by winword.exe, such as cmd.exe, powershell.exe, rundll32.exe, or mshta.exe.
Office processes writing executable content to %APPDATA%, %TEMP%, or %PUBLIC% directories.
Outbound network connections from winword.exe to unfamiliar domains or IP addresses shortly after document open events.
Crash events referencing heap corruption in winword.exe or associated Office DLLs in Windows Error Reporting.

Detection Strategies

Monitor process ancestry to flag Office applications launching script interpreters or living-off-the-land binaries.
Inspect inbound email attachments for Word documents containing anomalous OLE streams, oversized records, or crafted size fields.
Correlate document open telemetry with sudden memory region allocations and module loads inside the Office process.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office child process creation and executable content from email.
Forward Sysmon process, image load, and network events to a centralized data lake for retrospective hunting.
Alert on crashes in winword.exe with heap corruption exception codes such as 0xC0000374 or 0xC0000005.

How to Mitigate CVE-2026-108451

Immediate Actions Required

Apply the Microsoft security updates referenced in the Microsoft Security Update Guide to all affected Microsoft 365 Apps, Office 2021 LTSC, and Office 2024 LTSC installations.
Prioritize patching endpoints used by high-risk users such as executives, finance, and HR who frequently open external documents.
Validate that Office update channels are active and that deferred update rings do not delay deployment of this fix.

Patch Information

Microsoft released patches for this vulnerability through the standard Office servicing channels. Administrators should consult the Microsoft Security Update Guide for specific build numbers mapped to each affected SKU. Update Microsoft 365 Apps using the Office Click-to-Run client or managed deployment tooling, and apply the corresponding MSI or LTSC package for Office 2021 and Office 2024.

Workarounds

Enable Protected View and Office Block macros from the internet via Group Policy for all users.
Disable the Outlook Preview Pane for messages originating from external senders to reduce automatic parsing exposure.
Enforce Attack Surface Reduction rules that block Office applications from creating child processes and from injecting into other processes.
Restrict opening of Office documents from untrusted locations through File Block policies configured in the Trust Center.

bash

# Example: enable key ASR rules via PowerShell to reduce Office exploitation surface
Set-MpPreference -AttackSurfaceReductionRules_Ids `
  D4F940AB-401B-4EFC-AADC-AD5F3C50688A, `
  3B576869-A4EC-4529-8536-B80A7769E899, `
  75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84 `
  -AttackSurfaceReductionRules_Actions Enabled,Enabled,Enabled