CVE-2026-108450: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-108450 Overview

CVE-2026-108450 is a heap-based buffer overflow [CWE-122] in Microsoft Office Word. An unauthorized attacker can execute arbitrary code over a network by convincing a user to open a crafted document. The flaw affects Microsoft 365 Apps, Office LTSC 2021, and Office LTSC 2024 across both x86 and x64 builds.

Successful exploitation grants the attacker code execution in the context of the current user. If that user holds administrative rights, the attacker gains full control of the host, enabling data theft, lateral movement, and persistent compromise.

Critical Impact
Remote code execution through a weaponized Word document, requiring only that a target opens the file.

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-2026-108450 published to NVD
2026-10-19 - Last updated in NVD database

Technical Details for CVE-2026-108450

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow in Microsoft Office Word's document parsing path. Word writes attacker-controlled data past the bounds of a heap allocation, corrupting adjacent heap metadata or object pointers. An attacker who shapes the heap can convert that corruption into control of execution flow.

Exploitation requires user interaction. A victim must open a malicious document delivered through email, a web download, a shared drive, or a messaging platform. The attack succeeds across the network boundary because no authentication or local access to the target host is needed.

Microsoft classifies this vulnerability under CWE-122: Heap-based Buffer Overflow. The current EPSS probability is 0.432%, reflecting a modest but non-zero prediction of exploitation activity in the next 30 days.

Root Cause

The root cause is improper bounds checking during the processing of a specific structure within a Word document. The parser allocates a heap buffer sized from one field and then copies data governed by a different length field, allowing attacker-controlled data to overflow the allocation.

Attack Vector

The attack vector is a crafted .doc, .docx, .rtf, or related Office document. Delivery typically occurs through phishing. Preview Pane rendering can expand the attack surface for similar Office issues, so administrators should treat the Preview Pane as an exposed surface.

No public proof-of-concept code or in-the-wild exploitation has been reported at the time of writing. See the Microsoft Security Update Guide for the authoritative technical description.

Detection Methods for CVE-2026-108450

Indicators of Compromise

winword.exe spawning child processes such as cmd.exe, powershell.exe, wscript.exe, cscript.exe, mshta.exe, or rundll32.exe.
Unexpected network connections initiated by winword.exe to external hosts shortly after document open.
Office documents containing anomalous embedded objects, oversized structures, or malformed length fields.
New or modified files in %APPDATA%, %TEMP%, or Office startup folders following document execution.

Detection Strategies

Alert on Office applications writing executable content to disk or loading unsigned DLLs from user-writable paths.
Hunt for process chains originating from winword.exe that lead to script interpreters or regsvr32.exe with remote arguments.
Inspect inbound email attachments for Word documents with abnormal structure, obfuscated OLE streams, or embedded shellcode patterns.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office child processes and executable content creation.
Forward Office telemetry, Sysmon process and image-load events, and EDR alerts into a centralized data lake for correlation.
Track patch deployment status against the vulnerable Microsoft 365 Apps and Office LTSC builds listed by Microsoft.

How to Mitigate CVE-2026-108450

Immediate Actions Required

Apply the Microsoft security updates referenced in the Microsoft Security Update Guide to all affected Office installations.
Confirm that Microsoft 365 Apps clients are receiving updates on the Current, Monthly Enterprise, or Semi-Annual Enterprise channels as appropriate.
Prioritize patching for users who routinely process external documents, including finance, HR, and executive assistants.

Patch Information

Microsoft has released security updates addressing CVE-2026-108450 for Microsoft 365 Apps, Office LTSC 2021, and Office LTSC 2024. Review the Microsoft Security Update Guide for the current build numbers and deployment guidance for each channel.

Workarounds

Enable Protected View and Office Protected Mode for documents sourced from the internet or email attachments.
Disable the Outlook Preview Pane and the Windows Explorer preview handler for Office file types until patches are deployed.
Enforce Microsoft Defender ASR rules that block Office applications from creating child processes and injecting into other processes.
Block or quarantine Office documents from untrusted senders at the email gateway and strip active content where feasible.

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