CVE-2026-108449: Microsoft 365 Apps Buffer Overflow

CVE-2026-108449 Overview

CVE-2026-108449 is a heap-based buffer overflow [CWE-122] in Microsoft Office Word that allows an unauthorized attacker to execute arbitrary code over a network. The flaw affects multiple Office releases, including Microsoft 365 Apps, Microsoft 365 for macOS, Office LTSC 2021, and Office LTSC 2024. Exploitation requires a user to open or preview a crafted document, after which attacker-controlled code runs in the context of the current user. Microsoft published the advisory through the Microsoft Security Response Center (MSRC) update guide.

Critical Impact
Successful exploitation grants arbitrary code execution with the privileges of the targeted user, enabling full compromise of document confidentiality, integrity, and availability.

Affected Products

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

Discovery Timeline

2026-10-15 - CVE-2026-108449 published to the National Vulnerability Database (NVD)
2026-10-19 - Last updated in the NVD database

Technical Details for CVE-2026-108449

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow in the Microsoft Office Word parsing path. Word allocates a heap buffer sized according to a field in the document structure, then writes attacker-controlled content without validating the actual length against the allocation. The resulting out-of-bounds write corrupts adjacent heap metadata and object pointers.

An attacker who controls the overwritten region can hijack control flow, typically by targeting function pointers, virtual tables, or callback structures reachable during continued document processing. Because the overflow occurs in the renderer, code execution runs with the privileges of the user opening the file. The CWE-122 classification confirms the root issue is heap memory corruption rather than a logic flaw.

Root Cause

The root cause is improper validation of a size or length value during parsing of a Word document element. The code trusts an attacker-supplied field when sizing or copying data into a heap allocation, resulting in a write past the end of the buffer. Microsoft has not published source-level details, and no public proof-of-concept has been released.

Attack Vector

The attack vector is network-based but requires user interaction. Delivery methods include email attachments, cloud-shared documents, SharePoint and Teams links, or web downloads using .docx, .doc, .rtf, or related Word-compatible formats. Preview Pane rendering may also trigger the vulnerable parsing path depending on configuration.

The vulnerability requires user interaction to open or preview the file. No valid credentials are needed on the target system.

Detection Methods for CVE-2026-108449

Indicators of Compromise

Unexpected child processes spawned by winword.exe or WINWORD.EXE, such as cmd.exe, powershell.exe, rundll32.exe, mshta.exe, or wscript.exe.
Crashes of winword.exe with heap corruption exceptions (for example STATUS_HEAP_CORRUPTION, 0xC0000374) coinciding with document opens.
Inbound Word documents from untrusted senders containing malformed or heavily nested object streams.
Outbound network connections from winword.exe to previously unseen domains or IP addresses shortly after document open.

Detection Strategies

Hunt for Office applications writing executables, scripts, or scheduled tasks under %APPDATA%, %TEMP%, or %PUBLIC%.
Alert on process tree anomalies where winword.exe is the parent of LOLBins or interpreters.
Inspect inbound email attachments and SharePoint uploads for anomalous Word documents and detonate suspicious samples in a sandbox.
Monitor Windows Error Reporting telemetry for winword.exe faults with heap-related exception codes across the fleet.

Monitoring Recommendations

Enable and forward Microsoft Defender for Office 365 Safe Attachments telemetry to the SIEM.
Collect Sysmon Event ID 1 (process create) and Event ID 11 (file create) scoped to Office processes.
Correlate endpoint process telemetry with mail gateway logs to trace document provenance.
Baseline normal winword.exe child-process and network behavior to surface deviations quickly.

How to Mitigate CVE-2026-108449

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-108449 Update Guide across all affected channels.
Prioritize patching for users who routinely process external documents, including executives, finance, HR, and legal teams.
Verify Microsoft 365 Apps update channels are current and that macOS Office builds have received the corresponding update.
Block or quarantine inbound Word documents from untrusted external senders until patching is complete.

Patch Information

Microsoft has released fixes through the standard servicing channels for Microsoft 365 Apps, Microsoft 365 for macOS, Office LTSC 2021, and Office LTSC 2024. Refer to the Microsoft CVE-2026-108449 Update Guide for the specific build numbers that remediate the issue on each platform and channel.

Workarounds

Open untrusted documents in Protected View and keep Protected View enabled for files from the internet, Outlook attachments, and unsafe locations.
Disable the Outlook Preview Pane for external mail to prevent automatic rendering of attachments.
Enforce Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and from spawning executable content.
Deploy email gateway rules to strip or sandbox Word attachments from external senders pending patch deployment.

bash

# Enable ASR rule: Block Office applications from creating child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled

# Enable ASR rule: Block Office applications from creating executable content
Set-MpPreference -AttackSurfaceReductionRules_Ids 3B576869-A4EC-4529-8536-B80A7769E899 -AttackSurfaceReductionRules_Actions Enabled

# Enforce Protected View for files originating from the internet
Set-ItemProperty -Path 'HKCU:\Software\Microsoft\Office\16.0\Word\Security\ProtectedView' -Name DisableInternetFilesInPV -Value 0