CVE-2026-69759: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-69759 Overview

CVE-2026-69759 is a stack-based buffer overflow [CWE-121] in Microsoft Office Word that allows an unauthenticated attacker to execute arbitrary code over a network. Exploitation requires user interaction, typically by opening or previewing a crafted document. Successful exploitation yields code execution in the context of the current user, compromising confidentiality, integrity, and availability of the target system. Microsoft has published guidance and a security update through the Microsoft Security Response Center (MSRC).

Critical Impact
An attacker who convinces a user to open a malicious Word document can execute arbitrary code on the target host, enabling malware deployment, credential theft, and lateral movement across Microsoft 365, Office 2019, Office 2021, and Office 2024 deployments.

Affected Products

Microsoft 365 Apps for Enterprise (x64 and x86)
Microsoft Office 2019, Office 2021 LTSC, and Office 2024 LTSC (Windows and macOS)
Microsoft Word 2016 (x64 and x86)

Discovery Timeline

2026-09-08 - CVE-2026-69759 published to the National Vulnerability Database
2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-69759

Vulnerability Analysis

The flaw is a stack-based buffer overflow within Microsoft Office Word's document parsing logic. When Word processes a specially crafted file, malformed structures cause data to be written past the bounds of a fixed-size stack buffer. This corrupts adjacent stack memory, including saved return addresses and structured exception handlers.

An attacker who controls the overflow contents can redirect execution to attacker-supplied code. Because the vulnerability is triggered during document processing, exploitation runs inside the Word process with the privileges of the logged-in user. Attackers commonly chain this class of flaw with sandbox escapes or privilege escalation vulnerabilities to broaden impact.

The issue affects Word rendering across Windows and macOS builds, indicating the vulnerable parsing path is shared across the modern Office code base.

Root Cause

The root cause is missing or insufficient bounds validation on attacker-controlled length or offset fields inside a Word document format. When these values exceed the destination stack buffer, the parser writes beyond the allocated region [CWE-121]. Compiler mitigations such as stack cookies and Control Flow Guard reduce reliability but do not eliminate exploitability, especially when combined with information disclosure primitives.

Attack Vector

Delivery is network-based and requires user interaction. Typical attack chains include phishing emails with a Word attachment, links to documents hosted on attacker-controlled or compromised web servers, and delivery through collaboration platforms such as SharePoint or Teams. Preview handlers and Protected View reduce but do not eliminate exposure, particularly when users click Enable Editing.

The vulnerability manifests during Word document parsing. No public proof-of-concept exploit is available at the time of publication, and the flaw is not listed in the CISA Known Exploited Vulnerabilities catalog. Refer to the Microsoft Security Update guidance for authoritative technical details.

Detection Methods for CVE-2026-69759

Indicators of Compromise

Word documents (.doc, .docx, .rtf) with anomalous structure, oversized records, or malformed OLE streams delivered via email or download
Unexpected child processes spawned by winword.exe, including cmd.exe, powershell.exe, wscript.exe, mshta.exe, or rundll32.exe
Outbound network connections initiated by winword.exe to previously unseen or low-reputation domains shortly after document open
Crash telemetry or Windows Error Reporting events implicating Word document parsing modules

Detection Strategies

Hunt for process lineage where winword.exe is the parent of scripting or LOLBin processes
Alert on Office applications writing executables, DLLs, or scheduled tasks to disk
Inspect email gateway telemetry for documents containing malformed embedded objects or unusually large records
Correlate document open events with subsequent credential access or persistence behavior on the same host

Monitoring Recommendations

Ingest endpoint process, file, and network telemetry into a centralized analytics platform for behavioral correlation
Monitor for exploitation attempts against Office on both Windows and macOS builds
Track patch deployment status across all Microsoft 365 Apps, Office 2019, 2021, and 2024 installations
Enable macro and script execution logging, including AMSI events and PowerShell script block logging

How to Mitigate CVE-2026-69759

Immediate Actions Required

Apply the security update referenced in the Microsoft Security Update guidance across all affected Office SKUs
Prioritize patching for users who routinely handle external documents, including finance, HR, and executive staff
Reinforce phishing awareness and instruct users not to disable Protected View on untrusted files
Validate that email gateways scan and sandbox inbound Office attachments

Patch Information

Microsoft has issued fixes through standard servicing channels for Microsoft 365 Apps, Office 2019, Office 2021 LTSC, Office 2024 LTSC, and Word 2016. Consult the MSRC advisory for CVE-2026-69759 for build numbers and macOS package versions. Deploy updates through Microsoft Update, Configuration Manager, Intune, or the Office click-to-run channel appropriate to your environment.

Workarounds

Enforce Protected View and Office Application Guard for documents originating from the internet and email
Configure Attack Surface Reduction rules to block Office applications from creating child processes and writing executable content
Restrict opening of legacy binary formats such as .doc and .rtf using File Block Settings in Group Policy
Deliver documents through browser-based Office Web viewers where feasible until patches are fully deployed

bash

# Example: Enable Attack Surface Reduction to block Office child processes (PowerShell)
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled

# Example: Block legacy Word binary formats via Group Policy registry key
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Word\Security\FileBlock" `
    /v Word2000Files /t REG_DWORD /d 2 /f