CVE-2026-69764: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-69764 Overview

CVE-2026-69764 is a heap-based buffer overflow [CWE-122] in Microsoft Office Word that allows an unauthorized attacker to execute code over a network. The flaw affects multiple supported Office releases including Microsoft 365 Apps, Office 2016, 2019, 2021, and 2024. Exploitation requires a user to open or preview a crafted Word document, and successful attacks yield high impact to confidentiality, integrity, and availability. Microsoft published the advisory on the MSRC portal.

Critical Impact
A crafted Word document processed by a vulnerable Office client can trigger heap corruption and lead to arbitrary code execution in the user's security context.

Affected Products

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

Discovery Timeline

2026-09-08 - CVE-2026-69764 published to NVD
2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-69764

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow classified under [CWE-122] in the Word document parsing path. When Word processes malformed structures inside a crafted document, it writes past the bounds of a heap-allocated buffer. The corruption can overwrite adjacent heap metadata or object pointers used later during rendering.

An attacker can use the corruption primitive to hijack control flow and execute arbitrary code in the context of the current user. Because Word is often used with elevated document trust or on privileged workstations, the impact extends to credential theft, lateral movement, and persistence. The attack is network-reachable when documents are delivered by email, chat, or web download, though it requires the recipient to open the file.

Root Cause

The root cause is missing or incorrect bounds validation when Word allocates and populates a heap buffer from attacker-controlled length fields inside a document structure. The parser trusts input-derived sizes, allowing an oversized copy into an undersized allocation. Detailed component-level information has not been published beyond the Microsoft advisory.

Attack Vector

Exploitation is remote but requires user interaction. An attacker crafts a malicious .doc, .docx, or RTF file and delivers it through phishing, a web link, or a shared document repository. Preview handlers and the Outlook reading pane can also process document content, increasing the risk surface. Once the file is opened by a vulnerable Word client, the heap corruption executes payload code with the user's privileges.

See the Microsoft Security Update for CVE-2026-69764 for technical details.

Detection Methods for CVE-2026-69764

Indicators of Compromise

Unexpected child processes spawned by WINWORD.EXE such as cmd.exe, powershell.exe, mshta.exe, rundll32.exe, or wscript.exe.
Word crash events in the Windows Application event log referencing wwlib.dll or module faults with heap corruption exception codes such as 0xC0000374.
Outbound network connections initiated by Office processes to previously unseen or low-reputation domains shortly after a document open.
Newly written executable, script, or DLL files in %APPDATA%, %TEMP%, or Office Startup folders following document interaction.

Detection Strategies

Hunt for Office-to-shell process ancestry patterns using EDR telemetry and MITRE ATT&CK technique T1566.001 (Spearphishing Attachment) correlation.
Inspect inbound email attachments for anomalous OOXML or RTF structures using content disarm and reconstruction or sandbox detonation.
Alert on Office applications loading unusual DLLs or invoking Win32 APIs commonly abused for shellcode execution such as VirtualAlloc followed by CreateThread.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction rules that block Office applications from creating child processes and injecting into other processes.
Forward Sysmon process, image load, and file creation events to a central SIEM for cross-host correlation.
Monitor Protected View bypass attempts and audit trust center macro and ActiveX policy changes across the fleet.

How to Mitigate CVE-2026-69764

Immediate Actions Required

Apply the Microsoft security update referenced in the MSRC advisory to all Office and Microsoft 365 Apps installations.
Confirm that Microsoft 365 Apps update channels are current and that clients have restarted to load the patched binaries.
Prioritize patching for users with elevated privileges, executive assistants, and finance or HR staff who routinely handle external documents.

Patch Information

Microsoft has released fixes tracked in the Security Update Guide for CVE-2026-69764. Patches are available for Microsoft 365 Apps, Office 2016, Office 2019, Office LTSC 2021, and Office LTSC 2024 across x64 and x86 architectures. Verify installation using winver for click-to-run builds or Microsoft Update history for MSI installations.

Workarounds

Enforce Protected View and Office Protected Mode for files originating from the internet, email attachments, and unsafe locations.
Block or strip legacy Word document formats at the email gateway when they are not required for business processes.
Disable the Outlook reading pane preview for messages from external senders until patches are deployed.
Apply the ASR rule Block all Office applications from creating child processes (GUID D4F940AB-401B-4EFC-AADC-AD5F3C50688A) in block mode.

bash

# Enable ASR rule to block Office child process creation via PowerShell
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled