CVE-2026-50301: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-50301 Overview

CVE-2026-50301 is a heap-based buffer overflow vulnerability [CWE-122] affecting multiple versions of Microsoft Office. An attacker who successfully exploits this flaw can execute arbitrary code locally on the target system with the privileges of the current user. Exploitation requires user interaction, typically by opening a crafted document. Microsoft rates the issue with a CVSS 3.1 base score of 7.8.

The vulnerability impacts Microsoft 365 Apps, Office 2016, Office 2019, Office 2021 LTSC, and Office 2024 LTSC across both x86 and x64 builds. No public proof-of-concept code or in-the-wild exploitation has been reported as of publication.

Critical Impact
Successful exploitation grants local code execution in the context of the user, enabling malware installation, credential theft, and lateral movement from a single opened Office document.

Affected Products

Microsoft 365 Apps (Enterprise, x86 and x64)
Microsoft Office 2016, 2019, 2021 LTSC, and 2024 LTSC (x86 and x64)
Any deployment using vulnerable Office file parsers on Windows endpoints

Discovery Timeline

2026-07-14 - CVE-2026-50301 published to the National Vulnerability Database
2026-07-16 - Last updated in NVD database

Technical Details for CVE-2026-50301

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow [CWE-122] in Microsoft Office document processing. Office allocates a heap buffer of insufficient size when parsing attacker-controlled fields inside a crafted document. Subsequent write operations exceed the allocated boundary and corrupt adjacent heap metadata or object pointers.

An attacker who controls the corrupted memory can hijack control flow and execute arbitrary code. Because execution occurs in the Office process, the resulting code runs with the privileges of the user who opened the file. Impact on confidentiality, integrity, and availability is high, as reflected in the CVSS vector.

Root Cause

The defect stems from improper validation of a size or length field used to size a heap allocation, or during the copy operation that follows. Microsoft has not released detailed internal analysis. Refer to the Microsoft Security Advisory for authoritative technical guidance.

Attack Vector

Exploitation is local and requires user interaction. An attacker delivers a specially crafted Office document by email, chat, cloud share, or web download. When the victim opens the file in a vulnerable Office application, the malformed structure triggers the overflow.

Common delivery methods include phishing attachments, drive-by downloads, and macro-enabled lures. The Preview Pane in Outlook and Explorer may also render Office content, expanding the attack surface without an explicit open action. No specific privileges are required on the target system beyond the ability to run Office.

No verified public exploit code is available. See the Microsoft Security Advisory for further details.

Detection Methods for CVE-2026-50301

Indicators of Compromise

Office applications (WINWORD.EXE, EXCEL.EXE, POWERPNT.EXE) spawning unexpected child processes such as cmd.exe, powershell.exe, wscript.exe, or rundll32.exe
Office processes writing executable content to %TEMP%, %APPDATA%, or %PROGRAMDATA%
Unexpected outbound network connections initiated by Office processes shortly after a document is opened
Crash events for Office binaries logged in the Windows Application event log referencing heap corruption

Detection Strategies

Hunt for anomalous parent-child process relationships originating from Office applications across the fleet
Correlate Office document open events with subsequent process creation, file writes, and network activity within short time windows
Inspect inbound email attachments and shared documents for malformed Office file structures using sandbox detonation

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office from creating child processes and from injecting into other processes
Ingest Sysmon process creation, image load, and file creation events into a centralized SIEM for retrospective hunting
Monitor Office telemetry for repeated crashes on the same user or document, which may indicate exploitation attempts

How to Mitigate CVE-2026-50301

Immediate Actions Required

Apply the security updates listed in the Microsoft Security Advisory to all affected Office installations
Prioritize patching for users who routinely open external documents, including finance, HR, legal, and executive assistants
Verify that Microsoft 365 Apps clients are configured to receive updates on the Current or Monthly Enterprise Channel

Patch Information

Microsoft has issued fixes through its standard update channels for Microsoft 365 Apps, Office 2016, 2019, 2021 LTSC, and 2024 LTSC. Administrators should consult the Microsoft Security Advisory for exact build numbers by product edition and architecture, then validate patch deployment through Microsoft Endpoint Configuration Manager, Intune, or WSUS reporting.

Workarounds

Enable Protected View for documents originating from the internet and other untrusted locations
Block or strip Office document attachments at the email gateway when the sender is untrusted or the file fails structural validation
Disable the Outlook Preview Pane and Windows Explorer preview handlers for Office file types until patches are deployed
Enforce Microsoft Defender ASR rules that block Office child process creation as a compensating control

bash

# Enable ASR rule: Block all 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 injecting code into other processes
Add-MpPreference -AttackSurfaceReductionRules_Ids 75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84 -AttackSurfaceReductionRules_Actions Enabled

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