CVE-2026-55022: Microsoft 365 Apps RCE Vulnerability

CVE-2026-55022 Overview

CVE-2026-55022 is a type confusion vulnerability [CWE-843] in Microsoft Office that allows an unauthorized attacker to execute code locally. The flaw stems from Office accessing a resource using an incompatible type, corrupting memory in a way that supports arbitrary code execution in the context of the current user.

Exploitation requires user interaction, typically opening a crafted document. Successful attacks compromise confidentiality, integrity, and availability on the affected host.

Critical Impact
A local attacker who convinces a user to open a malicious Office file can execute arbitrary code with the privileges of the current user, enabling malware installation, data theft, and lateral movement.

Affected Products

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

Discovery Timeline

2026-07-14 - CVE-2026-55022 published to NVD
2026-07-16 - Last updated in NVD database

Technical Details for CVE-2026-55022

Vulnerability Analysis

The vulnerability is a type confusion issue [CWE-843] in Microsoft Office parsing routines. Type confusion occurs when code allocates or accesses a resource assuming one data type, but the underlying object is a different, incompatible type. The mismatched interpretation leads to out-of-bounds memory reads or writes and can hand attackers control over object pointers or virtual function tables.

Exploitation requires local access and user interaction, which aligns with the typical Office attack chain of email delivery followed by document open. No authentication is required. Because the attacker executes code in the context of the user opening the file, follow-on activity such as credential theft or persistence is trivial once code execution is achieved.

Root Cause

Office components fail to validate that an object retrieved during document parsing matches the expected type before dereferencing it. When a crafted document supplies a manipulated object header or embedded structure, Office operates on the object as if it were the anticipated type. The resulting confusion between the actual and expected memory layout enables controlled memory corruption and code execution.

Attack Vector

An attacker crafts a malicious Office document containing structures designed to trigger the type confusion condition. The attacker delivers the file through email, chat, cloud sharing, or drive-by download. When the target opens the document in a vulnerable version of Word, Excel, PowerPoint, or another affected Office application, the parser reaches the vulnerable code path and executes attacker-controlled payload in the user's session.

No verified public proof-of-concept code is available for CVE-2026-55022 at the time of this writing. Refer to the Microsoft Security Update CVE-2026-55022 advisory for authoritative technical details.

Detection Methods for CVE-2026-55022

Indicators of Compromise

Office applications (WINWORD.EXE, EXCEL.EXE, POWERPNT.EXE) spawning unusual child processes such as cmd.exe, powershell.exe, mshta.exe, wscript.exe, or rundll32.exe.
Office processes performing outbound network connections to unfamiliar or newly registered domains shortly after a document is opened.
Unexpected writes to Startup folders, Run keys, or Office Trusted Locations following document access.
Crash dumps or Windows Error Reporting events for Office binaries with heap or access-violation exceptions.

Detection Strategies

Hunt for Office child-process anomalies mapped to MITRE ATT&CK T1204.002 (User Execution: Malicious File) and T1566.001 (Spearphishing Attachment).
Correlate document open events with subsequent script interpreter execution and outbound network traffic.
Inspect inbound email attachments and cloud file uploads for Office documents containing anomalous embedded objects or oversized structures.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction rules that block Office from creating child processes and from injecting into other processes.
Collect Sysmon Event ID 1 (process creation) and Event ID 3 (network connection) with Office binaries as the parent, and forward to your SIEM for hunting.
Alert on Office application crashes in Windows Event Log source Application Error referencing Office modules.

How to Mitigate CVE-2026-55022

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-55022 advisory to all Windows and macOS Office installations.
Prioritize patching for users who routinely process external documents, including finance, HR, legal, and executive assistants.
Verify Microsoft 365 Apps update channels are configured to receive current security builds, and confirm Office 2016, 2019, 2021, and 2024 installations report the patched build.

Patch Information

Microsoft has released fixes through the July 2026 security update cycle. Deploy the corresponding updates for Microsoft 365 Apps, Office 2016, Office 2019, Office 2021 LTSC, and Office 2024 LTSC on both Windows and macOS. Confirm patch application by validating file versions of core Office binaries against the versions listed in the vendor advisory.

Workarounds

Enforce Protected View and Office Application Guard for documents originating from the internet and email attachments.
Disable or restrict macros, ActiveX, and embedded OLE objects through Group Policy where business processes allow.
Use Attack Surface Reduction rules to block Office applications from launching child processes and creating executable content.
Route inbound email attachments through a sandbox or content disarm and reconstruction (CDR) solution.

bash

# Enable Attack Surface Reduction: Block Office from creating child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled

# Enforce Protected View for files from the internet (Word example)
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" `
    /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f