CVE-2026-78509: Microsoft Office Outlook Remote Code Execution Vulnerability

Overview

Severity

Critical (CVSS 9.8)

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

Category

Remote Code Execution

Exploit Status

Not Exploited

Exploitation Likelihood

Less Likely

Patch Tuesday

2026-Sep

Released

2026-09-08

Last Updated

2026-09-17

EPSS Score

0.97% (percentile: 61.7%)

Description

Heap-based buffer overflow in Microsoft Office Outlook allows an unauthorized attacker to execute code over a network.

FAQ

How could an attacker exploit this vulnerability? An attacker could send a specially crafted email message to a target user. Simply viewing the message in the Outlook Reading Pane could trigger the vulnerability, without the recipient opening the message or clicking anything in it. Successful exploitation could allow the attacker to execute code on the recipient's system. Network access is required; authentication and user interaction are not required. Is the Preview Pane an attack vector for this vulnerability? No, the Preview Pane is not an attack vector. Are the updates for Microsoft Office LTSC for Mac 2021 and 2024 currently available? Yes. As of September 16, 2026, the security update for Microsoft Office LTSC for Mac 2021, 2024 and Microsoft Office 365 for Mac are available. Customers running Microsoft Office LTSC for Mac 2021, 2024 and Microsoft Office 365 for Mac should ensure the update is installed to be protected from this vulnerability.

Affected Products (13)

Microsoft Office

• Microsoft Office 365 for Mac
• Microsoft Office 2019 for 32-bit editions
• Microsoft Office 2019 for 64-bit editions
• Microsoft 365 Apps for Enterprise for 32-bit Systems
• Microsoft 365 Apps for Enterprise for 64-bit Systems
• Microsoft Office LTSC for Mac 2021
• Microsoft Office LTSC 2021 for 64-bit editions
• Microsoft Office LTSC 2021 for 32-bit editions
• Microsoft Office LTSC 2024 for 32-bit editions
• Microsoft Office LTSC 2024 for 64-bit editions
• Microsoft Office LTSC for Mac 2024
• Microsoft Word 2016 (32-bit edition)
• Microsoft Word 2016 (64-bit edition)

Security Updates (3)

• Release Notes
• Release Notes
• 5002923

Acknowledgments

Haifei Li with <a href="https://pub.expmon.com/">EXPMON</a>

Revision History

• 2026-09-08: Information published.
• 2026-09-17: Microsoft is announcing the availability of the security updates for Microsoft Office for Mac. Customers running affected Mac software should install the update for their product to be protected from this vulnerability. Customers running other Microsoft Office software do not need to take any action. See the Release Notes for more information and download links.