CVE-2026-78505: Microsoft Office Remote Code Execution Vulnerability

Overview

Severity

High (CVSS 8.8)

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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

0.82% (percentile: 56.8%)

Description

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

FAQ

Is the Preview Pane an attack vector for this vulnerability? Yes, the Preview Pane is an attack vector. How could an attacker exploit this vulnerability? An attacker could convince a user to open and save a specially crafted Microsoft Office document. Successful exploitation could allow the attacker to execute code in the context of the user. Authentication is not required, but user interaction is required. 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 Office 2016 (32-bit edition)
• Microsoft Office 2016 (64-bit edition)

Security Updates (3)

• Release Notes
• Release Notes
• 5002916

Acknowledgments

Kentaro Kawane with GMO Cybersecurity by Ierae, Inc., <a href="https://x.com/lmksecurity">Jeongmin Choi</a>, <a href="https://x.com/seadragnol">SeaDragnoL</a> with <a href="https://x.com/qriousec">Qrious Secure</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.