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

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 the Microsoft Office LTSC for Mac currently available? The security update for Microsoft Office LTSC for Mac 2021 and 2024 are not immediately available. The updates will be released as soon as possible, and when they are available, customers will be notified via a revision to this CVE information.

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 (1)

Acknowledgments

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

Revision History

  • 2026-09-08: Information published.