Summary
Impact: Remote Code Execution. Microsoft rating: Critical. Not known to be exploited — Exploitation Less Likely. Customer action: Required.
Details
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.
Security updates
16.113.26091433 — https://www.microsoft.com/en-us/microsoft-365/mac/microsoft-365-for-mac?msockid=35f9adb0e74b61392038b90de6fe608c
16.0.10417.20207 — https://docs.microsoft.com/en-us/officeupdates/microsoft365-apps-security-updates
16.0.20326.20138 — https://docs.microsoft.com/en-us/officeupdates/office365-proplus-security-updates
16.113.26091433 — https://go.microsoft.com/fwlink/p/?linkid=831049
16.0.5569.1000 — https://support.microsoft.com/help/5002923
Vulnerabilities 1
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Critical 9.8
Microsoft Office Outlook Remote Code Execution Vulnerability
Heap-based buffer overflow in Microsoft Office Outlook allows an unauthorized attacker to execute code over a network.
Revision history
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#2
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](https://go.microsoft.com/fwlink/p/?linkid=831049) for more information and download links.
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#1
Information published.
References
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CVE-2026-78509 Microsoft Office Outlook Remote Code Execution Vulnerability - HTML
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-78509
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Microsoft Exploitability Index
https://www.microsoft.com/en-us/msrc/exploitability-index?rtc=1
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Microsoft Support Lifecycle
https://support.microsoft.com/lifecycle
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Common Vulnerability Scoring System
https://www.first.org/cvss