CVE-2026-78509: Microsoft 365 Apps

Critical severity, CVSS 9.8. EPSS: 1% chance of exploitation in the next 30 days.

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

Affected products

• Microsoft 365 Apps: affected versions not specified
• Microsoft Microsoft 365: affected versions not specified
• Microsoft Office 2019: affected versions not specified
• Microsoft Office 2021: affected versions not specified
• Microsoft Office 2024: affected versions not specified
• Microsoft Word: version 2016 only

Published 2026-09-08. Last modified 2026-09-17.