CVE-2026-21511: Microsoft 365 Apps

High severity, CVSS 7.5. EPSS: 4% chance of exploitation in the next 30 days.

Deserialization of untrusted data in Microsoft Office Outlook allows an unauthorized attacker to perform spoofing over a network.

Affected products

• Microsoft 365 Apps: affected versions not specified
• Microsoft Office: version 2019 only
• Microsoft Office Long Term Servicing Channel: version 2021 only; version 2024 only
• Microsoft SharePoint Server: before 16.0.19127.20518 (fixed in 16.0.19127.20518); version 2016 only; version 2019 only
• Microsoft Word: version 2016 only

Published 2026-02-10. Last modified 2026-06-17.