CVE-2026-21511: Microsoft Outlook Spoofing Vulnerability

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 4.0% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-10 · updated 2026-08-19
Weakness: CWE-502

Description

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

Affected products

Microsoft 365 Apps: all versions
Microsoft Office: 2019
Microsoft Office LTSC for Mac 2021: 2021, 2024
Microsoft SharePoint Server: before 16.0.19127.20518; 2016, 2019 — fixed in 16.0.19127.20518
Microsoft Word for Android: 2016
Microsoft 365 Apps: from 16.0.1 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases
Microsoft Office LTSC 2021: from 16.0.1 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases
Microsoft Office LTSC for Mac 2024: from 16.0.0 before 16.106.26020821 — fixed in 16.106.26020821
Microsoft SharePoint Server Subscription Edition: from 16.0.0 before 16.0.19127.20518 — fixed in 16.0.19127.20518
Microsoft Word 2016: from 16.0.1 before 16.0.5539.1002 — fixed in 16.0.5539.1002
Microsoft Office: from 19.0.0 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases
Microsoft Office LTSC 2024: from 16.0.0 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases
Microsoft Office LTSC for Mac 2021: from 16.0.1 before 16.106.26020821 — fixed in 16.106.26020821
Microsoft Sharepoint Enterprise Server 2016: from 16.0.0 before 16.0.5539.1002 — fixed in 16.0.5539.1002
Microsoft SharePoint Server: from 16.0.0 before 16.0.10417.20097 — fixed in 16.0.10417.20097

How to fix

Upgrade to a fixed version: Microsoft SharePoint Server 16.0.19127.20518; Microsoft 365 Apps https://aka.ms/OfficeSecurityReleases; Microsoft Office LTSC 2021 https://aka.ms/OfficeSecurityReleases; Microsoft Office LTSC for Mac 2024 16.106.26020821; Microsoft SharePoint Server Subscription Edition 16.0.19127.20518; Microsoft Word 2016 16.0.5539.1002; Microsoft Office https://aka.ms/OfficeSecurityReleases; Microsoft Office LTSC 2024 https://aka.ms/OfficeSecurityReleases; Microsoft Office LTSC for Mac 2021 16.106.26020821; Microsoft Sharepoint Enterprise Server 2016 16.0.5539.1002; Microsoft SharePoint Server 16.0.10417.20097. Check the vendor advisory for mitigations.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21511

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