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CVE-2026-21260: Microsoft Outlook Spoofing Vulnerability

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.5% 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-200

Description

Exposure of sensitive information to an unauthorized actor 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 Outlook: 2016
• Microsoft SharePoint Server: before 16.0.19127.20518; 2016, 2019 — fixed in 16.0.19127.20518
• 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 SharePoint Server Subscription Edition: from 16.0.0 before 16.0.19127.20518 — fixed in 16.0.19127.20518
• 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 Outlook 2016 for Mac: from 16.0.0.0 before 16.0.5539.1002 — fixed in 16.0.5539.1002
• 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 SharePoint Server Subscription Edition 16.0.19127.20518; Microsoft Office https://aka.ms/OfficeSecurityReleases; Microsoft Office LTSC 2024 https://aka.ms/OfficeSecurityReleases; Microsoft Outlook 2016 for Mac 16.0.5539.1002; 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-2026-21260

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