CVE-2026-21259: Microsoft Excel Elevation of Privilege Vulnerability

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% 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-122

Description

Heap-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to elevate privileges locally.

Affected products

Microsoft 365 Apps: all versions
Microsoft Excel 2013 Service Pack 1: 2016
Microsoft Office: 2019
Microsoft Office LTSC for Mac 2021: 2021, 2024
Microsoft Office Online Server: before 16.0.10417.20097 — fixed in 16.0.10417.20097
Microsoft 365 Apps: from 16.0.1 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases
Microsoft Excel 2016: from 16.0.0.0 before 16.0.5539.1002 — fixed in 16.0.5539.1002
Microsoft Office LTSC 2021: from 16.0.1 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases
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 Online Server: from 16.0.0.0 before 16.0.10417.20097 — fixed in 16.0.10417.20097

How to fix

Upgrade to a fixed version: Microsoft Office Online Server 16.0.10417.20097; Microsoft 365 Apps https://aka.ms/OfficeSecurityReleases; Microsoft Excel 2016 16.0.5539.1002; Microsoft Office LTSC 2021 https://aka.ms/OfficeSecurityReleases; Microsoft Office https://aka.ms/OfficeSecurityReleases; Microsoft Office LTSC 2024 https://aka.ms/OfficeSecurityReleases; Microsoft Office Online Server 16.0.10417.20097. Check the vendor advisory for mitigations.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21259

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