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CVE-2026-55121: Microsoft Office Information Disclosure Vulnerability

• Severity: medium (CVSS 5.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-14 · updated 2026-10-28
• Weakness: CWE-125

Description

Out-of-bounds read in Microsoft Office allows an unauthorized attacker to disclose information locally.

Affected products

• Microsoft 365 Apps: all versions
• Microsoft 365: all versions
• Microsoft Office 2016: all versions
• Microsoft Office 2019: all versions
• Microsoft Office 2021: all versions
• Microsoft Office LTSC 2024: all versions
• Microsoft SharePoint Server: before 16.0.19725.20434; 2016, 2019 — fixed in 16.0.19725.20434
• Microsoft 365 Apps: from 16.0.1 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases
• Microsoft Office 365 for Mac: from 1.0.0 before 16.111.26071215 — fixed in 16.111.26071215
• 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 16.111.26071215 — fixed in 16.111.26071215
• Microsoft SharePoint Server Subscription Edition: from 16.0.0 before 16.0.19725.20434 — fixed in 16.0.19725.20434
• Microsoft Office: from 16.0.0 before 16.0.5561.1000 — fixed in 16.0.5561.1000
• 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 16.111.26071215 — fixed in 16.111.26071215
• Microsoft Sharepoint Enterprise Server 2016: from 16.0.0 before 16.0.5561.1001 — fixed in 16.0.5561.1001
• Microsoft SharePoint Server: from 16.0.0 before 16.0.10417.20175 — fixed in 16.0.10417.20175

How to fix

Upgrade to a fixed version: Microsoft SharePoint Server 16.0.19725.20434; Microsoft 365 Apps https://aka.ms/OfficeSecurityReleases; Microsoft Office 365 for Mac 16.111.26071215; Microsoft Office LTSC 2021 https://aka.ms/OfficeSecurityReleases; Microsoft Office LTSC for Mac 2024 16.111.26071215; Microsoft SharePoint Server Subscription Edition 16.0.19725.20434; Microsoft Office 16.0.5561.1000; Microsoft Office LTSC 2024 https://aka.ms/OfficeSecurityReleases; Microsoft Office LTSC for Mac 2021 16.111.26071215; Microsoft Sharepoint Enterprise Server 2016 16.0.5561.1001; Microsoft SharePoint Server 16.0.10417.20175. Check the vendor advisory for mitigations.

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55121

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