KB5121645: Overview with user sentiment and feedback

Overview

KB5121645 is a cumulative security update for .NET Framework 3.5, 4.7.2, and 4.8 released on August 11, 2026, targeting Windows 10 version 1809 and Windows Server 2019. This update addresses six critical security vulnerabilities including multiple remote code execution and elevation of privilege flaws. The update is distributed through Windows Update, Microsoft Update, and the Microsoft Update Catalog.

General Purpose

• Addresses CVE-2026-65810 and CVE-2026-62872 elevation of privilege vulnerabilities in .NET Framework
• Resolves three remote code execution vulnerabilities (CVE-2026-62886, CVE-2026-62897, CVE-2026-70354)
• Patches CVE-2026-62902 information disclosure vulnerability
• Implements enhanced security protections for font handling and XPS document processing
• Provides cumulative quality and reliability improvements to .NET Framework components

General Sentiment

The update addresses significant security concerns with multiple critical vulnerabilities, which is positive for system security posture. However, community feedback indicates distribution challenges, with users reporting the update is not appearing in the Microsoft Update Catalog for some systems. Additionally, the update introduces two known WPF application compatibility issues affecting printing and PDF/XPS generation, which has generated concern among affected users. The workarounds available require configuration changes that temporarily disable the security protections being introduced.

Known Issues

• WPF applications may fail with System.IO.FileFormatException when printing or generating PDF/XPS content using certain fonts (including Calibri); workaround available via AppContext switch but disables security protections
• WPF applications printing or displaying print preview from in-memory XPS documents registered with System.IO.Packaging.PackageStore may fail with System.IO.FileFormatException; workaround requires code changes or AppContext configuration
• Update not appearing in Microsoft Update Catalog for some Windows Server 2019 x64 systems despite successful installation of related cumulative updates

Disclaimer: We take measures to ensure that AI-generated content is of the highest possible quality, but we cannot guarantee its accuracy and recommend that users do their own independent research. Generated on 2026-09-05 01:22 AM

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