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KB5121647: Overview with user sentiment and feedback

Last Updated September 25, 2026

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Overview

KB5121647 is a cumulative security and quality update for .NET Framework 3.5, 4.8, and 4.8.1 released on August 11, 2026, targeting Windows 10 Version 22H2. This update addresses multiple critical security vulnerabilities including elevation of privilege and remote code execution issues, along with reliability improvements for the .NET Framework stack.

General Purpose

  • Addresses six security vulnerabilities including CVE-2026-65810 and CVE-2026-62872 (elevation of privilege), CVE-2026-62886, CVE-2026-62897, and CVE-2026-70354 (remote code execution), and CVE-2026-62902 (information disclosure)
  • Provides quality and reliability improvements to .NET Framework components
  • Applies to Windows 10 Version 22H2 systems running .NET Framework 3.5, 4.8, or 4.8.1
  • Available through Windows Update, Microsoft Update, and WSUS deployment channels

General Sentiment

The update addresses significant security concerns with multiple critical vulnerabilities, making it an important installation for security posture. However, the patch introduces two documented issues affecting WPF applications, particularly those handling fonts and XPS documents, which may impact production environments. Microsoft acknowledges these issues and indicates resolution in subsequent updates, suggesting a measured approach to deployment timing.

Known Issues

  • WPF applications may fail with System.IO.FileFormatException when printing or generating PDF/XPS content using certain fonts including Calibri; resolved in September 2026 update
  • WPF applications printing from in-memory XPS documents registered with System.IO.Packaging.PackageStore may fail with System.IO.FileFormatException when using pack scheme URIs; status is investigating

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 2026-09-25 07:31 PM

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