Microsoft August .NET Framework Updates Break Printing and PDF Export in WPF Apps

August Updates Cause WPF Application Failures

Microsoft has confirmed that the August 2026 Patch Tuesday .NET Framework updates are leading to problems with printing and PDF export functionality in certain applications. Specifically, applications utilizing the Windows Presentation Foundation (WPF) UI framework are experiencing System.IO.FileFormatException errors when attempting to print or generate PDF/XPS content, particularly with fonts like Calibri.

Affected Platforms and Scope

The issue impacts a broad range of Microsoft platforms, including both client releases such as Windows 10 and Windows 11, and server versions from Windows Server 2012 up to Windows Server 2025. This widespread effect means a significant number of enterprise and consumer applications could be affected by this bug.

Temporary Workaround with Caveats

While Microsoft investigates a permanent solution, it has provided a temporary workaround. This involves enabling the Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection AppContext switch in the application's config file. However, Microsoft warns that implementing this workaround will disable security protections introduced with the August 2026 .NET Framework update, potentially exposing systems to vulnerabilities that the update was meant to address. Microsoft advises using this workaround only as a temporary measure and when absolutely necessary.

Prior Incidents and Related Issues

This is not the first time Microsoft updates have caused issues with WPF applications; a similar problem occurred in February 2021, leading to crashes in WPF apps and Visual Studio. Additionally, Microsoft recently shared a workaround for a separate issue caused by August 2026 Windows 11 updates, which are causing games like ARC Raiders, MARVEL Tōkon: Fighting Souls, and The Finals to crash and freeze.