Microsoft confirms printing and PDF export errors in WPF applications

Microsoft has confirmed that the August 2026 .NET Framework cumulative updates contain a bug affecting applications built on the Windows Presentation Foundation (WPF) framework. The issue causes certain WPF applications to fail with a ‘System.IO.FileFormatException’ when attempting to print or export content to PDF or XPS formats, particularly when using specific fonts such as Calibri.

The bug impacts a wide range of operating systems, including Windows 10, Windows 11, and Windows Server versions ranging from 2012 to 2025.

Microsoft is currently investigating the issue and has provided a temporary workaround. This workaround involves enabling the ‘AppContext’ switch ‘MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection’ in the application’s configuration file. However, Microsoft warned that this bypass disables security protections introduced in the August update, potentially leaving systems vulnerable to attacks that the update was intended to prevent. The company advises using this workaround only temporarily and when absolutely necessary.