Overview
- The August Patch Tuesday .NET Framework cumulative update caused some WPF apps to throw System.IO.FileFormatException errors when printing or exporting PDF/XPS from documents using fonts such as Calibri.
- Microsoft says the bug affects WPF applications running on current Windows 10 and Windows 11 clients and on Windows Server editions from 2012 through 2025.
- The company acknowledged the issue, is investigating, and published a temporary AppContext workaround that restores printing but removes protections added by the August security update.
- Reporters and testers found other fonts that trigger the error (for example Cambria, Constantia, Corbel) and identified safer temporary alternatives such as Arial, Times New Roman, Verdana, Tahoma, Segoe UI, or Calibri Light.
- Because WPF and Calibri are widely used in business documents, the regression can disrupt workflows and force organizations to choose between restored printing and reduced security while Microsoft develops a permanent fix.