August .NET Framework updates can break WPF printing and PDF export

On August 24 BleepingComputer reported that August 2026 Patch Tuesday .NET Framework updates can throw System.IO.FileFormatException when WPF apps print or export PDF or XPS with certain fonts, including Calibri. Affected systems include Windows 10 and 11 and Windows Server 2012 through 2025.

What we know

• August 2026 Patch Tuesday .NET Framework updates can throw System.IO.FileFormatException in WPF print, PDF, and XPS flows with certain fonts, including Calibri.
• The breakage is reported on Windows 10 and 11 and on Windows Server 2012 through 2025.
• The AppContext switch Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection disables protections introduced in the same update.
• BleepingComputer also noted a workaround for Windows 11 updates that crash some games.

Takeaways

• The failure shows up at print and export time, not at install.
• Calibri is enough to trip the new font-table checks.
• The documented switch turns those new protections off rather than patching the apps.

Source: BleepingComputer