GoKawiilMicrosoft has confirmed that .NET Framework updates released as part of the August 2026 Patch Tuesday are breaking printing and PDF export in some applications.
In a Windows release health alert seen by BleepingComputer, Microsoft says this known issue affects only apps that use the Windows Presentation Foundation (WPF) UI framework, an open-source graphical subsystem for building Windows desktop client applications.
"After installing the August 2026 .NET Framework cumulative update, some WPF applications may fail with a System.IO.FileFormatException when printing or generating PDF/XPS content that uses certain fonts, including Calibri," Microsoft says.
The complete list of impacted platforms includes both Windows client releases (including the latest versions of Windows 10 and Windows 11) and Windows Server (from Windows Server 2012 up to Windows Server 2025).
Microsoft says it is still investigating the issue and, until it can ship a permanent solution, has provided a temporary fix to help affected users work around these printing problems.
This workaround requires enabling the Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection AppContext switch in the application config file by adding the following:
<configuration> <runtime> <AppContextSwitchOverrides value="Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection=true"/> </runtime> </configuration>
However, the company warned that doing this will also disable protections introduced with the August 2026 .NET Framework update, exposing the system to attacks that could exploit vulnerabilities addressed by this month's security updates.
"Microsoft recommends using this workaround only as a temporary measure and only when required to address this issue," it warned.
Roughly five years ago, in February 2021, Microsoft addressed another known issue that caused WPF apps and Visual Studio to crash after installing Windows 10 cumulative updates.
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