Microsoft has confirmed that .NET Framework updates released as part of its August 2026 Patch Tuesday are causing printing and PDF export failures in certain applications. The issue specifically impacts applications that utilize the Windows Presentation Foundation (WPF) UI framework, an open-source graphical subsystem for developing Windows desktop client applications.
According to a Windows release health alert from Microsoft, affected WPF applications may encounter a `System.IO.FileFormatException` when attempting to print or generate PDF/XPS content that incorporates specific fonts, including Calibri.
The problem is not limited to a single operating system version, affecting a broad range of platforms. This includes various Windows client releases, such as the latest versions of Windows 10 and Windows 11, as well as Windows Server versions from Windows Server 2012 up to Windows Server 2025.
Microsoft is currently investigating the root cause of the issue and is working towards a permanent solution. In the interim, the company has provided a temporary workaround for users experiencing these printing difficulties.
The workaround involves enabling the `Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection` AppContext switch. This is done by adding a specific configuration entry to the application's config file: `<configuration><runtime><AppContextSwitchOverrides value="Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection=true"/></runtime></configuration>`.
However, Microsoft has issued a caution regarding this temporary fix. Implementing the workaround will disable protections that were introduced with the August 2026 .NET Framework update. This action could potentially expose systems to vulnerabilities that were addressed by this month's security updates.
Given this risk, Microsoft advises that the workaround should be considered a temporary measure only, to be used strictly when necessary to mitigate the immediate printing and PDF export problems.
This is not the first time Microsoft has addressed issues related to WPF applications following updates. Approximately five years prior, in February 2021, a known issue caused WPF apps and Visual Studio to crash after installing Windows 10 cumulative updates. More recently, Microsoft also shared a temporary workaround for a separate issue triggered by August 2026 Windows 11 updates, which caused games like ARC Raiders, MARVEL T kon: Fighting Souls, and The Finals to crash and freeze.
