Microsoft breaks WPF printing with .NET update

Having some unexpected printer problems? Microsoft's August 2026 .NET update might have broken printing on your device.

According to Microsoft, "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." Ah yes, Calibri, the default font in some versions of that little-used application, Microsoft Word.

The cumulative update for .NET Framework released on August 11 came jam-packed with fixes, but also printer problems afflicting the company's client operating systems (Windows 10 and 11) and Windows Server 2012 through 2025.

Microsoft's workaround is a tweak in the app config file to enable Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection.

However, as the company notes, doing so carries with it its own risks: "This switch disables security protections introduced in the August 2026 update and may increase exposure to the vulnerabilities addressed by that update.

"Microsoft recommends using this workaround only as a temporary measure and only when required to address this issue." Microsoft says it's still investigating the issue.

Printing can be enough of a pain without Microsoft rolling out updates to make the experience even more unpleasant.

However, the mention of "certain fonts" suggests the problem is more about how and which fonts are processed during XPS/PDF content generation by WPF applications.

August has marked a return to form for Microsoft when it comes to updates breaking systems.

The August 11 security update for Windows, for example, broke several games running on Microsoft's flagship operating system.

While the exact cause remains unclear, the Windows-maker reckons it has something to do with peripherals or internal device components with RGB lighting features.

Microsoft published a registry edit to disable the suspect driver, inpoutx64, to resolve the game crashes, although editing the Windows registry carries with it its own risks.

The company said it "is investigating the interaction between these RGB-related components and the affected games.

Additional details will be provided as they become available." As for the printing problem after the .NET update installation, the only workaround on offer is to turn off the security protections that arrived in the August 2026 update whenever the affected applications need to print on patched devices. ®