Microsoft's August 2026 cumulative update for .NET Framework has broken printing for some WPF (Windows Presentation Foundation) applications, causing them to fail with a System.IO.FileFormatException when printing or generating PDF/XPS content that uses certain fonts, including Calibri. The issue affects Windows 10, Windows 11 and Windows Server 2012 through 2025, and is significant because Calibri remains a widely used default font in applications such as Microsoft Word, meaning the bug could hit many users carrying out routine printing tasks.

Microsoft's only current fix is a workaround involving an app config file tweak to enable a setting called Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection, which disables security protections introduced in the same update and thus increases exposure to the vulnerabilities it was meant to fix. The company says this should only be used temporarily and only when necessary, and that it is still investigating the root cause. The update released on 11 August also caused separate problems, breaking several games on Windows, an issue Microsoft has linked to RGB-lighting-related drivers such as inpoutx64, for which it has issued a registry-edit workaround.