"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.
What Microsoft has reported
Microsoft confirmed in a Windows release health alert, seen by BleepingComputer, that updates shipped as part of the August 2026 Patch Tuesday are causing printing and PDF export failures in some desktop applications. The company narrowed the problem to applications built on the Windows Presentation Foundation (WPF) UI framework, describing WPF as an open-source graphical subsystem for building Windows desktop client applications.
Platforms and error details
According to Microsoft, the failure presents as a System.IO.FileFormatException during printing or when generating PDF/XPS content that uses certain fonts, with Calibri explicitly named. The company listed the impacted platforms as both Windows client releases — including the latest versions of Windows 10 and Windows 11 — and Windows Server editions from Windows Server 2012 up to Windows Server 2025.
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See what we buildThe temporary workaround: AppContext switch
While Microsoft continues to investigate, it has supplied a temporary workaround that requires changing the application configuration. The vendor instructs affected applications to enable the AppContext switch Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection by adding the following to the application config file:
<configuration> <runtime> <AppContextSwitchOverrides value="Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection=true"/> </runtime> </configuration>
Microsoft framed this as a stopgap intended to restore printing and PDF/XPS generation functionality until a permanent fix can be shipped.
Security trade-off and Microsoft’s recommendation
Microsoft warned that enabling the workaround will also disable protections that were introduced with the August 2026 .NET Framework update. The company explicitly said this exposes systems 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," the alert stated.
How technologists, procurement leaders, and end users will respond
- Technologists and security teams: Expect to weigh immediate application availability against increased exposure. Microsoft’s advisory leaves the fix space constrained to either applying the AppContext workaround — which restores printing but removes new protections — or waiting for a permanent patch from Microsoft while accepting interrupted printing and PDF/XPS workflows.
- Procurement and enterprise application owners: Organizations running WPF-based line-of-business applications will need to inventory affected builds, determine which apps rely on fonts Microsoft identified (including Calibri), and decide whether to authorize configuration changes that weaken the recent security protections.
- End users and desktop application owners: Users experiencing print or PDF export failures in affected WPF applications should expect guidance from their IT teams. Microsoft’s alert names the problem and the workaround but frames the change as temporary and cautions that it increases vulnerability to exploitation.
Microsoft also noted a separate, contemporaneous issue affecting Windows 11 games following the same Patch Tuesday: the company shared a temporary workaround for a known issue that was causing games such as ARC Raiders, MARVEL Tōkon: Fighting Souls, and The Finals to crash and freeze. In addition, Microsoft referenced an earlier, similar incident from February 2021 when a known issue caused WPF apps and Visual Studio to crash after Windows 10 cumulative updates — underscoring that updates to platform components can sometimes produce regressions in desktop application behaviors.
Microsoft is still investigating the August 2026 .NET Framework printing issue and has left the immediate decision to administrators: restore functionality by relaxing the new font protections for affected applications, or wait for a permanent update and accept the disruption. The company’s guidance is clear — the workaround is temporary, comes with a security cost, and should be used only when required.
