Microsoft has confirmed that the August 2026 Patch Tuesday .NET Framework cumulative update is causing printing and PDF/XPS export failures in certain WPF‑based applications. A temporary workaround is available, but it disables newly added security protections, raising exposure risk.

  • August 2026 .NET update breaks printing/PDF in WPF apps
  • Temporary fix requires enabling an AppContext switch
  • Workaround disables security mitigations, increasing attack surface

What Went Wrong

In a Windows release health alert viewed by BleepingComputer, Microsoft disclosed that after installing the August 2026 .NET Framework cumulative update, some Windows Presentation Foundation (WPF) applications throw a System.IO.FileFormatException when printing or generating PDF/XPS content that uses certain fonts, notably Calibri.

Affected Platforms

The issue spans all current Windows client releases—including the latest builds of Windows 10 and Windows 11—as well as Windows Server editions from Server 2012 through Server 2025. Both consumer and enterprise environments are therefore at risk.

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Temporary Workaround

Microsoft’s interim fix involves adding an AppContext switch to the application’s configuration file:

<configuration>

<runtime>

<AppContextSwitchOverrides value="Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection=true"/>

</runtime>

</configuration>

Enabling this switch restores printing and PDF export functionality but also disables the security protections introduced with the August 2026 update, potentially exposing systems to exploits addressed by the same patch.

Why This Matters

BozokMedia analysis shows that enterprise‑grade desktop software relying on WPF for reporting, invoicing, or design tools can become unstable, leading to operational downtime and possible data loss. The issue is especially critical for organizations that depend on PDF/XPS generation for compliance documentation.

"When the core font rendering engine in WPF breaks after an update, it directly undermines the reliability of mission‑critical business applications," said a software security analyst.

Historical Background

Microsoft has faced similar challenges before. In February 2021, a Windows 10 cumulative update caused WPF apps and Visual Studio to crash. Earlier this month, a separate Windows 11 update triggered crashes in games such as ARC Raiders, MARVEL Tōkon, and The Finals, prompting a temporary fix.

Did You Know?: WPF was first introduced in 2006 and remains one of the oldest UI frameworks in the .NET ecosystem.

Frequently Asked Questions

Q1: Does applying the workaround increase my system’s security risk?

A: Yes, it disables the security mitigations bundled with the August .NET update, so it should be used only as a short‑term measure when absolutely necessary.

Q2: When will Microsoft release a permanent fix?

A: Microsoft has not announced a timeline yet but is actively investigating and plans to address the issue in an upcoming update.