August .NET Update Breaks WPF Printing

An August 2026 Patch Tuesday update is causing headaches for users of Windows Presentation Foundation (WPF) applications. Microsoft has confirmed that the .NET Framework cumulative update released this month can break printing and PDF export in some WPF apps, throwing a wrench into document workflows for affected organizations.

What's Going Wrong

According to a Windows release health alert seen by BleepingComputer, the issue stems from a new known issue in the August .NET Framework cumulative update. The problem affects only applications built on the WPF UI framework, an open-source graphical subsystem used for Windows desktop client applications.

When users attempt to print or generate PDF/XPS content that uses certain fonts, including Calibri, the affected WPF applications may fail with a System.IO.FileFormatException. This is a specific error code that indicates a problem with file format handling, not a generic crash.

Affected Systems

The known issue spans a wide range of supported platforms. Microsoft lists 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. This broad footprint means the problem could hit a significant number of enterprise environments where WPF apps are still in active use.

Temporary Workaround

Microsoft says it is still investigating the root cause and has not yet shipped a permanent fix. In the meantime, the company has provided a temporary workaround that involves enabling a specific AppContext switch in the application configuration file. The switch, named Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection, can be set by adding the following to the configuration file:

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

Security Trade-off

Microsoft warns that enabling this switch will disable protections that were introduced with the August .NET Framework update. Those protections were designed to mitigate vulnerabilities addressed in this month's security updates. By turning them off, systems could become exposed to attacks that exploit those very flaws.

"Microsoft recommends using this workaround only as a temporary measure and only when required to address this issue," the company said in its alert. The recommendation is clear: administrators should weigh the operational need for printing against the security risk of disabling protections.

Not the First Time

This is not the first time WPF applications have clashed with Windows updates. Roughly five years ago, in February 2021, Microsoft addressed another known issue that caused WPF apps and Visual Studio to crash after installing Windows 10 cumulative updates. That incident, while different in its symptoms, demonstrates a pattern of compatibility problems emerging after system updates.

Related Gaming Issue

The WPF problem is not the only issue Microsoft has had to tackle this month. On Friday, the company also shared a temporary workaround for a known issue triggered by Windows 11 updates released during the August 2026 Patch Tuesday. That issue caused games like ARC Raiders, MARVEL Tōkon: Fighting Souls, and The Finals to crash and freeze.

What to Consider

For IT administrators and developers, the immediate concern is identifying which applications are affected and whether printing or PDF export is critical to daily operations. The workaround restores functionality but at a security cost. Until Microsoft ships a permanent solution, organizations must decide whether to accept the risk or disrupt their workflows.

The timing is also awkward. Enabling the switch disables protections for vulnerabilities that attackers may already be targeting. In an environment where valid credentials are often the first step in an attack, weakening those protections could have serious consequences.

Why It Matters

This incident highlights the delicate balance between keeping systems patched and maintaining software stability. For enterprises that depend on WPF applications for document generation or customer-facing processes, the choice between a broken workflow and a potential security hole is not a comfortable one. As Microsoft continues its investigation, this could mean that some organizations will hold off on deploying the August update on critical systems until a permanent fix arrives, or they will implement the workaround with strict monitoring and compensating controls.

The fact that Microsoft has had to address WPF-specific issues in the past suggests that compatibility problems like this are not uncommon, and organizations might want to plan for similar hiccups in future update cycles.