In the latest round of updates from Microsoft, the company has released its monthly Patch Tuesday security updates, accompanied by a series of additional releases, including dynamic updates and new .NET Framework versions. However, this month’s updates have not come without their share of complications. Users across all Windows platforms, including Windows 11, Windows 10, and Windows Server, are experiencing issues related to printing.
Printing Problems and Legacy App Modernization
The irony of this situation is not lost on observers, particularly given Microsoft’s recent discussions on the ease of modernizing legacy applications built on the .NET framework to ensure compatibility with contemporary Windows environments. Following the installation of the August 2026 .NET Framework cumulative updates (CUs), users have reported that some Windows Presentation Foundation (WPF) applications are failing during printing tasks or when generating PDF or XPS documents. The issue seems to be linked to specific fonts, with Calibri being notably identified as a trigger for the printing failures.
When these errors occur, affected applications may throw a System.IO.FileFormatException, indicating that the ramifications extend beyond conventional printing tasks to include any applications that generate PDF or XPS content. This broad impact underscores the significance of the issue for both users and developers alike.
Fortunately, Microsoft has stepped in with a temporary workaround while it investigates the root cause of the problem. Developers can implement a specific AppContext switch, Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection, in their application’s configuration file. This adjustment can be made using the following code:
// Insert code here for developers to use However, it is crucial to note a significant caveat associated with this workaround. By enabling this switch, developers effectively disable certain security protections that were part of the August 2026 .NET Framework update. This action could potentially expose applications to vulnerabilities that the update was designed to mitigate.
As a result, Microsoft strongly advises that this workaround be employed only as a temporary solution, utilized strictly when necessary to resolve the printing or document generation issues. At present, the company has not issued a permanent fix or provided a timeline for when one might be available. Consequently, users and developers affected by this bug will need to rely on the temporary workaround until further notice.
