Critical Bug: August .NET Framework Updates Break WPF Printing and PDF Export

The August 2026 cumulative updates for the .NET Framework were intended to deliver crucial security and reliability improvements. However, they inadvertently introduced a severe regression affecting applications built with Windows Presentation Foundation (WPF). Specifically, WPF applications that rely on font subsetting for printing or generating PDF/XPS content are encountering a System.IO.FileFormatException.

This error typically occurs when the application tries to process certain fonts, with Calibri being prominently cited as an affected font, alongside others such as Cambria, Constantia, and Corbel. The symptom is a complete failure of printing or document export functionality, often presenting an error message indicating that a font file 'does not conform to the expected file format specification.'

The problem is not with the fonts themselves being corrupt, as Windows can still load and display them normally. Instead, the failure is triggered specifically when WPF attempts to create a subset of the font for XPS or printing, a process that was altered by the recent updates. This impacts a wide range of applications, from general business software to specialized tools that depend on accurate document output. The affected updates include, but are not limited to, KB5120708 for Windows 11 25H2 and Windows Server 2025, and KB5120705 for Windows Server 2022, as well as corresponding August .NET security updates across various Windows client and server platforms.

Microsoft has confirmed that the underlying cause of this issue is a regression within the WPF TrueType font subsetter. The specific component affected is System.Windows.Media.GlyphTypeface.ComputeSubset, which now incorrectly throws a FileFormatException when processing the aforementioned fonts. This change was a consequence of security enhancements introduced in the August 2026 updates, aimed at addressing integer overflow vulnerabilities (CVE-2026-62886 and CVE-2026-62897).

The security updates included bounds validation within the font processing logic. While intended to fortify the framework against potential exploits, this validation appears to be overly strict or misconfigured for certain legitimate font structures, leading to the erroneous `FileFormatException`. This means that fonts that were previously considered valid and functioned correctly are now being rejected by the updated WPF font subsetter. The regression affects not only stock Windows fonts but also some older third-party fonts that previously worked without issue.

The scope of affected platforms is broad, encompassing various versions of Windows client releases, including Windows 10 and Windows 11, and Windows Server editions from 2012 up to 2025. This widespread impact underscores the importance of a swift and stable resolution from Microsoft, as many enterprise applications rely heavily on these printing and export functionalities for daily operations.

Recognizing the critical nature of this bug, Microsoft has provided a temporary workaround for affected developers and users. The solution involves enabling a specific AppContext switch in the application's configuration file. Developers can mitigate the issue by adding the Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection=true setting within the <AppContextSwitchOverrides> element under the <runtime> section of their application's .config file.

Here's how the configuration snippet looks:

While this workaround can restore critical printing and PDF export functionality, it comes with a significant caveat: it disables the security protections introduced by the August 2026 .NET Framework updates. These protections were put in place to address several vulnerabilities, including remote code execution and elevation of privilege flaws (e.g., CVE-2026-62886, CVE-2026-62897, CVE-2026-70354, CVE-2026-62902).

By applying the workaround, applications may become exposed to the very security risks that the August updates aimed to mitigate. Microsoft explicitly states that this workaround should only be used as a temporary measure and only when absolutely necessary to address the printing issue. Developers and IT administrators must carefully weigh the immediate need for printing functionality against the increased security exposure. It is crucial to monitor Microsoft's official channels for updates on a permanent fix and to remove the workaround as soon as a corrected update is available. Uninstalling the entire security update is generally not recommended due to the significant security implications.