WPF Printing Broken After August 2026 .NET Framework Patch
Microsoft has confirmed that .NET Framework cumulative updates released as part of August 2026 Patch Tuesday are breaking printing and PDF export in some applications. In a Windows release health alert, Microsoft says this known issue affects only apps that use the Windows Presentation Foundation (WPF) UI framework, an open-source graphical subsystem for building Windows desktop client applications. 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. The complete list of impacted platforms includes both Windows client releases (including the latest versions of Windows 10 and Windows 11) and Windows Server (from Windows Server 2012 up to Windows Server 2025).
Technical Root Cause: ComputeSubset Function Font Validation
The bug stems from tightened font validation in the ComputeSubset function inside MS.Internal.TrueTypeSubsetter, which is part of the security hardening shipped in the August 2026 update. This internal function now applies stricter validation to font glyph tables, and as a side effect, it rejects otherwise legitimate Calibri-based fonts when WPF applications attempt to print or generate PDF or XPS content. The problem is isolated to the subsetting logic used when serializing XPS content or sending jobs to a printer — the fonts themselves load correctly in other contexts such as Microsoft Office.
Microsoft has confirmed the issue surfaces specifically in WPF applications that rely on TrueType font subsetting during printing or document export operations. Community testing has traced the bug to the ClearType font family shared with Microsoft Office, meaning Calibri, Cambria, Constantia, and Corbel all trigger the crash, while fonts like Arial, Segoe UI, Times New Roman, Verdana, Tahoma, Consolas, and even Calibri Light continue to work normally.
Affected Platforms and KB Articles
The regression is not confined to a single Windows release. Microsoft's release health alerts confirm it affects Windows 10, Windows 11, and Windows Server versions spanning Windows Server 2012 through Windows Server 2025. The full list of impacted platforms includes both Windows client releases (including the latest versions of Windows 10 and Windows 11) and Windows Server (from Windows Server 2012 up to Windows Server 2025).
Six vulnerabilities were patched in the August 2026 .NET Framework rollup, including remote code execution flaws tracked as CVE-2026-62886, CVE-2026-62897, and CVE-2026-70354, along with elevation-of-privilege issues CVE-2026-65810 and CVE-2026-62872, and an information disclosure flaw, CVE-2026-62902. The tightened font validation introduced to close these gaps is what now causes the subsetter to reject otherwise legitimate Calibri-based fonts.
Affected KB articles include:
- KB5120710 (Windows 11 version 24H2)
- KB5120708 (Windows 11 version 25H2 / Server 24H2)
- KB5120705 (Windows Server 2022)
Temporary Workaround: AppContext Switch
Until Microsoft ships a permanent fix, the company has provided a temporary workaround requiring enabling the Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection AppContext switch in the application config file. Administrators must add the following configuration to their application configuration file:
<configuration>
<runtime>
<AppContextSwitchOverrides
value="Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection=true"/>
</runtime>
</configuration>
However, Microsoft warns that doing this will also disable protections introduced with the August 2026 .NET Framework update, exposing the system 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 the printing issue.
Security Implications of the Workaround
The workaround trades functional printing capability for reduced security posture. By disabling the Cmap and Sbit overflow protection mechanisms, affected systems become more exposed to the very vulnerabilities the August update was meant to patch. Microsoft is explicit that this switch should be treated strictly as a stopgap and removed once an official patch ships.
For organizations with high security requirements, Microsoft recommends font substitution as an alternative approach. Switching document templates from Calibri to Arial or Times New Roman sidesteps the subsetting bug entirely without touching security protections. This is particularly relevant since Calibri remains the default font in Microsoft Office and many enterprise templates.
Microsoft's Status and Timeline
Microsoft's status for this known issue remains listed as "Investigating" across all three affected KB articles, with no confirmed timeline for a permanent resolution. IT teams managing enterprise patch management strategies are advised to test the AppContext workaround in staging environments before deploying it broadly, monitor Microsoft's release health dashboard for updates, and prioritize font substitution where document security matters most until Redmond ships a corrected build.
The issue was first reported on August 24, 2026, via BleepingComputer, and has since been confirmed through multiple tech publications including Daily.dev and Cybersecurity News. Until a permanent fix arrives, affected users face a choice between broken printing/PDF export and temporarily weakened security protections.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-august-updates-break-printing-pdf-export-in-wpf-apps/, https://daily.dev/posts/microsoft-august-updates-break-printing-pdf-export-in-wpf-apps-mtbuvqlmn, https://cybersecuritynews.com/microsoft-august-2026-update-pdf-xps/
