Affected Systems
.NET Framework cumulative updates released August 2026 Patch Tuesday. Affects Windows Presentation Foundation (WPF) applications on Windows 10, Windows 11, Windows Server 2012 through Windows Server 2025. Applications using certain fonts (including Calibri) for printing or PDF/XPS generation fail with System.IO.FileFormatException.
Exploitation Status
No exploitation. This is a regression bug introduced by security patches, not a vulnerability. The underlying security fixes addressed unspecified vulnerabilities; no CVE or exploitation details provided for those.
Business Impact
WPF applications cannot print or generate PDF/XPS documents when using affected fonts. Impacts line-of-business desktop applications, reporting tools, and document workflows. Microsoft's workaround disables security protections introduced in August 2026, creating a choice between functionality and exposure to patched vulnerabilities. No permanent fix timeline provided.
Urgency
🟠Within 24 hours
Recommended Actions
- Inventory WPF-based applications in your environment and test printing and PDF/XPS export functionality after August 2026 .NET Framework updates
- For critical business applications affected, apply the AppContext switch workaround (Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection=true) only on isolated systems with compensating controls
- Consider rolling back August 2026 .NET Framework updates on systems running mission-critical WPF applications until Microsoft releases a permanent fix, weighing security risk against operational impact
- Monitor Microsoft's Windows release health dashboard and .NET Framework update channels for a corrective patch
- Document all systems where the workaround is applied and prioritize them for immediate patching once Microsoft ships a fix