Microsoft August 2023 Update Breaks PDF/XPS Generation

Key Takeaways

• Microsoft’s August 2026 .NET Framework updates are causing critical printing and PDF/XPS generation failures in Windows Presentation Foundation (WPF) applications.
• The issue specifically impacts applications using TrueType font subsetting, particularly with fonts like Calibri, leading to System.IO.FileFormatException errors.
• Affected systems include Windows 10, Windows 11, and various Windows Server versions from 2012 to 2025.
• The bug is a side effect of security hardening measures intended to patch six vulnerabilities, including RCE flaws.
• A temporary workaround exists, but it disables crucial security protections; font substitution is a safer alternative until a permanent fix is released.

Microsoft August 2026 Update Breaks PDF/XPS Generation

Microsoft has acknowledged a significant operational issue following its August 2026 cumulative updates for the .NET Framework. The updates are causing widespread failures in Windows Presentation Foundation (WPF) applications, specifically disrupting printing capabilities and the generation of PDF and XPS documents. This flaw emerges just days after a series of critical security vulnerabilities were addressed.

The problem originates from the .NET Framework updates deployed on August 11, 2026. These include KB5120710 (Win 11 24H2), KB5120708 (Win 11 25H2/Server 24H2), and KB5120705 (Server 2022). When an affected application attempts to print or create PDF/XPS content, particularly when utilizing certain fonts like Calibri, it triggers a System.IO.FileFormatException.

Technical Details of the Regression

Microsoft’s official advisory details that the issue specifically impacts WPF applications that employ TrueType font subsetting during document export or printing operations. Further investigation by the community has identified the ClearType font family, commonly used in Microsoft Office, as the primary culprit. Fonts such as Calibri, Cambria, Constantia, and Corbel are known to cause the crashes, while others like Arial, Segoe UI, and Times New Roman remain unaffected.

Developers have pinpointed the root cause to the ComputeSubset function within MS.Internal.TrueTypeSubsetter. This function, now enforcing stricter validation on font glyph tables as part of the update’s security enhancements, rejects fonts that were previously considered valid. Since these fonts function correctly in other contexts, such as Microsoft Word, the problem is isolated to the subsetting logic applied during XPS content serialization or print job submission.

Broad Impact Across Windows Ecosystems

The bug’s reach is extensive, affecting a wide range of Windows platforms, including Windows 10, Windows 11, and various Windows Server versions from 2012 through 2025. This broad impact means critical business operations could be disrupted, from generating invoices and reports using line-of-business WPF applications to third-party software like Crystal Reports, which relies on the same underlying printing mechanisms. Organizations heavily dependent on document management, accounting, or reporting platforms built on the .NET Framework are particularly vulnerable, especially given Calibri’s status as the default font in Microsoft Office and numerous enterprise templates.

This regression is an unintended consequence of genuine security enhancements. The August 2026 .NET Framework rollup addressed six vulnerabilities, including severe remote code execution flaws (CVE-2026-62886, CVE-2026-62897, CVE-2026-70354), elevation of privilege issues (CVE-2026-65810, CVE-2026-62872), and an information disclosure vulnerability (CVE-2026-62902). The tightened font validation implemented to mitigate these security gaps is precisely what now causes the subsetter to reject legitimate, Calibri-based fonts.

What You Should Do

• Evaluate the Workaround Carefully: Microsoft has provided a temporary workaround involving an AppContext switch (Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection) to restore functionality. However, this switch disables the very security protections introduced in the August update, increasing exposure to patched vulnerabilities. Implement this only as a short-term measure after thorough testing in staging environments.
• Prioritize Font Substitution: A safer alternative is to modify document templates and applications to use unaffected fonts like Arial, Times New Roman, or Segoe UI. This completely bypasses the subsetting bug without compromising security. This should be the preferred immediate action, especially for sensitive documents.
• Monitor Microsoft Advisories: Microsoft currently lists the issue as “Investigating” with no timeline for a permanent fix. Regularly check Microsoft’s release health dashboard and official support advisories for updates and the eventual release of a corrected patch.
• Test in Staging: Before deploying any workaround or implementing font changes across your production environment, always test thoroughly in a controlled staging environment to prevent further disruptions.

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