Microsoft August 2026 Update Breaks When Generating PDF/XPS Content

Microsoft has confirmed that its August 2026 .NET Framework cumulative updates are causing printing failures and PDF/XPS generation errors in Windows Presentation Foundation (WPF) applications, creating fresh headaches for enterprises just days after patching a batch of security flaws.

The issue stems from the August 2026 updates released on August 11, 2026, including KB5120710 for Windows 11 version 24H2, KB5120708 for Windows 11 version 25H2 and Microsoft server operating system 24H2, and KB5120705 for Windows Server 2022.

Affected systems throw a System.IO.FileFormatException whenever an application tries to print or generate PDF or XPS content using certain fonts, most notably Calibri.

Microsoft August 2026 Update Breaks PDF/XPS Generation

According to Microsoft’s official support advisory, 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.

Developer investigation further narrowed the root cause to the internal ComputeSubset function inside MS.Internal.TrueTypeSubsetter, which now applies stricter validation to font glyph tables as part of the security hardening shipped in this update cycle.

Because the fonts themselves are not corrupted and load correctly in other contexts such as Word, the problem is isolated to the subsetting logic used when serializing XPS content or sending jobs to a printer.

The bug 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.

This broad footprint means the issue can disrupt everyday business workflows, from generating invoices and reports through line-of-business WPF applications to third-party software such as Crystal Reports-based tools that depend on the same underlying printing pipeline.

Organizations running document management, accounting, or reporting platforms built on the .NET Framework are particularly exposed, since Calibri remains the default font in Microsoft Office and many enterprise templates.

The regression is a side effect of genuine security fixes bundled into the same update. The August 2026 .NET Framework rollup patches six vulnerabilities, 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.

Update / Component Metric	Operational Detail	Technical & Security Impact
Cumulative Update KBs	KB5120710 (Win 11 24H2), KB5120708 (Win 11 25H2/Server 24H2), KB5120705 (Server 2022)	Triggers System.IO.FileFormatException on PDF/XPS export and printing
Affected Framework	.NET Framework 3.5 and 4.8.1 / WPF Applications	Failure within MS.Internal.TrueTypeSubsetter.ComputeSubset
Problematic Fonts	Calibri, Cambria, Constantia, Corbel	ClearType font families fail TrueType glyph table validation
Unaffected Fallback Fonts	Arial, Times New Roman, Segoe UI, Verdana, Tahoma, Consolas	Successfully bypasses TrueType font subsetting errors
Patched Vulnerabilities	CVE-2026-62886, CVE-2026-62897, CVE-2026-70354, CVE-2026-65810, etc.	High-severity RCE, Elevation of Privilege, and Information Disclosure
Temporary Workaround	AppContext switch: Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection	Disables security overflow validation until permanent patch is released

While a permanent fix is still pending, Microsoft has published a workaround that developers can apply directly to an application’s configuration file.

Adding the Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection AppContext switch under the runtime section restores printing and PDF/XPS generation functionality.

However, Microsoft is explicit that this switch rolls back the very security protections the August update introduced, increasing exposure to the vulnerabilities it was designed to fix.

As a result, the company recommends treating the switch strictly as a stopgap and removing it once an official patch ships.

Administrators who prefer not to weaken security posture can instead substitute affected fonts, such as switching document templates from Calibri to Arial or Times New Roman, which sidesteps the subsetting bug entirely without touching security settings.

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 an enterprise patch management strategy 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.

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