.NET Framework August update breaks WPF printing and PDF export on Windows 11 (August 24, 2026)
Chapters
What Microsoft confirmed
Microsoft has confirmed that its August 2026 .NET Framework cumulative update breaks printing in WPF applications. The wording is specific: "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."
Calibri is named among the affected fonts. An application that cannot export a PDF is not a cosmetic problem: it is an invoice run, a report batch or a statement print that stops.
The confirmation names the August 2026 .NET Framework cumulative update rather than a per-platform KB number, so I am not going to guess which KB landed on your build. What is on the record as of August 24, 2026 is the fault, the exception, the font, the platform list and a workaround with a cost.
TL;DR
The August 2026 .NET Framework cumulative update stops WPF applications printing and exporting PDF or XPS files on Windows 11 and Windows 10, and Microsoft's confirmation of it was published on August 24, 2026 by BleepingComputer. Affected applications throw System.IO.FileFormatException on certain fonts, with Calibri named. Microsoft says it is "still investigating the issue" and offers one workaround, an AppContext switch that also turns off the security protections the same update added.
- Microsoft's wording: "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."
- Calibri is named among the affected fonts.
- The scope runs from Windows 10 and Windows 11 to Windows Server 2012 through Windows Server 2025.
- The only workaround is the AppContext switch Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection, set per application, which Microsoft says also disables the protections the August update introduced.
- No fix had shipped as of August 24, 2026, and Microsoft says it is still investigating.
Where the fault sits
The exception is raised in the print and document export path, on fonts that path has to subset. WPF builds a reduced copy of a TrueType font when it prints or writes PDF or XPS output, and that is where the file-format error comes from. The name of Microsoft's own workaround switch, Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection, points at the same place.
The scope is wider than a desktop problem. Windows Server 2012 through Windows Server 2025 are all in range alongside Windows 10 and Windows 11, so unattended document generation is in scope, not only someone pressing Ctrl and P.
The workaround, and its price
Microsoft's workaround is an AppContext switch, set in the configuration file of the application that fails: Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection. Setting it restores printing in that application.
It also undoes part of the update. Microsoft says the switch "will also disable protections introduced with the August 2026 .NET Framework update", and tells administrators to use it only temporarily. So this is not a choice between broken and fixed. It is a choice between an application that cannot print and a machine missing the hardening the August update added.
Two things follow. Set it per application rather than machine-wide, so only the process that needs it loses the protection. And record where it is set, because a temporary switch nobody wrote down becomes a permanent one.
| Item | Detail |
|---|---|
| Exception | System.IO.FileFormatException |
| Where it appears | WPF printing and PDF or XPS export |
| Fonts | certain fonts, Calibri named by Microsoft |
| Client platforms | Windows 10 and Windows 11 |
| Server platforms | Windows Server 2012 through Windows Server 2025 |
| Microsoft status on August 24, 2026 | still investigating |
| Fix | none shipped |
| Workaround | AppContext switch, per application, disables August hardening |
The second open fault from the same Patch Tuesday
This is not the only thing from August still open. As of August 24, 2026 Microsoft's known issues list for Windows 11, version 25H2 (Microsoft Learn) still carried the KB5121003 game crashes tied to RGB-lighting drivers as unresolved.
I mention it because the two faults pull in opposite directions. The game crashes have a workaround that removes a third-party driver. The printing fault has a workaround that removes Microsoft's own security hardening. Only one of those is comfortable to leave in place for a month.
What you can do today
If printing or PDF export broke in an application this month, work in this order.
1. Confirm the exception. The type to look for is System.IO.FileFormatException, raised while printing or while generating PDF or XPS output. Application logs and crash dialogs usually carry the type name.
2. Test with a different font. Calibri is the font named in Microsoft's confirmation, so re-exporting the same document in another font is the quickest way to tell this apart from a printer or driver problem.
3. Try the same print from an application that is not WPF. Microsoft's confirmation covers WPF applications, so printing the document from something else is often cheaper than changing any settings.
4. Only if a business process is stopped, set Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection in that one application's configuration file, and nowhere else.
5. Book the removal now. Microsoft says the switch is temporary and that it disables protections the August 2026 .NET Framework update introduced. Put a date in the calendar to check for a fix and take the switch back out.
6. Do not uninstall the .NET Framework update as a reflex. It closed vulnerabilities, and the route Microsoft published is the per-application switch, not removal.
If you want the affected applications found and the switch tracked without doing it yourself, Get XLAnt.
Frequently asked questions
Should I uninstall the August .NET Framework update instead?
Microsoft's published route is the per-application AppContext switch, not removal. The update closed vulnerabilities, and the switch disables only the protections that update introduced, in the one application you set it for. Removing the update takes those protections off the whole machine.