On August 11, Microsoft shipped its monthly .NET Framework security updates. Within a day, businesses were reporting that printing and PDF export had stopped working inside programs that had run perfectly the day before.
The failure is easy to misread. An application looks healthy, opens files, renders text on screen, then throws a System.IO.FileFormatException the moment someone clicks print or exports a document. Because the error surfaces at the print button, the printer, the driver and the spooler get blamed first. None of them are involved.
What broke is font handling inside Windows Presentation Foundation, the framework behind much business desktop software. August’s update hardened the bounds checking in WPF’s native TrueType subsetting code, which rewrites font data whenever a document is printed or serialized to XPS. The new validation is stricter than the fonts it has to read. In several stock Windows fonts the legacy kern table carries a length value that has wrapped, and rather than skipping that one optional table, the check now rejects the entire font.
The fonts involved are not exotic. According to the tracking issue on the WPF repository, fifteen stock faces trigger it, covering Calibri, Cambria, Constantia and Corbel. None of these are unusual choices in a business document. Fonts that still worked in third party testing include Arial, Segoe UI and Times New Roman.
The updates involved depend on the Windows version. KB5120708 covers Windows 11 version 25H2 and the matching server release, KB5120710 covers Windows 11 version 24H2, and KB5120705 covers Windows Server 2022, with separate packages including KB5120701 and KB5120709 for Windows 10. Microsoft’s release health information describes the problem as reaching Windows 10, Windows 11 and Windows Server releases from 2012 through 2025, although the support pages for the Windows 10 packages still say Microsoft is not aware of any issues in them. That gap has not been explained.
Microsoft acknowledged the known issue on August 24, thirteen days after the update shipped, and says it is still investigating. No fix date has been announced. The company has documented a workaround for developers and administrators: an AppContext switch, set in an application’s configuration file, that turns the new protection off for that program only. Microsoft states plainly that doing so disables protections introduced in the August update and recommends it strictly as a temporary measure, which is a fair warning given that the same package carries several .NET security fixes.
If your printing broke this month
Start by ruling out the hardware, because the instinct to reinstall drivers will cost you an afternoon and fix nothing. If the failure appeared after mid August, affects one specific program rather than everything on the machine, and mentions a file format error, this is the likely cause. It is the second Windows servicing bug to hit users this month, after a Windows 11 bug that broke IE Mode in Edge.
From there, the safest thing an ordinary user can do is change the font in the document or template that fails. Moving from Calibri to Arial or Times New Roman avoids the broken code path entirely and keeps the security update installed, which uninstalling the update does not.
Removing the update is the option to resist. It restores printing by removing that whole set of fixes, and the configuration switch belongs to whoever builds and supports the software, not to the person trying to print an invoice. Report the failure to the vendor, and note that Microsoft says a permanent fix is in progress, with the known issues section of the support page as the place it will appear.
Until then the sensible position is an unglamorous one. Keep the update, change the font, and let the vendor handle the configuration file. Our Windows and operating system coverage carries the rest of this month’s Windows changes.
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