August 2026 .NET Update Breaks Printing, PDF Export in Some Apps

If one of your Windows business apps started throwing an error every time someone clicks Print or saves to PDF, while everything else prints fine, hold off on calling the copier technician. Microsoft has confirmed that its August 2026 .NET Framework update can break printing and PDF export in some apps, and the trigger is as ordinary as the font on the page.

Here's what happened, how to tell whether it's hitting your office, and why Microsoft's quick workaround comes with a security catch.

What happened with the August .NET Framework update?

Microsoft released its August 2026 Patch Tuesday updates, including a cumulative update for .NET Framework, on Tuesday, August 11, 2026, as its release notes show and as Qualys and Windows Latest reported.

Microsoft has since confirmed a known issue with that update: some Windows Presentation Foundation (WPF) apps may fail with a System.IO.FileFormatException when printing or generating PDF/XPS content that uses certain fonts, including Calibri. BleepingComputer, PCWorld and BetaNews all reported on the breakage.

In plain English, WPF is a Microsoft technology for building Windows desktop apps. When an affected app tries to print a document, or turn it into a PDF or XPS file, and that document uses one of the problem fonts, the job can fail with an error instead of producing a page.

Note Microsoft's exact wording: "certain fonts, including Calibri." Calibri is the one it names, so don't assume it's the only one.

Which Windows versions and updates are involved?

Affected platforms span Windows 10, Windows 11 and Windows Server 2012 through Windows Server 2025, according to Microsoft, BleepingComputer and Cybersecurity News. That covers staff PCs as well as the servers some offices use to run shared business apps.

The August .NET update ships as different packages for different versions of Windows. Relevant ones include:

Neowin and BleepingComputer reported the same KB numbers. Other Windows versions get their own packages, so match each machine against Microsoft's release notes rather than hunting for a single KB number.

Is it the printer or the app?

If the pattern below fits, it's the app. Because Microsoft's description covers generating PDF/XPS content as well as printing, a failed save to PDF from the same app is a strong clue: no printer is involved in that step at all.

Quick triage checklist:

• The error appears in one business app, such as a billing, reporting, practice-management or tax program, not across the board.
• Word, a web browser and a Windows test page still print normally to the same printer.
• The same app also fails when exporting or saving to PDF.
• The error message mentions FileFormatException.
• The problem started after the August updates were installed.

If most of those match, reinstalling printer drivers or swapping the printer is unlikely to help. If they don't match, you may have a genuine driver or connectivity problem, and our guides to printer driver management and network printer setup are a better starting point.

What is Microsoft's workaround, and what's the catch?

Microsoft's interim workaround is an AppContext switch added to the affected app's configuration file. Its name is a mouthful, Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection, but the important part is what Microsoft says about it: the switch disables security protections introduced in the August 2026 update, and Microsoft recommends using it only temporarily and only when required. Wccftech and Cybersecurity News flagged the same trade-off.

Put simply, the switch turns off, for that app, security protections that the August update added. You get printing back; that app gives up the protection the update was meant to add.

That can still be the right call for an office that can't bill clients or close the month without the app. The point is to make it a deliberate, documented decision rather than a quick fix nobody remembers:

• Apply it only to the specific app that's failing, not to every program "just in case."
• Have your IT provider or software vendor make the change, and record where and when it was made.
• Set a reminder to remove the switch once a proper fix is installed.
• Don't treat uninstalling the August security update as a shortcut. Removing the update removes its protections too, which is the same trade-off on a bigger scale.

What this means for San Fernando Valley offices

The apps that could be at stake are the everyday tools Valley businesses rely on: billing and practice-management software at law firms and medical and dental offices, tax and accounting programs, and reporting tools at real estate brokerages, production companies and warehouses. Not all of them are built on WPF, so the only reliable way to know whether yours is affected is to ask the vendor.

The bigger risk for small offices without in-house IT is misdiagnosis. A failed print job looks like a printer problem, so it's tempting to call the copier dealer first, but a dealer can't fix a .NET Framework issue. The other trap is a well-meaning helper who "fixes" it by removing security updates.

If your business app runs on an office server rather than on each PC, check the server as well, since Windows Server versions are on the affected list. We help offices across the San Fernando Valley communities we serve sort out exactly this kind of "is it the printer?" question.

What to do now

1. Confirm the pattern: try printing and saving to PDF from the failing app, then print from Word or a browser to the same printer.
2. Write down the exact error text, looking for FileFormatException, and note which fonts the failing document or template uses.
3. Check each affected PC's and server's Windows Update history for the August .NET Framework update, such as KB5120708, KB5120710 or KB5120705.
4. Contact your software vendor, ask whether the app is WPF-based and affected, and get their recommended fix in writing.
5. If your vendor or IT provider recommends Microsoft's workaround, limit it to the affected app, document it, and schedule its removal.
6. Keep the August security updates installed, and read the release notes for each new .NET Framework update to see whether this issue is addressed.
7. Going forward, install updates on one test PC first and print a sample invoice and PDF before rolling them out office-wide.

Not sure whether it's the printer, the driver or the app? Book a free virtual consultation and we'll help you figure out which one actually needs fixing.

Update (September 30, 2026)

Microsoft says this issue is now fixed. According to Microsoft's .NET Framework release notes, the printing and PDF-export failure "is resolved in the September 2026 .NET Framework cumulative update and later updates." Microsoft's September release notes show that update was released on September 8, 2026; on Windows 11 24H2 and 25H2 and Windows Server 2025, the package is KB5126052.

The fix comes from Microsoft's own documentation, and we haven't seen independent reports confirming it yet, so test before you rely on it. Install the latest .NET Framework update, confirm that printing and PDF export work in the affected app, and then remove the AppContext switch if your office applied it, so the August security protections are turned back on. If the app still can't print or preview after updating, go back to your software vendor and check Microsoft's release notes for any other known issues.