A Windows/.NET update from August 2026 broke printing and PDF export in some desktop apps.
If you cannot print PDF after update installation in a WPF app, check whether it now throws a System.IO.FileFormatException. Microsoft lists this as a known issue affecting printing and PDF/XPS creation with certain fonts.
Microsoft documents the bug in some apps built with WPF (Windows Presentation Foundation). Word, Excel, and web browsers are not part of this reported WPF issue.
Microsoft’s temporary workaround is an AppContext setting in the affected application’s configuration file. There is no single click-to-fix setting for every program, so contact the software vendor or your IT team if you do not manage the application.
Check Windows Update first. Microsoft may ship a permanent fix that removes the need for any workaround.
Only apps built with WPF, Microsoft’s desktop UI framework, are affected. If one program cannot print PDF after update installation while other apps still print normally, check whether the affected program uses WPF.
Word, Excel, most browsers, and everyday printing still work normally.
What Error You’ll See
Affected apps throw a System.IO.FileFormatException when printing or exporting to PDF or XPS.
The crash is tied to certain fonts, including Calibri.
Microsoft also documents a separate WPF failure involving an in-memory PackageStore XPS document. It can produce the same exception but has a different cause and workaround. If your error mentions a pack:// URI or PackageStore, follow the matching known-issue guidance in Microsoft’s KB instead of assuming the font switch applies.
The Root Cause
The August 2026 .NET Framework update added stricter validation for TrueType fonts.
Some fonts fail that new check, and instead of just flagging it, the print or export process crashes. This is why some affected WPF apps cannot print PDF after update installation.
Which Updates Are Involved
The applicable KB varies by Windows release and installed .NET Framework version. For Windows 11 version 25H2 and Microsoft server operating system version 24H2, Microsoft documents the issue in KB5120708. Check the August 2026 .NET update page for your exact Windows version before applying a workaround.
If you’re also chasing a different update failure, see our guide to Windows Update error 0x80073712.
Fix: Microsoft’s Official Workaround
Microsoft’s fix disables the new font-validation check that’s causing the crash.
For an app you or your IT team control, add this to the app’s .exe.config file:
<configuration><runtime><AppContextSwitchOverrides value="Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection=true"/></runtime></configuration>
This workaround turns off a security protection Microsoft added in the same update.
Use it only temporarily, and only for apps where printing or PDF export is genuinely broken.
If You Don’t Control the App’s Config
Many everyday apps — PDF readers, accounting software, line-of-business tools — don’t expose this setting to regular users.
In that case, contact the app’s vendor or your IT department. Do not edit another program’s configuration file unless its vendor provides instructions for that exact application.
Check for a Permanent Fix First
Microsoft is actively investigating this issue and may release a proper fix through Windows Update.
Before applying any workaround, open Settings > Windows Update and install the latest available update.
For other update-related errors, see our guide to diagnosing common Windows errors.
For everyday printer issues unrelated to this update, see our general printer troubleshooting guide.
Frequently Asked Questions
Does this affect Microsoft Word or Excel?
No. The bug is specific to apps built with WPF. Office apps print normally.
Is Calibri the only font affected?
It’s the most commonly reported one, but other TrueType fonts can trigger the same error.
Is the AppContext workaround safe?
It’s an official Microsoft workaround, but it removes a security check added in the same update. Use it only until a real fix ships.
Will a future update fix this automatically?
Likely. Microsoft has acknowledged the issue and is investigating a permanent fix.
What if my app has no config file I can edit?
Contact the software vendor or your IT department for guidance specific to that application. Do not copy a configuration file from another program.
Bottom Line
This bug is narrow but disruptive: only WPF apps, only certain fonts, but a real productivity blocker if it hits you.
Check Windows Update first. If the app still cannot print PDF after update installation, use Microsoft’s temporary workaround only when you manage that application directly.
