Microsoft's August .NET patch broke printing in Windows apps. Here's what to do.

Key points

• Microsoft confirmed that its August 2026 Patch Tuesday updates for .NET Framework break printing and PDF export in some Windows desktop apps.
• The bug hits apps built on Windows Presentation Foundation, Microsoft's toolkit for building Windows desktop software, when they use certain fonts including Calibri.
• Every current Windows version is affected, from Windows 10 and Windows 11 on desktops to Windows Server 2012 through Windows Server 2025.
• Microsoft's temporary workaround disables part of the very security protection the August update added, so applying it reopens the door to the attacks the patch was meant to stop.
• No permanent fix has shipped yet, and Microsoft says the investigation is ongoing.

Microsoft has confirmed that its August 2026 security update for .NET Framework, the software layer many Windows business apps run on, is breaking printing and PDF export in a chunk of desktop programs.

The failure mode is narrow but painful. Apps built with Windows Presentation Foundation (WPF), the graphical toolkit Microsoft ships for building Windows desktop software, crash with a file format error when a user hits print or tries to save a document as a PDF or XPS file. Documents using Calibri, the default Office font for years, are among those that trigger it. BleepingComputer first reported the alert Microsoft posted to its Windows release health dashboard.

Who is actually affected?

Anyone running a business app built on WPF that prints receipts, invoices or medical forms. That covers a lot of line-of-business software written in the last decade. If your accounts package or point-of-sale tool suddenly throws an error when you press print, and your IT team pushed August's Windows updates, this is very likely the cause.

Home users mostly won't notice. Office itself isn't WPF. But third-party tools built on .NET Framework are everywhere in hospitals and small businesses, and they're exactly the sort of thing nobody notices until it stops working.

Every current Windows version is on the list:

Why did a security patch break printing?

The August update added new protections inside the code that reads font files, specifically parts handling character maps and bitmap tables. That hardening is stricter than before, and it's rejecting some perfectly normal font data as if it were malformed. Calibri is one of the fonts caught in the net.

This is a familiar shape of bug. Security team tightens input validation. Real-world files that were always a bit sloppy now fail the stricter check. Users get an exception instead of a printed page. The post-mortem will note that the test matrix didn't cover enough real-world font files before shipping. We covered the broader August Patch Tuesday rollout on 11 August in our report on nearly 400 fixes, three zero-days, and a Lazarus Group kernel rootkit, so this regression lands on top of an already complicated month for Windows admins.

What is Microsoft's workaround, and is it safe?

Microsoft has published a config-file switch that disables the new font protection for a specific app. An administrator edits the app's configuration file and sets an AppContext flag called Switch.MS.Internal.TtfDelta.DisableCmapAndSbitOverflowProtection to true. Printing then works again.

The catch is significant. Flipping that switch also disables the security fix the August update introduced. Microsoft is blunt: doing this exposes the machine to the vulnerabilities this month's patches were meant to close. Use it per-app, use it only while you have to, then pull it once the real fix ships.

Microsoft hasn't given a date for the permanent update. The company also flagged a separate August issue crashing several PC games including ARC Raiders and The Finals; we reported on that investigation on 20 August.

If you rolled August's .NET update to production before validating your business apps, you already know. If you haven't rolled it yet, stage it against your printing workflows first.