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

Key points

• Microsoft confirmed on 15 August 2026 that its August 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 switches off part of the very security protection the August update added, so turning it on 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 admitted that its August 2026 security update for .NET Framework, the software layer that many Windows business apps sit on top of, is breaking printing and PDF export in a chunk of desktop programs.

The failure mode here is narrow but painful. Apps built with Windows Presentation Foundation (WPF), which is 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 PDF or XPS. Documents using Calibri, the default font in Office for years, are among the ones 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, medical forms or quotes. That covers a lot of line-of-business software written in the last decade. If your accounts package, dispensary system, 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 will not notice. Office itself is not WPF. But third-party tools written on top of .NET Framework are everywhere in hospitals, law firms, schools and small businesses, and they are 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 the parts handling character maps and bitmap tables. That hardening is stricter than the old code, and it is rejecting some perfectly normal font data as if it were malformed. Calibri is one of the fonts caught in the net.

In practice, 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. One thing the post-mortem will say: the test matrix did not cover enough real-world font files before shipping.

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

Microsoft has published a config-file switch that turns off 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 switches off the security fix the August update introduced. Microsoft is blunt about it: doing this exposes the machine to the vulnerabilities this month's patches were meant to close. Use it per-app, use it briefly, and take it off once the real fix ships.

Microsoft has not given a date for the permanent update. The company also flagged a separate August issue that is crashing several PC games including ARC Raiders and The Finals, with its own temporary workaround.

Operational takeaway: if you rolled August's .NET update to production before validating your business apps, you already know. If you have not rolled it yet, stage it against your printing workflows first.