Formate eVo Printing can be affected by MS Update KB5120708

Windows Update breaks WPF TrueType font subsetting (Calibri.ttf format specification error) KB5120708

What’s the Issue?

This is a troublesome Windows/.NET issue, particularly if you have business applications that use WPF for printing or PDF generation (for example: Cystal Reports and other reporting systems, ERP systems creating PDFs using WPF/XPS generation, Formate eVo, etc).

In simple terms

The August 2026 Windows update KB5120708 changed part of the .NET/WPF code that handles TrueType fonts.

The problem is that the updated code has become more draconian about the internal structure of some fonts. Unfortunately, some perfectly legitimate Windows fonts, especially Calibri contain an old font table structure that the new code doesn't like.

So, Windows effectively says:

"This Calibri font doesn't conform to the expected format."

even though the Calibri font isn't actually broken. It still displays normally in Word, Windows, browsers etc.

What is "font subsetting"?

This is the important bit.

Suppose your application is producing a printable document containing:

Invoice No. 12345

It doesn't necessarily need to embed the entire Calibri font, which can be quite large. Instead, WPF can create a subset containing only the characters/glyphs actually used:

I n v o i c e N 1 2 3 4 5

That smaller font is then embedded in the XPS document.

This is called TrueType font subsetting.

The WPF process looks roughly like:

Your application → WPF → XPS → font subsetting → printer/PDF

KB5120708 has introduced a problem in the font-subsetting stage.

Why does this cause printing to fail?

This is the crux of the issue:

Your application may be perfectly capable of:

• loading Calibri

• displaying Calibri

• measuring Calibri

• putting Calibri on the screen

But when it tries to print or generate XPS, WPF attempts to create a subset of the font.

That calls:

GlyphTypeface.ComputeSubset()

and that's where the exception occurs.

The resulting error is something like:

Calibri.ttf does not conform to the expected file format specification

Microsoft has confirmed that the problem is specifically in the WPF TrueType font-subsetting path.

Why Calibri?

Calibri is particularly affected because of an old kern table inside the font.

The kern table contains information about kerning — how the spacing between particular pairs of letters should be adjusted.

The affected Microsoft fonts appear to contain a legacy kern structure whose length information doesn't fit neatly into the way the newer WPF code validates the font. The new code rejects it rather than simply ignoring the problematic optional table.

The currently reported affected Microsoft fonts include:

• Calibri

• Cambria

• Constantia

• Corbel

Interestingly, fonts such as Arial, Times New Roman, Segoe UI, Verdana and Consolas appear to work. Calibri Light also appears to work, despite being part of the Calibri family.

Why is this suddenly happening?

This is the important distinction:

The Windows Update hasn't necessarily changed your Calibri font.

Instead:

Before KB5120708

Calibri.ttf → WPF subsetter → works

After KB5120708

Calibri.ttf → newer WPF subsetter → rejects font → exception

Microsoft/community testing has demonstrated that the same font can work before the update and fail afterwards.

Who is likely to be affected?

This is particularly relevant to business software built using WPF/.NET.

For example:

• WPF applications

• Crystal Reports applications

• Formate eVo

• applications using PrintDialog

• XPS generation

• PDF generation that goes through WPF/XPS

• reporting systems

• document-generation software

• applications using GlyphTypeface

• older .NET Framework 4.8 applications

One of the reports Microsoft is investigating involves Crystal Reports, but Crystal isn't actually the underlying problem. It is simply one of the applications that eventually calls the WPF font-subsetting functionality.

And importantly, this isn't necessarily restricted to .NET Framework 4.8. There are reports of the same behaviour affecting applications targeting newer .NET versions where the relevant WPF code is involved.

I’m a Formate eVo user, what can you do?

There are currently three main options.

• Change Fonts used by the Formate eVo form types

• Wait for an MS fix to be released

• Roll back the update causing the problem

1. Change the font

The easiest workaround is to replace Calibri with something that still subsets correctly, such as:

Arial, Segoe UI, Times New Roman, Verdana or Consolas.

Obviously, this isn't a two minute job if you have hundreds of form templates that currently use Calibri, but it's a solid fix.

2. Wait for an MS fix to be released

At this stage, no timeline for an MS fix is available, so the other options will need to be considered if you have this problem.

3. Uninstall KB5120708

This restores the previous WPF behaviour and has been confirmed as a workaround.

However, this is a temporary emergency measure rather than a solution, because KB5120708 is a security update.

More Information is available if you search using the link below, or check out the Formate eVo blog in Dashboard.

More information

If you have any further questions, please contact the Document Genetics support team.