Font enumeration leaks on Windows

Windows ships a font catalog that no clean laptop would invent from scratch. Enumeration APIs and DirectWrite both leak that catalog. A page does not need to list every face in Settings. It can call document.fonts.check, measure a hidden span, or compare offsetWidth across a probe list. The set of hits is the fingerprint.

A random bag of faces looks false. An allowlist that could exist on the OS you claimed looks like a computer.

How the probe works

Scripts keep a list of likely names. For each name they ask whether the face exists and how wide a test string draws. Widths are layout. That is why this surface sits next to client rects and subpixels. A face that exists but measures like a substitute is a tell. A face that a Windows 11 box should have, and does not, is a tell.

DirectWrite is the raster path under Chromium on Windows. The inject script can constrain document.fonts.check and the measurement fallbacks. It cannot fully rewrite every glyph the GPU already knows. When the custom Chromium core is present, launch can pass a MASKWRIGHT_FONT_LIST into a patched raster. That path is partial. When the core is missing, you have the inject only.

Why a random set fails

Operators copy a macOS list onto a Windows UA. Or they hide Calibri and keep a Windows Client Hint. Or they allow a designer catalog that a stock laptop would not have. The page does not need a rare font to catch that. It needs two APIs that disagree.

A user agent string will not save a lying catalog. Client Hints will make the lie louder if platform says Windows and the faces say otherwise.

The allowlist should sound like the family you claimed: desktop Windows, not a phone, not a Mac. MaskWright profiles are desktop families. Do not dress a Windows host in a foreign catalog and call it isolation.

Probe list versus the installed catalog

A page does not need queryLocalFonts. That API is gated and loud. The old method is enough: a list of a few hundred names, document.fonts.check, and a width comparison against a fallback face. If the width changes, the face exists.

Windows stock faces are the boring set: Segoe UI, Calibri, Cambria, Consolas, and the rest of a clean 10 or 11 install. Designer catalogs, Adobe suites, and games add hits that a stock laptop would not have. Those hits identify a machine family. Hiding them with an allowlist can be coherent. Hiding Segoe UI on a Windows UA cannot.

The raster path is the leftover risk. DirectWrite still knows faces the allowlist denied if the custom core and MASKWRIGHT_FONT_LIST are not in play. Canvas will paint what the raster has. Keep the two notes in one head even though they are two slugs.

Measurement fallbacks are the second list

When document.fonts.check is patched, probes fall back to DOM measurement. They set a face on a hidden span, compare width to a known fallback, and infer existence. That is why the inject also covers offsetWidth, offsetHeight, and getBoundingClientRect when they are used as font probes. A wrap on check alone is a half door.

I still expect the allowlist to be short. A 400-face allowlist that includes every designer font you ever installed is a machine family, not a stock Windows story.

Without the custom core, treat DirectWrite as host truth with a JavaScript allowlist on top. That is honest. It is also why a random Mac catalog on a Windows process fails twice: the inject can hide names, and the raster can still paint them.

Neighboring surfaces

Fonts leak into canvas paint. They also leak into bounding rects. AudioContext is unrelated math. WebRTC is unrelated addresses. WebRTC modes will not change a face list. hardwareConcurrency should still sound like the same class of PC that would own this catalog.

How I use the allowlist

I want a short, boring Windows set that stays put across launches. I do not want a new random catalog when I reopen the profile. I do not want faces that cannot exist on the UA.

The parent note is what browser fingerprinting actually measures. The Browser fingerprinting cluster keeps enumeration here so canvas can stay on paint. The list and the raster are related. They are not the same API.

FAQ

Is hiding Segoe UI a good idea on Windows? No. A Windows UA without Segoe UI is a cheap catch.

Does the inject rewrite DirectWrite? Only partly, and only when the custom core is present with a font list. Otherwise you have a JavaScript allowlist on host raster.

Will WebRTC modes change my font list? No. Those are address policies.