# Why OS spoofing fails on fonts: a Mac makes Mac profiles > Why OS spoofing fails on fonts: they come from the computer, not the claim. A Linux profile made on our Mac showed 1 font where macOS showed 159. Source: https://mosaicbrowser.com/blog/a-mac-makes-mac-profiles ## Why OS spoofing fails on fonts: a Mac makes Mac profiles Why OS spoofing fails on fonts: they come from the computer, not the claim. A Linux profile made on our Mac showed 1 font where macOS showed 159. ![159: Same Mac, two claims: 159 fonts against 1](https://mosaicbrowser.com/covers/a-mac-makes-mac-profiles.webp) OS spoofing fails on fonts because a browser can change what it says about the operating system, but not the fonts that system has installed or how it draws them. On our Mac, with our own engine, a profile claiming macOS was read by Pixelscan as consistent with 159 fonts found. The same Mac claiming Linux was read as `Masking detected`, with 1 font found. ## What a page measures when it looks at fonts A fingerprinting script does not ask the browser for a font list. It draws a short string in a font family it wants to test, then draws the same string in a generic fallback, and compares the widths. If they differ, the font is installed. Repeat that for a few hundred family names and the page has a list, built from the computer's own font files. It also gets more than names. The widths come out of each font's own metrics, and the pixels, if the script draws onto a canvas, come out of the platform's own text renderer, with its own way of smoothing edges. So there are three layers a page can read: which fonts exist, how wide they set, and how they are drawn. A User-Agent string touches none of them. Pixelscan probes the fonts of the platform the browser claims. When a Mac claims Linux, almost none of the Linux families are there, and the count falls to 1. That single number is the whole verdict. ## Why OS spoofing fails on fonts, even inside the engine Tesseract Chrome, our engine, changes fingerprint values in C++ through a set of `--fingerprint-*` switches: the GPU renderer string, the screen, the thread count, the timezone, the language. Each of those is a value the browser reports, so the engine can report a different one. [Why we built our own Chromium](https://mosaicbrowser.com/blog/why-we-built-our-own-chromium) explains why that has to happen inside the engine rather than in page JavaScript. Fonts are not a reported value. They are files on disk and a renderer in the operating system, and the page measures the result. No switch in the engine reaches either. We could make the browser call itself Windows in every header and every `navigator` field, and the text it drew would still be a Mac's text. Mosaic is a desktop app Email yourself the link and open it on your computer. [How it works: Machines that exist](https://mosaicbrowser.com/features/real-fingerprints) ## What we measured On the shipped engine, this Mac as the host, one fresh profile per row and no proxy on any of them, run on Pixelscan: ``` claimed macOS (host) consistent, no masking twice claimed Linux inconsistent, masking detected claimed Windows still "scanning…" after 90 s three times ``` The Windows rows were worse than a fail. A scan that never finishes is a page that could not make sense of the browser at all. An earlier engine gave the same answer on IPHey, one fresh profile each: claiming macOS scored 100 of 100, Trustworthy, and claiming Windows scored 80, Unreliable. ![A profile fingerprint in Mosaic: GPU, screen, fonts, timezone and language matched to the exit](https://mosaicbrowser.com/shots/fingerprint-detail.png) *A profile that claims the host's own OS* ## A warning is not a door For a long time our audit warned about a profile claiming another desktop OS, and that was all it did. The window showed the warning. The REST API and MCP did not: a caller asking for `os: "windows"` got a 200, a Windows User-Agent and no word that the browser underneath was a Mac. A rule that only one surface shows is a rule the other two never see. So we moved the check into the generator, the one function all three surfaces go through, and until November 2026 Mosaic refused to create a desktop profile for a platform the computer was not running. On a Mac that meant macOS and Android, and nothing else. ## What changed on 3 November 2026 We stopped refusing, and kept the honesty. A Mac can now make Windows and Linux profiles. Each one is drawn for the OS it claims, with that platform's GPU, screen and threads, because a Windows User-Agent beside an Apple chip would be a louder contradiction than the fonts. And the editor says the cost once, where the choice is made: > We can't promise this matches as well as macOS; fonts and rasterisation come from this computer, so a consistency scanner can read the difference. Library health counts such profiles as information rather than a fault. Repair hardware leaves the claimed OS alone now: a Windows profile you chose on purpose stays Windows. If you would rather one matched this computer, pick macOS for it in the editor and the hardware is drawn again to fit; its cookies stay where they are. ![Creating a profile in Mosaic: a checked proxy, and the hardware and locale drawn to match its exit](https://mosaicbrowser.com/shots/profile-editor.png) *The editor, where the platform is chosen* ## Android is the measured exception Mobile profiles were never refused, and fonts are the reason. A phone's font set is small and aliased, and a desktop can actually produce it. Eight detectors were measured passing on an Android profile made on this same Mac. How the rest of a profile is drawn from real machines is on [real fingerprints](https://mosaicbrowser.com/features/real-fingerprints). So the sentence we use now is the measured one: Mosaic makes profiles for any desktop OS or Android, and the strongest match is always the OS your own computer runs. If you are choosing a browser for this, [what an anti-detect browser is](https://mosaicbrowser.com/guides/what-is-an-anti-detect-browser) covers what any of them can and cannot change, and [pricing](https://mosaicbrowser.com/pricing) has the plans. ## Questions ### Why does OS spoofing fail on fonts? The browser can say any operating system in its User-Agent, but the fonts installed, their metrics and the way glyphs are drawn come from the computer underneath. A page that measures text finds the real system's fonts, and they disagree with the claimed one. ### Can a browser switch change which fonts a page finds? Not on our engine. Tesseract Chrome has switches for the GPU string, the screen, threads, timezone and more, and none of them reaches the font files on disk or the platform's text renderer. ### Can Mosaic make a Windows profile on a Mac? Yes, since 3 November 2026. The profile gets Windows hardware drawn to fit, and the editor says plainly that it can't promise it matches as well as macOS, because fonts and rasterisation come from the Mac. ### Why do Android profiles on a Mac work better than Windows ones? A phone's font set is small, and it is one a desktop can actually produce. Eight detectors were measured passing on an Android profile made on our Mac; a Windows claim on the same Mac never finished Pixelscan's scan. ## Related [Machines that exist: fingerprints drawn from real hardware](https://mosaicbrowser.com/features/real-fingerprints): Mosaic draws each profile's GPU, screen, CPU, memory and fonts from one real device, so the parts of a fingerprint agree with each other and with the host. [Why we built our own Chromium instead of injecting JavaScript](https://mosaicbrowser.com/blog/why-we-built-our-own-chromium): Mosaic changes fingerprint values in C++ inside its own engine. Three measured differences between Chrome and plain Chromium explain why that matters. [What is an anti-detect browser?](https://mosaicbrowser.com/guides/what-is-an-anti-detect-browser): An anti-detect browser gives each profile its own fingerprint, proxy and cookies, so sites see separate visitors. What it changes, how, and its limits. [Introducing Mosaic, an anti-detect browser that lives on your machine](https://mosaicbrowser.com/blog/introducing-mosaic): Mosaic runs its own Chromium, draws each profile from a real machine and matches it to the proxy exit. Profiles stay on disk. One plan, no per-profile fees. [A detector that never answered because its tab was in the background](https://mosaicbrowser.com/blog/requestanimationframe-background-tab): Background tab throttling stops requestAnimationFrame. Our test tab was in the background, two detectors never answered, and Page.bringToFront fixed both. [When the FingerprintJS demo can't name your browser](https://mosaicbrowser.com/blog/fingerprintjs-cannot-name-browser): FingerprintJS read 'Chromium-Based Browser Not Available' for our engine. We blamed the version; the real variable was Google's branding. [A Russian exit and a profile speaking Mexican Spanish](https://mosaicbrowser.com/blog/accept-language-and-proxy-country): A profile matched to a Russian exit kept sending es-MX in Accept-Language. Leaving the language alone was right. Saying nothing about it was the bug. [IPHey's "(whale)" signal, and what it was really about](https://mosaicbrowser.com/blog/iphey-whale-signal): IPHey's inconsistent browser fingerprint (whale) looked like a version problem. We followed it through branding and JavaScript to a Widevine certificate. ## Mosaic, on your own computer The browser this blog is about: its own Chromium, fingerprints from real machines and a proxy exit per profile. Free holds 3 profiles. Pro is $49 a month for any number. Mosaic is a desktop app Email yourself the link and open it on your computer.