Perhaps more importantly, websites can determine which fonts are installed on your system (regardless of which you're enforcing), making fingerprinting much easier.
this post was submitted on 25 Jan 2024
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Privacy
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RFP probably ensures you give out a predetermined set of fonts as installed.
Literally why is this necessary on a technical level. I'm a web developer, whenever I need my JavaScript to access a resource I have to spell out where to find it. I've never had a need to scan the browser or the system to make shit happen so why should I be able to in the first place.
First of all, you can assume the server can infer this in a number of ways - there is actually no way to fully block it, but we can try.
The main issue for privacy is that it makes your browser behave in ways that are a bit too specific (i.e. less private by comparison with the rest of the browsers in the known universe).
As for techniques the site can use
- javascript can test the geometry of something that was rendered to draw conclusions - was this font actually used? test several options and check for variations
- measure font work between network events i.e. generate a site that makes the browser use unique links for 1) fetches a font 2) renders text and 3) only then another fetch - measure the time between 1) and 3) and draw conclusions. Repeat for test cases and draw conclusions - e.g. is the browser really fast using monospace vs custom huge font? not a great method, but not completely worthless
- some techniques can actually do some of this without Javascript, provided you can generate some weird CSS/HTML that conditionally triggers a fetch
By the away not downloading the fonts also makes you "less private". Some of this is a stretch but not impossible.
Now for a more practical problem. Lots of sites use custom fonts for icons. Which means some sites will be very hard to use, because they only display buttons with an icon (actually a letter with a custom font).
FWIW these two lines are in my Firefox profile to disable downloads and skip document provided fonts:
user_pref("gfx.downloadable_fonts.enabled", false);
user_pref("browser.display.use_document_fonts", 0);
If someone has better/different settings please share.
Finally the Tor browser folks did good work on privacy protections over FF. Maybe their issue tracker is a good source of inspiration https://gitlab.torproject.org/tpo/applications/tor-browser/-/issues/18097
The websites will just think that it's your default font.