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ProTip: If you enabled resistFingerprinting, you have to manually allow HTML5 canvas usage per website (mastodon.social)

submitted 7 months ago by [email protected] to c/[email protected]

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My apologies to the Bugzilla team for wasting their time holding my hand on this one. Would have honestly never noticed the little "HTML5" info icon to the left of the URL bar though without their help.

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[–] [email protected] 7 points 7 months ago (2 children)

How do u do fingerprinting with a canvas

[–] [email protected] 30 points 7 months ago (1 children)

https://en.m.wikipedia.org/wiki/Canvas_fingerprinting

When a user visits a page, the fingerprinting script first draws text with the font and size of its choice and adds background colors (1). Next, the script calls Canvas API’s ToDataURL method to get the canvas pixel data in dataURL format (2), which is basically a Base64 encoded representation of the binary pixel data. Finally, the script takes the hash of the text-encoded pixel data (3), which serves as the fingerprint ...

Variations in which the graphics processing unit (GPU), or the graphics driver, is installed may cause the fingerprint variation. The fingerprint can be stored and shared with advertising partners to identify users when they visit affiliated websites. A profile can be created from the user's browsing activity, allowing advertisers to target advertise to the user's inferred demographics and preferences.

By January 2022, the concept was extended to fingerprinting performance characteristics of the graphics hardware, called DrawnApart by the researchers.

[–] [email protected] 8 points 7 months ago* (last edited 7 months ago) (1 children)

Hmm seems like their would be ways to mitigate this. Why not just introduce some random human undetectable changes to every pixel ie change breightness color alpha etc by 1 or something so every time u call the ToDataURL it returns different data? Might break some things but since canvases dont seem to be predictable systems anyway whats the harm?

[–] [email protected] 14 points 7 months ago* (last edited 7 months ago)

Yes, that is one method to resist canvas fingerprinting, but only resist it. Notice the portion about GPU performance testing, that would work even if the data was fuzzed. The best method seems to be to return fake data that changes frequently, but even that is contested.

Here's a good document with other methods (but not focusing on just canvas fingerprinting): https://2019.www.torproject.org/projects/torbrowser/design/#fingerprinting-linkability

For what it's worth, Firefox does protect against canvas fingerprinting by default it seems. I am not exactly sure how, however.

[–] [email protected] 5 points 7 months ago

Different graphics stacks (graphics hardware, their drivers (of which there are different ones for all major OSes), the display server (on Linux)) draw things slightly differently. As I remember this especially applies to text rendering using different fonts, but slightly tilted lines, bent lines, and color blending is probably also part of it, and more.