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Fun fact: for quite a long time Chrome skipped over the user permission step in the Web MIDI spec, always allowing access and silently giving ad networks a list of connected USB MIDI devices with no user consent:


Here's what appeared on porn site xhamster.com once newer versions of Chromium got around to implementing the permission check (SFW-ish):


Guessing it was for additional browser fingerprinting.

That seems a bit far fetched.

Not sure why that's more odd than other crazy fingerprinting techniques actually in use. Keep in mind no midi devices would need to be present for fingerprinting. Different failure modes, etc.

Especially in the porn industry where the end users are likely using incognito mode or a VPN.

I still don't understand how WebMIDI would be used for fingerprinting of the vast majority of users who don't have any MIDI devices connected to their machine.

Because thats what you want when fingerprinting....the few users who have one connected gives you probably quite and accurate fingerprint for those users.

I'm sure there are fingerprint libraries that include every possible API that the browser provides. Does MIDI provide a good fingerprint alone? Probably not, but it can serve as a few more bits of information thrown into the mix when implementing fingerprinting. It's not like it would take many engineer hours to add it to an otherwise already functional fingerprinting system.

It's far fetched to think that google added web midi in this way just for a couple of bits of entropy which are essentially worthless (no ad network cares about identifying like 0.01% of people, if even that. Yes it's very valuable entropy if you want to identify those people specifically, but who actually wants to do that?)

The point is not to identify the people using Web MIDI but to identify individual users, regardless of what information exactly identifies them. To that end, every single piece of entropy helps. A good approach to it in general is to opportunistically consume every available API that can possibly divulge identifying information.

A lot of people also do have a virtual MIDI device installed whether they know about it or not. The name of this device differs between different operating systems and operating system revisions.

Well, in this case, a porn site wants to tie what it shows you to what you liked last time you visited. They aren't after your identity per se. They are after a conversion. And since you might be using incognito mode (no lingering cookies), they care about fingerprinting for that.

Edit: I see the disconnect now. I'm not saying Google/chrome added the midi API for fingerprinting. I'm saying the screenshot way up this thread is an example of a site using it for that purpose.

Here's a jsfiddle: https://jsfiddle.net/wj69s4fh/

I get different types of failures and messages from different versions of Chrome, Firefox, and IE. None of which have any midi devices. Those errors, or the structure of the resulting object if it succeeds, are all fingerprint inputs.

Yeah, ran it in Chrome, the browser didn't say a thing whatsoever and I see MIDIAccess object in JS console. Nice to know the browser just allows this entire API by default.

I would guess quite a few browsers or operating systems would implement at least one virtual MIDI device, so that sites wanting to play MIDI would work. Those virtual devices wouldn’t all be identical.

It might be a way to detect bots, even on headless browsers, that pretend to be Chrome but don’t implement the MIDI api. I’m sure crawlers are the bane of the porn industry.

It takes almost no work to check all available browser context, once you check for some of it.

Why? All kinds of details are already being used for fingerprinting

If this pun was intended, I'm phoning the World Court.

Please, make the call.

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