Thanks, I understand the issue now - I haven't thought about CDNs from a privacy perspective before.

I suppose with HTTP2 some of the benefits of serving JS through CDNs are gone anyway, so I guess it's time to stop using them.

Not to be dense but wasn't that always the purpose of running a CDN service for common scripts and libraries?

The script wouldn't have to be from a CDN to track people using the browser cache. I could infer whether you've visited a site that doesn't use CDNs or trackers by asking you to load something from that site and inferring whether you have that resource cached by the time it took you to load it.

This is true, but if you're running a CDN you have access to cross-domain user information just based on the headers, no?

The CDN is not the place you have to worried about.

If Site A loads a specific JavaScript file for users with an administrator account, Site B can check to see if the JavaScript file is in your cache, and infer that you must have an administrator account if the file is there.

The attack can happen with different types of resources (such as images).

This I understand, the risk of third-parties monitoring. The attacks are pretty obvious. My confusion is over what the business model of a commercial CDN is if not to track users across multiple sites? How do they pay for bandwidth?

The problem is not the CDN (or arbitrary shared source domain) being able to track you but the sites which use the CDN.

Furthermore a CDN can't track you as simple as you might think, it often would require thinks which need explicitly opt-in agreements on a per website basis to be legal.

Furthermore due to technical limitations you can only get that permission from the user after the CDN was already used.

CDNs can still track aggregated information to some degree but they can't legally act like a tracker cookie.

How serious is this type of threat? Compared to all the info about us that is already shared by data brokers?

There are several data-broker-esque "services" that actually do this already with FB, Google, etc assets (favicon.ico and similar, loggedIn urls, ...) to check whether you have visited those pages, or whether you are logged in to those services by trying to request a URL that might return a large image if logged in, or fail rapidly if logged out. -- This has been a thing for a long time: https://news.ycombinator.com/item?id=15499917

If you don't use any of those sites, you're considered higher risk/fraudulent user/bot.

Here's an example of a very short and easy way to see if someone is probably gay: https://pastebin.com/raw/CFaTet0K

On chrome, I consistently get 1-5 back after it's been cached, and 100+ on a clean visit. On Firefox with resistFingerprinting, I get 0 always.

Thank you, that was insightful.

> Here's an example of a very short and easy way to see if someone is probably gay

Ok, but now the resource is in my cache, so from now on they will think I'm gay?

> Ok, but now the resource is in my cache, so from now on they will think I'm gay?

This resource is just generic, so probably not, but if you actually visited grindr's site without adblocking heavily, they load googletagmanager and a significant number of other tracking services, which will almost certainly associate your advertising profile and identifiers as 'gay'

I also can't believe they send/sell your information to 3 pages worth of third party monetization providers/adtech companies for something that is this critically sensitive.

You could have run this on a private window of the browser (and in that case, they would surely think you're a closeted gay).

Could this not be solved by by Grindr setting up CORS properly for that resource? It's unlikely anyone would ever open the script directly in their browser.

CORS wouldn't help here. CORS prevents you from reading the response or making a cross origin XHR requests, not loading an external resource from a different domain in a script or img tag.

Fun fact: browsers put scary warnings in their dev console (and some web sites log warnings or console) because some people love copy-pasting code they got from sketchy people trying to bypass all the browser security.

It's being actively used by the ad networks to do user fingerprinting instead of cookies, since the latter are more and more blocked.

I guess as serious as any other privacy threats but one that doesn't get enough attention in my opinion. CDNs and web fonts are definitely being used to track us and can bypass mitigations like private mode in your browser and ad/tracker blockers by tracking your IP address across sites.