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Comment by mmcwilliams | Hacker News Reader

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.

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