How does TestFlightApp know what device I'm using via the web?

Question

We use TestFlight to test our iOS App binaries internally and with beta testers.

When TestFlight sends out an email alert to a user that a test binary is available, the web page it sends them to automatically knows if the user is using a registered device, but it seems to figure this out while in Safari. How is it doing this?

It has to match up UDIDs that we've supplied to their tool (via their website), but I was under the impression that websites could not grab data like UDIDs. How is TestFlight figuring that out?

Answer 1

When registering a device with an account and installing the testflight profile. It then will store the UDID information it received via the profile installation process in a session (and probably a cookie) which then can be accessed from the same website URL. So whenever you go back to the website it checks for the stored information.

This is roughly how it is done, detailed implementation can only be answered by the developers.