This is what we do. Store a 1st party httponly secure cookie jwt representing the user, then grab any extra data from the backend. Or just something like session cookie but persist it.

I wouldn't be surprised if this is one of the ways ad tracking tries to rebuild a universal identifier like the old urchin module. Might not be as easy as a cname but those might get blocked. It's always a game of cat and mouse. Could place uuid as 1st party httponly cookie. maybe uuid is domain scoped. then 'echo out' so accessible by 3rd party JS. Like a hash, one would need to know the global pooled uuid already and then combined with knowable domain could tie that uuid into the 2nd party tracking pool.