Frontend & browser·Browser security
the embedded iframe could not read its own cookie because it was not the site in the address bar.
Third-party cookie
Also calledcross-site cookie
A cookie set by an origin other than the one in the address bar, historically the mechanism behind cross-site tracking and behind most embedded session flows. Browsers have been restricting and partitioning them for years, so any feature relying on one is on a deadline. The replacements — storage partitioning, dedicated APIs, first-party redirect flows — all require the embedded product to change.