That only works if you explicitly set document.domain = "example.com" from the pages on example.com, as well as from the pages on the subdomain. See: https://de

That only works if you explicitly set document.domain = "example.com" from the pages on example.com, as well as from the pages on the subdomain. See: https://developer.mozilla.org/en-US/docs/DOM/document.domain https://developer.mozilla.org/en-US/docs/DOM/document.domain