Localstorage is limited to a domain, a common security model in the browser also used by cookies, and prevents cross-origin leaks... (unless a developer volunteers to expose the data via postmessage whose destination can also be limited to specific origins).

This is also why it is important to load your apps JS on your domain or same-origin and not offloaded to a 3rd party server which you might not control (libraries like jQuery CDNs and whatnot are still a minor risk, particularly from a privacy perspective, but not as bad, although I never saw the point with the large variety of versions).