demo · v133

SharedWorker and BroadcastChannel partitioning

Before partitioning, an embedded third-party iframe on news.example and on shop.example would attach to the same SharedWorker instance — a clean cross-site identity channel. Today, each top-level site gets its own SharedWorker instance for the same worker URL. Same for BroadcastChannel.

pre-115 (legacy)

// embedded on news.example
const sw1 = new SharedWorker("/track.js");
// embedded on shop.example
const sw2 = new SharedWorker("/track.js");
// sw1 === sw2 (same instance)
//   ↓
// the iframe can recognise the user
// across the two parents.

Chrome 133 (partitioned, default)

// embedded on news.example
const sw1 = new SharedWorker("/track.js");
// embedded on shop.example
const sw2 = new SharedWorker("/track.js");
// sw1 !== sw2 (separate instances)
// partition key: (top-level site, frame origin)
// no cross-site identification possible.
This page is embedded as the top-level site. Open the demo inside an iframe whose parent is a different site to see the partition keys diverge. The probe below shows the current partition perspective.

this document's partition perspective

top-level origin— frame origin— is top-level— SharedWorker support— BroadcastChannel ping—

see also