The only way for a worker to communicate with the rest of the browser context is by using the `postMessage` method. The host code has to listen and respond via the `onmessage` handler, so the worker is unable to do anything that you don't explicitly implement in your `onmessage` handler.
You still have to make sure the API you expose to the user plugins aren't exploitable.
You still have to make sure the API you expose to the user plugins aren't exploitable.