[whatwg] More on postMessage

[Jeff Walden]

Gorm Haug Eriksen wrote:
> I agree that postMessage should have been on the window and not on the 
> document, but why would you like to have the method on yourWindow 
> instead of the otherWindow you post the message to?


The benefit is that you don't have to punch holes through your existing security infrastructure to do it.  If you're in your code, you can have a reference to an |otherWindow| that's not same-origin as you, but you can't do any of the following (and probably more) with it:

  var secretProperty = otherWindow.secretProperty; // stolen!
  for (var i in otherWindow)
  {
    // if you get here, you know some of otherWindow's
    // properties -- information leak
  }
  otherWindow.trustedProperty = "subverted"; // oops!
  delete otherWindow.importantInfo; // DOS

For you to need to use postMessage on otherWindow, you need to be able to do many of these things -- but the entire browser security model is based on not allowing you to do this if the window you've called it on isn't same-origin with you.  You have to punch a hole in this security to allow getting, calling, or enumerating postMessage, but only if the object off which the property is gotten is a Window.  You also have to make the property appear ReadOnly/DontDelete externally, so you can't screw with windows that try to call postMessage on you.  Also, how does this restriction work with other windows which are same-origin?  Do they see only the original postMessage binding, or do they see any modifications that window makes to it?  What if a different window, same-origin, makes that modification?  What if windows pre-HTML5 wanted to communicate via a postMessage binding?  This gets complicated pretty quickly, and to do it all you have to punch a hole through security, and wi
th the fragility of that hole (only on Window, only if "postMessage", only with the original binding or only if it hasn't been overridden -- and I'm not at all sure that's enough) and the specific criteria for that hole to exist, it's going to be easy to accidentally allow more than you wanted to allow.

In contrast, passing a different-origin value into a function is already allowed, and you don't need to do anything special to make it possible.  The only security modification to allow the cross-origin-ness is to make postMessage ignore origins.  This is *vastly* simpler, easier to implement, and hence safer and more secure.

I'll agree that calling postMessage on the other window feels like a better and more intuitive API for users, but if implementers have to make such invasive and potentially-unsafe changes to do it, I think it's the wrong way to do it.

Jeff