Fictional situation for simplification:

• I have a grandparent page containing one parent iframe displaying the "parent page".
• This "parent page" contains two children iframes childA and childB.
• All the iframes are sandbox="allow-scripts" for security reasons.

parent sends and receive messages from/to grandparent, childA and childB.

When parent receives a message, I need to be able to identify the sender, but I can't find the right approach, as the origin of the event is null:

function onReceive(message) {
    // origin is null when the message comes from a sandboxed frame.
    console.log(message.origin);
}

Do you know a clean way to know who does the message come from?