postMessage spoofingEMBEDDED

Send forged messages to the parent page and check whether origin validation is missing.

Behavior by sandbox policy

Policy	Expected result
No sandbox	works
`sandbox="allow-scripts"`	works
`sandbox="" (strictest)`	blocked

Embed snippet

This snippet uses the dedicated embed page. Paste it into your own service and check rendering or blocking behavior.

<iframe src="https://xss-playground.com/embed/post-message?lang=en" title="XSS Playground - postMessage spoofing" width="600" height="420" loading="lazy" referrerpolicy="strict-origin-when-cross-origin"></iframe>

Run

target origin

// no logs

Explanation

postMessage is the intended cross-origin channel. SOP does not block it, so the parent must validate event.origin.
If the parent uses message listeners for a payment widget, iframe-resizer, the YouTube IFrame API, and similar integrations, mimicking that format becomes an attack surface.
Defense is event.origin validation plus message type/schema validation. In sandbox, only sandbox="" blocks postMessage itself.