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.

The current snippet has no sandbox restrictions. Use it to observe behavior, not as a production default.

<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.