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chromium
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chromium/third_party/blink/web_tests/http/tests/security/cookies/resources/innocent-victim-with-cookies.html 

<html>
<body>
<script>
k = Math.floor(Math.random() * 1000000);
document.cookie = "secretcookie=" + k + ";path=/";
parent.postMessage(k, "*");
</script>
</body>
</html>