• Points: 1500


We have now created a flag verifier service. Enter a flag to see if it matches the challenge you are trying to solve.

Visit https://15.adventofctf.com to start the challenge.


Upon opening the challenge website we’re greeted with some PHP code:


ini_set('display_errors', 0);


if (isset($_POST["flag"])) {
    $f = $_POST["flag"];

    if (strcmp($f, $flag) == 0 || sha1($flag) == sha1($f)) {
        echo $flag;

header("Location: /index.php?error=Wrong flag");

Besides this code, we also get an input field for the contents of the flag parameter.

Finding the vulnerability

When scanning this code, we see that $flag is compared to our input. It firstly does a strcmp, and, if it is not 0, it checks if the sha1 hashes of both variables are equal.

The thing with PHP and strcmp is that PHP will do some type juggling before checking the values. You can read more about PHP type juggling in the writeup of yesterday’s challenge.


This time, however, we have to use type juggling in a different way. In PHP, we can also pass arrays as a parameter. We do this by placing brackets after the parameter name like so: flag[]=a. And this is exactly how we solve it.

This works as strcmp("string", []) will always return 0 because PHP.

We can either use software like burp repeater or cURL to manually create a request, or change the contents of the name attribute to flag[].


After then making the request, we get the flag: NOVI{typ3_juggl1ng_f0r_l1fe_seriously}.

This flag can then be submitted for the challenge.


Because this challenge also has an XSS vulnerability, we can use it to solve the challenge automatically for us.

To do this, we firstly have to create some HTML code which executes some javascript code which then actually solves the challenge.

I came up with the following code:

  setTimeout(() => {
    let flagInput = document.getElementById("flag");
    flagInput.name = "flag[]";
    flagInput.value = "hi";
  }, 1000);

If we then put this in the error parameter in the URL, it will solve the challenge automatically. The resulting URL is the following: https://15.adventofctf.com/index.php?error=<script>setTimeout(()=>{let flagInput=document.getElementById("flag");flagInput.name="flag[]";flagInput.value="hi";flagInput.form.submit()},1000)</script>.