Add today's challenge

content/posts/adventofctf/challenge_3.md

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+author = "Maik de Kruif"
+title = "Challenge 3 - AdventOfCTF"
+date = 2020-12-03T09:29:12+01:00
+description = "Challenge 3 of AdventOfCTF."
+tags = [
+    "AdventOfCTF",
+    "challenge",
+    "ctf",
+    "hacking"
+]
+categories = [
+    "ctf",
+    "hacking",
+]
++++
+
+- Points: 300
+
+## Description
+
+For this challenge you will, again, need to bypass the login mechanism.
+
+Visit <https://03.adventofctf.com> to start the challenge.
+
+## Solution
+
+When opening the website we're provided with a login form. If we fill in the form with random data, nothing happens. Usually a website will do a `POST` request to a URL when submitting a form, but even that didn't happen. So my guess is that there is some javascript in play.
+
+Let's open the source and take a look at the form. Here we can see that when the form is submitted, a javascript function called `checkPass()` is called.
+
+```html
+<form action="/index.php" onsubmit='checkPass(); return false'>
+```
+
+To find this funtion, enter `checkPass` in the devtools console and click on the three dots at the bottom of the output.
+
+```js
+function checkPass()
+{
+    var username = document.getElementById('username').value;
+    var password = document.getElementById('password').value;
+
+    var novi = '-NOVI';
+
+    if (password == btoa(username + novi)) {
+        window.setTimeout(function() {
+            window.location.assign('inde' + 'x.php?username='+ username +'&password=' + password);
+        }, 500);
+    }
+}
+```
+
+We can see there is a check which checks if `password` is equal to `btoa(username + novi)`. But what is `btoa`? According to [w3schools](https://www.w3schools.com/jsref/met_win_btoa.asp):
+
+> The btoa() method encodes a string in base-64.
+
+To get the value of what the password should be, we have to know the output of `btoa(username + novi)`. Above this check, we see the `novi` variable is set to `'-NOVI'`. Now, we go to the devtools console and generate the password. In the console, enter `btoa("a" + "-NOVI")`. This returns `"YS1OT1ZJ"`, so lets try that combination. I used the username `"a"`. If we enter this combination in the form, we get redirected to a page with the flag.
+
+This flag is `NOVI{javascript_is_not_s@fe}`.
+
+This flag can then be submitted for the [challenge](https://ctfd.adventofctf.com/challenges#3-4).