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69 lines
2.5 KiB
69 lines
2.5 KiB
+++
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author = "Maik de Kruif"
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title = "Challenge 3"
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subtitle = "Challenge 3 - AdventOfCTF"
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date = 2020-12-03T09:29:12+01:00
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description = "A writeup for challenge 3 of AdventOfCTF."
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cover = "img/writeups/adventofctf/2020/4f5cc0afbb9e7ec6a57cdd68a92b9213.png"
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tags = [
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"AdventOfCTF",
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"challenge",
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"ctf",
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"hacking",
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"writeup",
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"web",
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"javascript",
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]
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categories = [
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"ctf",
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"writeups",
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"hacking",
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]
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+++
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- Points: 300
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## Description
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For this challenge you will, again, need to bypass the login mechanism.
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Visit <https://03.adventofctf.com> to start the challenge.
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## Solution
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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.
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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.
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```html
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<form action="/index.php" onsubmit="checkPass(); return false"></form>
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```
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To find this funtion, enter `checkPass` in the devtools console and click on the three dots at the bottom of the output.
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```js
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function checkPass() {
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var username = document.getElementById("username").value;
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var password = document.getElementById("password").value;
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var novi = "-NOVI";
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if (password == btoa(username + novi)) {
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window.setTimeout(function () {
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window.location.assign(
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"inde" + "x.php?username=" + username + "&password=" + password
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);
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}, 500);
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}
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}
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```
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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):
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> The btoa() method encodes a string in base-64.
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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.
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This flag is `NOVI{javascript_is_not_s@fe}`.
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This flag can then be submitted for the [challenge](https://ctfd.adventofctf.com/challenges#3-4).
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