CTF Challenges

Hack the Box Carrier: Walkthrough

Today we are going to solve another CTF challenge “Carrier”. It is a retired vulnerable lab presented by Hack the Box for helping pentester’s to perform online penetration testing according to your experience level; they have a collection of vulnerable labs as challenges, from beginners to Expert level.

Level: Expert

Task: To find user.txt and root.txt file

Note: Since these labs are online available therefore they have a static IP. The IP of Carrier is

Penetrating Methodology

  • Network scanning (Nmap)
  • Enumerating SNMP service port (161)
  • Surfing HTTPS service port (80)
  • Logging in through the Web portal
  • Finding command injection in web application
  • Getting reverse shell
  • Finding the first flag.
  • Finding Border Gateway Protocol
  • Border Gateway Protocol (BGP) Hijacking
  • Capturing FTP credentials
  • Logging in through SSH
  • Finding final flag


Let’s start off with our basic Nmap command to find out the open ports and services.

nmap -sC -sV -p-  -T4
nmap -sU --min-rate=5000 -T4

The Nmap scan shows us that there are 3 TCP ports are open: 21(FTP), 22(SSH), 80(HTTP) and 1 UDP port is open: 161(SNMP)

As port 161 is open we use snmpwalk to enumerate SNMP port and find a string called “SN#NET_45JDX23”.

snmpwalk -c public -v 1

As port 80 is open, we open the web services in the browser and we find a login page.

We try username “admin” and the string we find earlier as the password. But were unable to login but by using the password “NET_45JDX23”, we were able to login.

By checking the different options in the web application, in the diagnostic tab, we find something interesting. When we click on the “Verify status” button, we find that the server might be running “ps” command.

So further enumerate the web application, we use BurpSuite to capture the request and find inside the “check” parameter a base64 encoded string. When we decode the base64 encoded string we find the string to be called “quagga”. Now if check the web application, it is showing all the process that contains the string “quagga”. So that means the web application is running “ps” with “grep quagga” command.

Now to verify our theory, we change the check parameter to “root” and then encode it to base64 and then encode it to URL encode.

When we send the new request we find that the web application is displaying all the process that contains the string “root”.

Now we check if the web application is vulnerable to command injection or not. We try to run id command on the server.

By changing the parameter to “hack;id” and then encoding it with base64 encode and URL encode we forward the request to the server.

When we check the web application, we find that we are successfully able to run the “id” command that means the web application is vulnerable to command injection.

Now we replace the id command with nc reverse shell one-liner.

We encode the string with base64 encode and URL encode. We setup our listener and then forward the request.

As soon as we forward the request we get a reverse shell, we spawn a TTY shell and check for files in the current directory. Inside we find a file called “user.txt”, we open the file and find the first flag.

python -c "import pty; pty.spawn('/bin/bash')"

After getting a root shell we enumerated the machine, we do not find anything interesting. Going back to the tickets section on the web page, we find a hint that we need to check another subnet.

We use the ping command to find all the available machines on the subnet “”.

for i in {1..255}; do ping -c 1 10.120.15.$i | grep "bytes from" | cut -d " " -f4 | cut -d ":" -f1 ; done

Now we according to the ticket we know there is ftp server running on subnet “”. So we scan both the IP addresses and find port 21 is open on Further enumerating the system in cronjob we find that there is a bash script inside /opt/ directory called “restore.sh”. We take a look at the content of the file and find that the machine is working with Border Gateway Protocol (BGP) with Quagga. Now we can use a technique called BGP hijacking to take over the IP address. The bash script restores the BGP configuration every 10 minutes, so we remove executable permissions from the script so that we can make changes to the configuration of BGP

crontab -l
chmod -x /opt/restore.sh

Now we connect to the vty shell and check the current configuration.

show running-config

Now switch to configure mode, and to intercept the traffic we want to use our machine as the gateway.

configure terminal
ip prefix-list 0xdf permit
route-map to-as200 permit 10
match ip address prefix-list 0xdf
set community no-export
route-map to-as200 permit 20
route-map to-as300 deny 10
match ip address prefix-list 0xdf
route-map to-as300 permit 20
router bgp 100
network mask
clear ip bgp *

If we check our BGP routes we find that our machines will be used as a gateway.

show ip bgp neighbors advertised-routes

Now we will start collecting packets on port 21 using tcpdump, we will be using the interface eth2.

tcpdump -i eth2 -nnXSs 0 'port 21' -w hack.pcap

We wait for some time then interrupt the capture and check if the pcap file has been created. Now we transfer the file to our system and analyze it with Wireshark and find the password for FTP.

Password: BGPtelc0routing

We use this password to login through SSH on the target system and are successfully able to login. After logging in, we find a file called root.txt, we take a look at the content of the file and find the final flag.

ssh root@

Author: Sayantan Bera is a technical writer at hacking articles and cybersecurity enthusiastContact Here