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11 August 2020

THM write-up: KnockKnock

5 minutes to read



Note: The room has been discontinued

Hello there, welcome to another tryhackme CTF writeup. Today, I going to show you something special which is port knocking. Well, this is my first encounter on this privsec technique and I am amused. Basically, port knocking is a method to access a hidden port by attempting a correct connection sequence. In short, you need to knock the correct port in order to access a hidden port. Interesting huh? I can’t wait to tell you the story, let’s start the walkthrough.

Task 1: Capture the flag

Your task is to capture the flag.

Task 1-1/1-2: The webpage and the packet file

First and foremost, fire up your Nmap scanner with the following command.

nmap -Pn -p- -A -v <Machine IP>

Oh, there are only one port open publically inside the machine which is port 80 (HTTP).


Look like a pcap file download link. Guess I have to open the packet file using the Wireshark.

Task 1-3: The last knocked port

We have to find the user role based on the packet file.


By referring to the ping result, one can conclude that IP: is the client while IP: is the server. So now, we are only interested in how the client knocks the server. Let filter out the result with TCP port only.

wireshark sequence

Look like the client knock the port in the following sequence: 7000, 8000, 9000, 7000, 8000, 9000, 8888. Bear in mind that, the last port might be port access by the user. This made the sequence of 7000, 8000, 9000, 7000, 8000, 9000. Before you made a knock, made sure you download the knock script. If you are done, punch in the following command to reveal a secret message.

./knock <machine IP> 7000 8000 9000 7000 8000 9000 && telnet 8888

Task 1-4: The first hidden directory


After that, the terminal reveals a secret directory.

Task 1-5: Locate the second packet file

Visiting the secret directory reveal another message with a download link

more page

Download the packet file and open up with the Wireshark

more wireshark

This time, the packet look quite messy when compared to the first one. A port knocks usually one packet of request, response and then request, response until it stops. However, we have two response from the server (Port 8080) at the same time with a size of over 2000 bytes. Let’s take a look.

check pcap

The packet contains a readable ASCII character, Look like an ASCII image. In order to further our investigation, we can follow the TCP stream by right click the packet.

follow stream


A hidden message? It is written in the German language which translated as one three three seven or 1337. Knocking port 1337 only? Not going to work. How about Port 1 3 3 7 and telnet port 1337.

Task 1-6: The second directory

1 3 3 7

Well, it just works and we got the second hidden directory.

Task 1-7: Another message

more pagesss

Look like a base64 encoded message. Let’s decode it.

another port knock

Another port knocking sequence. This time, we are going to open an SSH shell instead of telnet. Punch in the following command.

./knock <Machine IP> 8888 9999 7777 6666 && ssh <Machine IP>

Task 1-8: Who inside the Machine

SSH cred

An SSH shell with butthead’s login credential. Great!.

Task 1-9: SSH problem

Without further ado, let’s login into butthead’s SSH shell.

SSH failed

Uh-oh, the shell instantly closed after we logged in. After a short internet searching and researching, I come across this help forum. We can force open a /bin/sh in SSH using the following command.

ssh butthead@<Machine IP> /bin/sh


Yes, we are now able to access the shell. But before that, let’s spawn a shell using the python cause we might need the TTY shell.

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

Task 1-10: Privilege escalate

I had tried the sudo -l, the GTFObin, crontabs, and none of them work. How about the infamous overlayfs exploit ? We need to check the kernel version just to be sure.


Great, the machine is vulnerable to overlayfs exploitation. I’m going to cut this short and you can visit my previous writeup on the overlayfs exploitation. From this point onward, I assumed you have the C script inside your tmp directory. Compile the script and launch it.


Congratulation, you are now rooted in the machine. Let’s capture the root flag.


That’s it, a short and simple CTF challenge. Hope you enjoy the write-up. Once again, see you another time ;).

tags: tryhackme - CTF - recon - privilege_escalate - port_knocking - wireshark

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