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)

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

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)

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.

Lets Get Started

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 lab machine.

If you haven't already looked at port scanning, I recommend checking out the Nmap room 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 already installed on the AttackBox, however if you need to install it on your own attacking machine, 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!

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’s already installed on the AttackBox, if you do need to install it on your own attacking machine, you can find the documentation here.

We can remotely access the SMB share using the syntax:

Syntax: smbclient //[IP]/[SHARE] -U [USERNAME] -p [PORT]

Example: smbclient //10.10.10.10/secrets -U Anonymous -p 445

SMBClient Commands

Once inside the share, you can view the available commands by typing "help". The most useful of which are:

• ls or dir: List files and directories
• cd [DIR]: Move to a different directory
• get [FILE]: Download the file to your AttackBox

Got it? Okay, let's do this!

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 where transmitted data is encrypted.
 
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]"

Lets Get Started

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

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 lab machine. Scan the machine with nmap.

Output

Let's see what's going on on the target server...

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:

• https://www.cvedetails.com/
• https://cve.mitre.org/

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 lab 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.

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 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. 

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.

Lets Get Started

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

By now, I don't think I need to explain any further how enumeration is key when attacking network services and protocols. You should, by now, have enough experience with nmap to be able to port scan effectively. If you get stuck using any tool- you can always use "tool [-h / -help / --help]" to find out more about it's function and syntax. Equally, man pages are extremely useful for this purpose. They can be reached using "man [tool]".

We're going to be exploiting an anonymous FTP login, to see what files we can access- and if they contain any information that might allow us to pop a shell on the system. This is a common pathway in CTF challenges, and mimics a real-life careless implementation of FTP servers.

Resources

As we're going to be logging in to an FTP server, we will need to make sure an FTP client is installed on the system. There should be one installed by default on most Linux operating systems, such as Kali or Parrot OS. You can test if there is one by typing "ftp" into the console. If you're brought to a prompt that says: "ftp>", then you have a working FTP client on your system. If not, it's a simple matter of using "sudo apt install ftp" to install one.

It's worth noting  that some vulnerable versions of in.ftpd and some other FTP server variants return different responses to the "cwd" command for home directories which exist and those that don’t. This can be exploited because you can issue cwd commands before authentication, and if there's a home directory- there is more than likely a user account to go with it. While this bug is found mainly within legacy systems, it's worth knowing about, as a way to exploit FTP.

This vulnerability is documented at: https://www.exploit-db.com/exploits/20745 

Now we understand our toolbox, let's do this.                 

Types of FTP Exploit

Similarly to Telnet, when using FTP both the command and data channels are unencrypted. Any data sent over these channels can be intercepted and read.

With data from FTP being sent in plaintext, if a man-in-the-middle attack took place an attacker could reveal anything sent through this protocol (such as passwords). An article written by JSCape demonstrates and explains this process using ARP-Poisoning to trick a victim into sending sensitive information to an attacker, rather than a legitimate source.

When looking at an FTP server from the position we find ourselves in for this machine, an avenue we can exploit is weak or default password configurations.

Method Breakdown

So, from our enumeration stage, we know:

    - There is an FTP server running on this machine

    - We have a possible username

Using this information, let's try and bruteforce the password of the FTP Server.

Hydra

Hydra is a very fast online password cracking tool, which can perform rapid dictionary attacks against more than 50 Protocols, including Telnet, RDP, SSH, FTP, HTTP, HTTPS, SMB, several databases and much more. Hydra is already installed on the AttackBox, however, if you need it on your own attacking machine, you can find the GitHub repository here.

The syntax for the command we're going to use to find the passwords is this:

Let's break it down:

SECTION             FUNCTION

hydra                   Runs the hydra tool

-t 4                    Number of parallel connections per target

-l [user]               Points to the user who's account you're trying to compromise

-P [path to dictionary] Points to the file containing the list of possible passwords

-vV                     Sets verbose mode to very verbose, shows the login+pass combination for each attempt

[machine IP]            The IP address of the lab machine

ftp / protocol          Sets the protocol

Let's crack some passwords!

Further Learning

There is no checklist of things to learn until you've officially learnt everything you can. There will always be things that surprise us all, especially in the sometimes abstract logical problems of capture the flag challenges. But, as with anything, practice makes perfect. We can all look back on the things we've learnt after completing something challenging and I hope you feel the same about this room.

Reading

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

• https://medium.com/@gregIT/exploiting-simple-network-services-in-ctfs-ec8735be5eef
• https://attack.mitre.org/techniques/T1210/
• https://www.nextgov.com/cybersecurity/2019/10/nsa-warns-vulnerabilities-multiple-vpn-services/160456/

Thank you

Thanks for taking the time to work through this room, I wish you the best of luck in future.