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!
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.
What does SMB stand for?
What type of protocol is SMB?
What do clients connect to servers using?
What systems does Samba run on?
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 target 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 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!
What ports is SMB running on?
Let's get started with Enum4Linux, conduct a full basic enumeration. For starters, what is the workgroup name?
What comes up as the name of the machine?
What operating system version is running?
What share sticks out as something we might want to investigate?
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!
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.
Does the share allow anonymous access? Y/N?
What service has been configured to allow him to work from home?
Okay! Now we know this, what directory on the share should we look in?
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?
Download this file to your local machine, and change the permissions to "600" using "chmod 600 [file]".
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.
What is the smb.txt flag?
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]"
What is Telnet?
What has slowly replaced Telnet?
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 target
machine. Scan the machine with nmap.
Output
Let's see what's going on on the target server...
How many ports are open on the target machine?
What port is this?
This port is unassigned, but still lists the protocol it's using, what protocol is this?
Now re-run the nmap scan, without the -p- tag, how many ports show up as open?
Here, we see that by assigning telnet to a non-standard port,
it is not part of the common ports list, or top 1000 ports, that nmap
scans. It's important to try every angle when enumerating, as the
information you gather here will inform your exploitation stage.
Always keep a note of information you find during your enumeration stage, so you can refer back to it when you move on to try exploits.
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.
Great! It's an open telnet connection! What welcome message do we receive?
Let's try executing some commands, do we get a return on any input we enter into the telnet session? (Y/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:
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.
Now, use the command "ping [local THM ip] -c 1" through
the telnet session to see if we're able to execute system commands. Do
we receive any pings? Note, you need to preface this with .RUN (Y/N)
Great! This means that we are able to
execute system commands AND that we are able to reach our local
machine. Now let's have some fun!
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:
Perfect. We're nearly there. Now all we need to do is start a netcat listener on our local machine. We do this using:
"nc -lvp [listening port]"
What would the command look like for the listening port we selected in our payload?
Great! Now that's running, we need to
copy and paste our msfvenom payload into the telnet session and run it
as a command. Hopefully- this will give us a shell on the target
machine!
Success! What is the contents of flag.txt?
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.
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.
What communications model does FTP use?
What's the standard FTP port?
How many modes of FTP connection are there?
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]".
MethodWe'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.
Run an nmap scan of your choice.
How many ports are open on the target machine?
What port is ftp running on?
What variant of FTP is running on it?
Great, now we know what type of FTP server we're dealing with we can check to see if we are able to login anonymously to the FTP server. We can do this using by typing "ftp [IP]" into the console, and entering "anonymous", and no password when prompted.
What is the name of the file in the anonymous FTP directory?
could be?
Great! Now we've got details about the FTP server and, crucially, a possible username. Let's see what we can do with that...
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
comes by default on both Parrot and Kali, however if you need it, you
can find the GitHub here.
The syntax for the command we're going to use to find the passwords is this:
"hydra -t 4 -l dale -P /usr/share/wordlists/rockyou.txt -vV 10.10.10.6 ftp"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 target machine
ftp / protocol Sets the protocol
Let's crack some passwords!
What is the password for the user "mike"?
Bingo! Now, let's connect to the FTP server as this user using "ftp [IP]" and entering the credentials when prompted
What is ftp.txt?
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.
~ PoloWell done, you did it!
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