What is brute forcing ?
In layman language, brute forcing means using a tool that picks passwords from a word list and tries them one by one until one works.

How to make a word list ?
A word list can consist of all possible combination's of letter,numbers,special characters. It can have some common or default passwords. You can download the word list generators or goggle the word-lists for brute-forcing and configure them according to yourself.

How fruitful attack can be ?
If we are try all possible combination's of letter,numbers,special characters, theoretically chances of success are 100%. But practically it is not possible to try every combination because it can take a lot of time. This attack just depends on the time you give,processing power and of course your luck.

Many tools are use for it hydra is one of them

Step 1
Download THC Hydra from here

Step 2
(a) Make a usename wordlist consisting of some common usernames like this

(b) Get a wordlist of passwords
(c) Copy both wordlists to your hydra folder

Step 3
(a)Open the command prompt and change directory to your hydra folder using cd command.


(b) Type "hydra" without quotes and it will show you the options to use.

(c) Now to start attak,

Type "hydra -L userslist.txt -P passlist.txt xxx.xxx.xxx.xxx ftp" and press enter

where userslist.txt is the list of usernames, passlist.txt is the list of passwords and xxx.xxx.xxx.xxx is the IP address of target, Now it will start cracking

To use a single username instead of wordlist , Replace capital L with small l , like this

Type "hydra -l username -P passlist.txt xxx.xxx.xxx.xxx ftp"

Note : Ftp port must be open.

Warning: I highly recommend you to use a chain of proxies to spoof your identity because proper logs of user's IP addresses who try to connect to ftp server is made on the server. Here is an example of the same. 

Countermeasures to protect yourself from this attack:
Use strong passwords

Thanks,,,,