In this paper, the authors report on a study of brute force attack on CFG passwords. Alphanumeric passwords are the common and usual mode of authentication for a range of online login. Human proclivities in creating Password draw hackers and enthusiastic password crackers to crack down password easily using various techniques, with accessible computing power and available large number of tools. Common attacks on passwords are brute force attack, dictionary attack and hybrid attack. A new method of alphanumeric password authentication for user login is "CFG Password".