Crack a linux password with John the Ripper

John the Ripper is an excellent password cracker available at http://www.openwall.com/john/

Identify weak passwords (e.g. MD5 passwords, shadow passwords, and NIS passwd maps that do not contain the password hash).

Installing on Linux:

1. wget http://openwall.com/john/john-1.6.tar.gz
2. tar zxvf john-1.6.tar.gz
3. cd john-1.6/src
4. make linux-x86-any-elf
5. cd ../run (change to directory containing john executable)

(or apt-get install john)

Note: if you are using shadow passwords (very likely on any modern UNIX system), you will need access to the /etc/shadow file on the target system. /etc/shadow contains the password hash needed to crack the password and is by default only readable and writable by the root user.

To crack a UNIX shadow file named shadow using John's default order of cracking modes: 

./john shadow

or run John in the background with a very low priority:

nice -n 20 ./john shadow &

Cracked passwords will be displayed on STDOUT (if running interactively) as well as stored in a file named john.pot. To display passwords already cracked by John, use the following command (assuming a password file named shadow):

./john -show shadow

Please employ the secure password practices listed above, and educate users on choosing strong passwords.


http://www.openwall.com/john/doc/EXAMPLES.shtml


john -i:digits shadowfile

Took 8 seconds to find a 4 digit password...