hacking an ubuntu system ...

This is a simple hack and works on systems where the root password has not been set by the user, which is hardly a thing a normal user does. You start by entering the recovery mode on system startup and and select the option that allows you to enter the root prompt. Once there simply type ....

>> passwd

and you will be asked to set the new root password.
Now login using the ...

username as root and the password just set.
Enjoy the root privileges ...