break into Ubuntu System

This morning, I got a spare machine from of of the labmates. The OS is ubuntu 12.04. I could not log into the system because it is under other person‘s account.

Attack

Just enter into recovery mode when I start the computer.

select root.

passwd [username]

Then there is an error information. authentication token manipulation error.

mount -rw -o remount /

passwd [username]

Countermeasure

[hide recovery mode]

vi /etc/default/grub

update-grub

[set root password]

sudo passwd root