The login into Run level 1,is by default the the ROOT login that is granted to the user,even without any password.Any unauthorised person can boot the system in RL 1 and just gain control of your system.
But don't take this as a hole,the security is just one file and couple of clicks away.All is needed is editing 'inittab' file present under /etc.And add a special line in there.
In steps it will be :
1. Be the root user.
2. Open the file in vi editor
vi /etc/inittab
3. Enter the 'Insert' mode,by pressing Insert button.
4. Append the line at last of the file
su:S:wait:/sbin/sulogin
5. Save the file and exit.
The back-door is now secured by the root password.






