I was trying to show my little brother how to root a RHEL6 box in single user mode today, and ran into a little issue. I booted the machine into init 1, but was not able to change the password. I could add a user, but could not assign a password. You could type: *passwd root * and the cmd prompt would just appear on the next line, as if the command was just blank.
Then I remembered SELINUX, my old buddy. Disabled selinux: setenforce 0 and bam! Password updated and life goes on.