The /var/yp directory should only be accessible by root. Change the permissions accordingly. If you are running TripWire, COPS or any other security tool, you should make it a part of the security audit process.
You should configure NIS to make its maps available only to certain networks. This can be done with the /var/yp/securenets file, here is an example:
255.255.255.0 10.10.20.0 255.255.255.0 10.10.21.0
To restrict availability to hosts, simply add the IP address of that host(s):
255.255.255.0 10.10.20.2 255.255.255.0 10.10.20.1
Secure your Root account
The root account should always be local! You should never allow it to be a part of NIS. If a hacker discovered the root password, he/she would have access to all of the machines within the NIS domain. Also, if NIS ever failed, you may not be able to login as root on any machine in the domain.
Move NIS Maps
NIS uses the /etc/passwd, /etc/shadow, /etc/inetd/netmasks files by default for its maps. Two problems with this are; anyone with login access to the system will be able to read all of the NIS maps; second, with /etc/passwd and /etc/shadow as NIS map sources, root login will be possible only if NIS is running properly. You should move these files out of the /etc directory.