Unfortunately, in some older implementations, it is possible to obtain a site's password file just by asking, if the yp domain name is known. Since many sites use the same yp domain as ip domain, this may be quite easy to guess. It may be possible to obtain the domain name from another service such as bootp, in any case. The password file thus obtained is vulnerable to password cracking.
Running unsecured yp is not, therefore, a good idea.
A.Daviel