Quoting from Building Internet Firewalls, O'Reilly:
NIS/YP is designed to provide distributed access to centralized administrative information (such as host tables, password files, site-wide email aliases, and so on) shared by machines at a site.
The main problem with NIS/YP is that its security isn't good enough to adequately protect some of the data it contains. In particular, a site's NIS/YP servers generally contain the shared password file (equivalent to the /etc/passwd file on a single system) for the site, complete with encrypted passwords. All you need to get data from an NIS/YP server is the NIS/YP domain name with which the data is associated. An attacker who can talk to a site's NIS/YP server, and who can guess what the site has chosen as its NIS/YP domain name (often the same as, or a derivative of, their regular Internet domain name), can request any information the server has. If the attacker gets their shared password file, these passwords can be cracked at the attacker's leisure.
NOTE: NIS/YP transfers include the encrypted passwords even if the machines are configured to use shadow passwords and the encrypted passwords are not readable on the NIS/YP server.
A few NIS/YP servers (notably Sun's) support a configuration file called securenets. This allows you to use IP address authentication to control which hosts your NIS/YP server will release data to.
...end quote. To see the kind of data which is being referred to above, on your host type
ypcat passwd.adjunct.byname(the map name may vary) and you will see the encrypted passwords for each user.