what is the default password for those build-in accounts, such as adm, sys, dhcp, oracle, guest?

if not set, can they be used to login? is that a security problem? how are they designed to be used?

thanks!

It's difficult to lump those all together since that includes a guest account and, unless I'm mistaken, an admin account. Talk about polar extremes. But system accounts like dhcp and oracle are generally configured as non-login accounts: no password, no home, not even a shell. Programs using this uid switched to it with a privileged setuid() call rather than any sort of login process.

I just used an oracle account to login into a solaris system with a password yesterday however.

you can look in the /etc/passwd file and that will show you what home directory, default shell and so on is set for each user. It will also let you know whether the user is allowed to login or not.

Here is a sample from my /etc/passwd file

as you can see the default shell when they login is /bin/false which means they have no shell access. I suspect yours would look much the same although depnding on your version you may see /sbin/nologin as the default shell, all does the same thing essentially

then how can I use these accounts? how are they usually used?

These accounts are not for logging in with and hence that is why they have their login shells set to /bin/false. They are used by the daemons that they relate so sshd is used by the SSH daemon as the description says.

If you login as root and superuser to them

you will see nothing happens.

As for you being able to login with Oracle, this is not unusual. It is sometimes used by DBAs to go in and use SQL Plus as it allows access to Orcale's components.