When you install CUBRID for the first time and try to connect to it using CUBRID Manager (CM) client tool, the username and password to login are admin/admin. Right after the first connection, CM Server will require to change the password. Once you change it, CM will ask if you want to reconnect with the new credentials. You do so and continue.
When the admin's password has been changed, CM Server will store it on the host machine in $CUBRID/conf/cm.pass where $CUBRID is a directory where CUBRID is installed. Originally for admin user and admin password it contains the following information:
The password part in the above line will change when you change your admin password.
Sometimes it happens that you do not remember the admin password and want to reset it manually. In this case, open this cm.pass file and reset the password to admin by replacing the existing content of the file with the above line as it was when you tried to login for the first time. This way you can reset your administrator's password and can login with admin/admin. Once logged in, do not forget to change your password to something secure.
Also you should remember that all CUBRID files under $CUBRID directory must have proper file permissions so that no one except you could change the administrator's password. Otherwise, you may run into serious problems.