The installation process for Oracle 10g and llg uses accounts with default passwords, which allows remote attackers to obtain login access by connecting to the Listener. NOTE: at the end of the installation, if performed using the Database Configuration Assistant (DBCA), most accounts are disabled or their passwords are changed.
http://www.securityfocus.com/bid/26425
http://www.securityfocus.com/archive/1/483652/100/200/threaded
http://www.oracle.com/technology/deploy/security/pdf/twp_security_checklist_db_database_20071108.pdf
http://www.davidlitchfield.com/blog/archives/00000030.htm