[oracle@ECP-UC-DB1 ~]$ sqlplus / as sysdba
SQL*Plus: Release 10.2.0.4.0 - Production on Mon Oct 17 23:37:51 2011
Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> alter user sys identified by "ab$";
User altered.
SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@ECP-UC-DB1 ~]$ sqlplus sys/ab$ as sysdba
SQL*Plus: Release 10.2.0.4.0 - Production on Mon Oct 17 23:38:53 2011
Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> show parameter name;
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_file_name_convert                 string
db_name                              string      test
db_unique_name                       string      test
global_names                         boolean     FALSE
instance_name                        string      test
lock_name_space                      string
log_file_name_convert                string
service_names                        string      test
SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@ECP-UC-DB1 ~]$ sqlplus sys/ab$abc as sysdba
SQL*Plus: Release 10.2.0.4.0 - Production on Mon Oct 17 23:39:05 2011
Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@ECP-UC-DB1 ~]$ sqlplus sys/ab$@abc as sysdba
SQL*Plus: Release 10.2.0.4.0 - Production on Mon Oct 17 23:40:06 2011
Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> show parameter name;
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_file_name_convert                 string
db_name                              string      test
db_unique_name                       string      test
global_names                         boolean     FALSE
instance_name                        string      test
lock_name_space                      string
log_file_name_convert                string
service_names                        string      test
SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@ECP-UC-DB1 ~]$ sqlplus sys/ab$@abc11 as sysdba
SQL*Plus: Release 10.2.0.4.0 - Production on Mon Oct 17 23:44:11 2011
Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> show parameter name;
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_file_name_convert                 string
db_name                              string      test
db_unique_name                       string      test
global_names                         boolean     FALSE
instance_name                        string      test
lock_name_space                      string
log_file_name_convert                string
service_names                        string      test
SQL>  

通过以上sql发现，把sys的密码改为ab$后，无论是什么tns都可以登录数据库，而且都是本地数据库，是不是感觉很诡异，其实你仔细观察发现，密码中有了$，使得$@的操作都变成了无效的，其实就是sqlplus sys/123(随意) as sysdba方式登录本地数据库

[oracle@ECP-UC-DB1 ~]$ sqlplus sys/123 as sysdba
SQL*Plus: Release 10.2.0.4.0 - Production on Mon Oct 17 23:48:23 2011
Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL>