Audit failed logon attempts

Step 1 :
sqlplus / as sysdba
Connected.
SQL> alter system set audit_trail=DB scope=spfile ;
OR
audit_trail=true

Step 2 :
 SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup ;
ORACLE instance started.

Step 3 :
sqlplus / as sysdba
 SQL> audit session whenever not successful ;

Step 4 :
Test auditing if it works

sqlplus scott/osama ;

ERROR:
ORA-01017: invalid username/password; logon denied
Warning: You are no longer connected to ORACLE.

Step 5:
sqlplus / as sysdba

Use the below Script :

select os_username,username,userhost,to_char(timestamp,'mm/dd/yyyy hh24:mi:ss') timestamp,returncode from dba_audit_session where action_name = 'LOGON' and returncode > 0
order by timestamp ;

http://osamamustafa.blogspot.com/2012/07/audit-failed-logon-attempts.html