User requests should come from a particular IP (i.e. that of Middleware). Request coming from any other IP will not be entertained.

 Here is the trigger to  entertained only one IP.

CREATE OR REPLACE TRIGGER system.check_ip_addresses 

AFTER LOGON 

ON DATABASE

DECLARE 

  l_ip_address VARCHAR2(16) := SYS_CONTEXT('USERENV', 'IP_ADDRESS'); 

BEGIN 

  IF USER IN ('SCOTT') THEN

    IF l_ip_address = '1*2.3*.3*.00' THEN

      NULL;

    ELSE

      RAISE_APPLICATION_ERROR(-20000, 'Can not log in from this IP address (' || l_ip_address || ')');

    END IF;

  END IF;

END; 

/

SQL> CREATE OR REPLACE TRIGGER system.check_ip_addresses

  2  AFTER LOGON

  3  ON DATABASE

  4  DECLARE

  5    l_ip_address VARCHAR2(16) := SYS_CONTEXT('USERENV', 'IP_ADDRESS');

  6  BEGIN

  7    IF USER IN ('SCOTT') THEN

  8      IF l_ip_address = '172.31.36.50' THEN

  9        NULL;

 10      ELSE

 11        RAISE_APPLICATION_ERROR(-20000, 'Can not log in from this IP address

(' || l_ip_address || ')');

 12      END IF;

 13    END IF;

 14  END;

 15  /

Trigger created.

SQL> conn scott/tiger -- Here i've try to connect using 127.0.0.1 IP.

ERROR:

ORA-00604: error occurred at recursive SQL level 1

ORA-20000: Can not log in from this IP address ()

ORA-06512: at line 8