DBLink's default user: postgres

Is this the regular behavior on DBLink?

rot=> SELECT user, current_database();
current_user | current_database
--------------+------------------
sa_rot | rot
(1 registro)

rot=> SELECT *
rot-> FROM dblink('dbname=escola',
rot(> 'SELECT user, current_database()')
rot-> AS (usr name, db name);
usr | db
----------+--------
postgres | escola
(1 registro)

This way, I fear DBLink functions should become a vulnerability issue on my
database.
Is there any way to protect or override this setting? Or it should be done
on pg_hba.conf only?

--
Regards,

Rodrigo Hjort
http://icewall.org/~hjort