Oracle database link
Credentials are not passed over the dblink connection.
When creating a CURRENT_USER dblink, a trust should be established between the databases. This requires setting up an enterprise domain, adding the databases to it, setting an SSL link between the databases and making the link CURRENT_USER enabled.
See here: http://download.oracle.com/docs/cd/B12037_01/network.101/b10772/asoeuscf.htm