Almost a decade ago I wrote about su in sqlplus. This 10gR2 “new” feature allows delegation à la sudo.
By checking the DBA_USERS in 12c I found PROXY_ONLY_CONNECT. According to Miguel Anjo, there is a secret syntax for allowing only the proxy user.
SQL> ALTER USER app_user PROXY ONLY CONNECT;
SQL> CONNECT app_user/xyz
ERROR:ORA-28058: login is allowed only through a proxy