
Outer Perimeter: The Listener is under Attack
Those who ever came across this database know that Oracle DB interacts with its external environment by using a listener, which is a kind of balancer. The listener listens to port 1521 and resolves incoming connections depending on the requested database. One listener allows you to serve different databases. Also, in some cases, it allows to launch DoS and RCE attacks against the server. The audit of an Oracle database usually begins with attacks against the listener service. A priority task that you need to resolve in order to connect to the listener is to get SID, a kind of unique database identifier. Without it, the listener will not communicate with you. Sh2kerr once wrote an excellent research paper on this subject (Different ways to guess Oracle database SID).