Check your EC2 security groups for inbound rules that allow unrestricted access (i.e. 0.0.0.0/0 or ::/0) to TCP port 1521 and restrict access to only those IP addresses that require it in order to implement the principle of least privilege and reduce the possibility of a breach. TCP port 1521 is used by the Oracle Database Server which is an object-relational database management system (RDBMS) server developed by Oracle Corporation: https://en.wikipedia.org/wiki/Oracle_Database
Allowing unrestricted Oracle Database access can increase opportunities for malicious activity such as hacking, denial-of-service (DoS) attacks and loss of data.
To determine if your EC2 security groups allow unrestricted Oracle Database access, perform the following:
To update your security groups inbound/ingress configuration in order to restrict Oracle Database access to specific entities (IP addresses, IP ranges, etc), perform the following: