Reduce the probability of a breach by checking EC2 security groups for inbound rules that allow unfettered access to TCP port 25 (used by Simple Mail Transfer Protocol (SMTP) for email transmission) and restrict access to IP addresses that require this port.
Rationale
Malicious activity, such as spamming, hacking, Shellshock, and denial-of-service (DoS) attacks, can occur when permitting unfettered access to this port.
Remediation
From the console
Follow the Security group rules docs to learn how to add a security group rule that will restrict access to a specific port.
From the command line
Run revoke-security-group-ingress to remove inbound rules that allow unrestricted access to port 25.
