Ensure no security groups allow ingress from 0.0.0.0/0 to SMTP port 25

Description:

Security groups act as virtual firewalls for Amazon EC2 instances and other AWS resources. Allowing unrestricted SMTP access (port 25) from the entire internet (0.0.0.0/0) poses a significant security risk, making resources susceptible to brute-force attacks and unauthorized access. It's critical to restrict SMTP access to known and trusted IP addresses or CIDR blocks.

Remediation:

AWS Management Console:

1. Click on the specific security group's ID link.
   Navigate to the Inbound Rules tab.
   Select the rule that allows traffic from 0.0.0.0/0 to port 25 and click Edit inbound rules.
   Modify the rule to restrict access to trusted IP addresses or CIDR blocks, or remove the rule entirely if it's not needed.
   Click Save rules.

AWS CLI:

To list security groups that have an open SMTP rule:

aws ec2 describe-security-groups --query "SecurityGroups[?IpPermissions[?ToPort==`25` && IpRanges[?CidrIp=='0.0.0.0/0']]]" --output table

2. Modify Inbound Rules:

AWS Management Console:

Follow the steps provided in the previous section.

AWS CLI:

To revoke an open SMTP rule for a particular security group:

aws ec2 revoke-security-group-ingress --group-id [SECURITY_GROUP_ID] --protocol tcp --port 25 --cidr 0.0.0.0/0

Replace [SECURITY_GROUP_ID] with the appropriate security group ID.

Terraform:

To ensure your security group does not have an open SMTP rule in Terraform:

resource "aws_security_group" "example" {
  # ... other configurations ...

  ingress {
    from_port   = 20
    to_port     = 21
    protocol    = "tcp"
    cidr_blocks = ["trusted_ip/32"]  # Replace 'trusted_ip/32' with your trusted CIDR block
  }

  # ... other configurations ...
}

In the code above, replace the placeholder trusted_ip/32 with your trusted IP or CIDR block. This ensures only the specified IPs can access the resource over SMTP.

3. Regularly Monitor and Audit:

Follow the monitoring and auditing steps provided in the previous section.

Recommendation:

It's a best practice to restrict SMTP access to specific IP addresses or CIDR blocks that require it. Implementing solutions like AWS Systems Manager Session Manager for secure access without opening SMTP ports to the public can enhance security. Ensure regular monitoring and auditing of security group configurations to adhere to security best practices.