Enabling public network access to cloud resources can affect an organization’s ability to protect its data or internal operations from data theft or disruption.

Depending on the component, inbound access from the Internet can be enabled via:

Deciding to allow public access may happen for various reasons such as for quick maintenance, time saving, or by accident.

This decision increases the likelihood of attacks on the organization, such as:

Ask Yourself Whether

This cloud resource:

There is a risk if you answered no to any of those questions.

Recommended Secure Coding Practices

Avoid publishing cloud services on the Internet unless they are intended to be publicly accessible, such as customer portals or e-commerce sites.

Use private networks (and associated private IP addresses) and VPC peering or other secure communication tunnels to communicate with other cloud components.

The goal is to prevent the component from intercepting traffic coming in via the public IP address. If the cloud resource does not support the absence of a public IP address, assign a public IP address to it, but do not create listeners for the public IP address.

Sensitive Code Example

For aws_cdk.aws_ec2.Instance and similar constructs:

from aws_cdk import aws_ec2 as ec2

ec2.Instance(
    self,
    "vpc_subnet_public",
    instance_type=nano_t2,
    machine_image=ec2.MachineImage.latest_amazon_linux(),
    vpc=vpc,
    vpc_subnets=ec2.SubnetSelection(subnet_type=ec2.SubnetType.PUBLIC) # Sensitive
)

For aws_cdk.aws_ec2.CfnInstance:

from aws_cdk import aws_ec2 as ec2

ec2.CfnInstance(
    self,
    "cfn_public_exposed",
    instance_type="t2.micro",
    image_id="ami-0ea0f26a6d50850c5",
    network_interfaces=[
        ec2.CfnInstance.NetworkInterfaceProperty(
            device_index="0",
            associate_public_ip_address=True, # Sensitive
            delete_on_termination=True,
            subnet_id=vpc.select_subnets(subnet_type=ec2.SubnetType.PUBLIC).subnet_ids[0]
        )
    ]
)

For aws_cdk.aws_dms.CfnReplicationInstance:

from aws_cdk import aws_dms as dms

rep_instance = dms.CfnReplicationInstance(
    self,
    "explicit_public",
    replication_instance_class="dms.t2.micro",
    allocated_storage=5,
    publicly_accessible=True, # Sensitive
    replication_subnet_group_identifier=subnet_group.replication_subnet_group_identifier,
    vpc_security_group_ids=[vpc.vpc_default_security_group]
)

For aws_cdk.aws_rds.CfnDBInstance:

from aws_cdk import aws_rds as rds
from aws_cdk import aws_ec2 as ec2

rds_subnet_group_public = rds.CfnDBSubnetGroup(
    self,
    "public_subnet",
    db_subnet_group_description="Subnets",
    subnet_ids=vpc.select_subnets(
        subnet_type=ec2.SubnetType.PUBLIC
    ).subnet_ids
)

rds.CfnDBInstance(
    self,
    "public-public-subnet",
    engine="postgres",
    master_username="foobar",
    master_user_password="12345678",
    db_instance_class="db.r5.large",
    allocated_storage="200",
    iops=1000,
    db_subnet_group_name=rds_subnet_group_public.ref,
    publicly_accessible=True, # Sensitive
    vpc_security_groups=[sg.security_group_id]
)

Compliant Solution

For aws_cdk.aws_ec2.Instance:

from aws_cdk import aws_ec2 as ec2

ec2.Instance(
    self,
    "vpc_subnet_private",
    instance_type=nano_t2,
    machine_image=ec2.MachineImage.latest_amazon_linux(),
    vpc=vpc,
    vpc_subnets=ec2.SubnetSelection(subnet_type=ec2.SubnetType.PRIVATE_WITH_NAT)
)

For aws_cdk.aws_ec2.CfnInstance:

from aws_cdk import aws_ec2 as ec2

ec2.CfnInstance(
    self,
    "cfn_private",
    instance_type="t2.micro",
    image_id="ami-0ea0f26a6d50850c5",
    network_interfaces=[
        ec2.CfnInstance.NetworkInterfaceProperty(
            device_index="0",
            associate_public_ip_address=False, # Compliant
            delete_on_termination=True,
            subnet_id=vpc.select_subnets(subnet_type=ec2.SubnetType.PRIVATE_WITH_NAT).subnet_ids[0]
        )
    ]
)

For aws_cdk.aws_dms.CfnReplicationInstance:

from aws_cdk import aws_dms as dms

rep_instance = dms.CfnReplicationInstance(
    self,
    "explicit_private",
    replication_instance_class="dms.t2.micro",
    allocated_storage=5,
    publicly_accessible=False,
    replication_subnet_group_identifier=subnet_group.replication_subnet_group_identifier,
    vpc_security_group_ids=[vpc.vpc_default_security_group]
)

For aws_cdk.aws_rds.CfnDBInstance:

from aws_cdk import aws_rds as rds
from aws_cdk import aws_ec2 as ec2

rds_subnet_group_private = rds.CfnDBSubnetGroup(
    self,
    "private_subnet",
    db_subnet_group_description="Subnets",
    subnet_ids=vpc.select_subnets(
        subnet_type=ec2.SubnetType.PRIVATE_WITH_NAT
    ).subnet_ids
)

rds.CfnDBInstance(
    self,
    "private-private-subnet",
    engine="postgres",
    master_username="foobar",
    master_user_password="12345678",
    db_instance_class="db.r5.large",
    allocated_storage="200",
    iops=1000,
    db_subnet_group_name=rds_subnet_group_private.ref,
    publicly_accessible=False,
    vpc_security_groups=[sg.security_group_id]
)

See