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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:

   a boolean value that explicitly allows access to the public network. 
   the assignment of a public IP address. 
   database firewall rules that allow public IP ranges. 

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:

   data breaches. 
   intrusions into the infrastructure to permanently steal from it. 
   and various malicious traffic, such as DDoS attacks. 

Ask Yourself Whether
This cloud resource:

   should be publicly accessible to any Internet user. 
   requires inbound traffic from the Internet to function properly. 

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

   AWS Documentation - Amazon EC2 instance IP
  addressing 
   AWS Documentation - Public and
  private replication instances 
   AWS Documentation - VPC Peering 
   CWE - CWE-284 - Improper Access Control 
   CWE - CWE-668 - Exposure of Resource to Wrong Sphere 
   STIG Viewer - Application Security and
  Development: V-222620 - Application web servers must be on a separate network segment from the application and database servers.