AWS VPC Security Hardening: Network Isolation & IAM Best Practices

AWS Virtual Private Cloud (VPC) represents the foundation of your network security boundary in the cloud. Unsecured or default configurations leave database ports exposed and allow unrestricted ingress paths. In this engineering guide, we walk through the steps required to secure your cloud perimeter.

1. Multi-Tier Subnet Architecture

An enterprise-ready VPC should segregate workloads into three distinct tiers:

1. Public Subnet Tier: Holds internet-facing resources (ALBs, NAT Gateways).
2. Private App Subnet Tier: Holds application services (Next.js backends, container runtimes).
3. Isolated Database Subnet Tier: Completely cut off from the internet, containing databases (MongoDB, PostgreSQL) and cash nodes.

                  ┌───────────────────────┐
                  │   Internet Gateway    │
                  └───────────┬───────────┘
                              │
            ┌─────────────────▼─────────────────┐
            │   Public Subnets (ALB, NAT GW)    │
            └─────────────────┬─────────────────┘
                              │ NAT Routed
            ┌─────────────────▼─────────────────┐
            │   Private App Subnets (Express)   │
            └─────────────────┬─────────────────┘
                              │ Internal Only
            ┌─────────────────▼─────────────────┐
            │  Isolated DB Subnets (MongoDB)   │
            └───────────────────────────────────┘

2. Stateless vs Stateful Rules

We configure:

• NACLs on subnet borders to block known bad IPs or restrict CIDR blocks.
• Security Groups to enforce tight microsegmentation between specific resource groups.

3. Terraform Code Snippet

Here is how we define the private VPC setup with Terraform:

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.0.0"

  name = "ajitdev-prod-vpc"
  cidr = "10.0.0.0/16"

  azs             = ["us-east-1a", "us-east-1b"]
  private_subnets = ["10.0.1.0/24", "10.0.2.0/24"]
  public_subnets  = ["10.0.101.0/24", "10.0.102.0/24"]

  enable_nat_gateway = true
  single_nat_gateway = true
}