EC2 instance has public IP

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Metadata

Id: terraform-aws-ec2-instance-has-public-ip

Provider: AWS

Platform: Terraform

Severity: Medium

Category: Networking and Firewall

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Description

EC2 instances should not be assigned public IP addresses directly, as this exposes them to the internet and increases the risk of unauthorized access and attacks. In Terraform, this is configured using the associate_public_ip_address attribute. Setting associate_public_ip_address = true on an aws_instance resource will result in a public IP being attached, whereas associate_public_ip_address = false ensures the instance is only accessible within the private network, reducing the attack surface and enhancing the security posture of the environment. Instances that leave the attribute unset are not flagged, because they follow the subnet’s map_public_ip_on_launch setting, which is disabled by default outside default VPCs.

Compliant Code Examples

data "aws_ami" "ubuntu" {
  most_recent = true

  filter {
    name   = "name"
    values = ["ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-*"]
  }

  filter {
    name   = "virtualization-type"
    values = ["hvm"]
  }

  owners = ["099720109477"] # Canonical
}

resource "aws_instance" "web" {
  ami           = data.aws_ami.ubuntu.id
  associate_public_ip_address = false
  instance_type = "t3.micro"

  tags = {
    Name = "HelloWorld"
  }
}

module "ec2_instance" {
  source  = "terraform-aws-modules/ec2-instance/aws"
  version = "~> 3.0"

  name = "single-instance"

  ami                    = "ami-ebd02392"
  instance_type          = "t2.micro"
  key_name               = "user1"
  monitoring             = true
  vpc_security_group_ids = ["sg-12345678"]
  subnet_id              = "subnet-eddcdzz4"
  associate_public_ip_address = false

  tags = {
    Terraform   = "true"
    Environment = "dev"
  }
}

module "ec2_instance" {
  source  = "terraform-aws-modules/ec2-instance/aws"
  version = "~> 3.0"

  name = "single-instance"

  ami                    = "ami-ebd02392"
  instance_type          = "t2.micro"
  key_name               = "user1"
  monitoring             = true
  vpc_security_group_ids = ["sg-12345678"]
  subnet_id              = "subnet-eddcdzz4"

  network_interface {
    network_interface_id = aws_network_interface.this.id
    device_index         = 0
  }

  tags = {
    Terraform   = "true"
    Environment = "dev"
  }
}

resource "aws_network_interface" "this" {
  subnet_id       = var.private_subnet_id
  security_groups = [aws_security_group.this.id]
}

resource "aws_security_group" "this" {
  name        = "example"
  description = "Example Security Group"
}

Non-Compliant Code Examples

data "aws_ami" "ubuntu1" {
  most_recent = true

  filter {
    name   = "name"
    values = ["ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-*"]
  }

  filter {
    name   = "virtualization-type"
    values = ["hvm"]
  }

  owners = ["099720109477"] # Canonical
}

resource "aws_instance" "web2" {
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t3.micro"

  tags = {
    Name = "HelloWorld"
  }
}

resource "aws_instance" "web3" {
  ami           = data.aws_ami.ubuntu.id
  associate_public_ip_address = true
  instance_type = "t3.micro"

  tags = {
    Name = "HelloWorld"
  }
}