Network ACL with unrestricted access to RDP

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Metadata

Id: a20be318-cac7-457b-911d-04cc6e812c25

Cloud Provider: AWS

Platform: Terraform

Severity: High

Category: Networking and Firewall

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Description

This check ensures that network ACLs don’t allow unrestricted access to RDP (TCP port 3389) from the public internet (0.0.0.0/0). Exposing RDP to the entire internet significantly increases the risk of brute force attacks and unauthorized access to your instances, potentially leading to data breaches or system compromise. Instead of using a wide-open CIDR block like 0.0.0.0/0, restrict RDP access to specific IP ranges as shown in the following secure configuration: ingress = [{ protocol = "tcp", rule_no = 100, action = "allow", cidr_block = "10.3.0.0/18", from_port = 3389, to_port = 3389 }].

Compliant Code Examples

provider "aws" {
  region = "us-east-1"
}

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "3.52.0"
    }
  }
}

resource "aws_network_acl" "negative3" {
  vpc_id = aws_vpc.main.id

  egress {
      protocol   = "tcp"
      rule_no    = 200
      action     = "allow"
      cidr_block = "10.3.0.0/18"
      from_port  = 443
      to_port    = 443
  }

  ingress {
      protocol   = "tcp"
      rule_no    = 100
      action     = "allow"
      cidr_block = "10.3.0.0/18"
      from_port   = 3389
      to_port     = 3389
  }

  tags = {
    Name = "main"
  }
}

provider "aws" {
  region = "us-east-1"
}

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 3.0"
    }
  }
}

resource "aws_network_acl" "negative2" {
  vpc_id = aws_vpc.main.id

  tags = {
    Name = "main"
  }
}

resource "aws_network_acl_rule" "negative2" {
  network_acl_id = aws_network_acl.negative2.id
  rule_number    = 100
  egress         = false
  protocol       = "tcp"
  rule_action    = "allow"
  from_port      = 3389
  to_port        = 3389
  cidr_block     = "10.3.0.0/18"
}

provider "aws" {
  region = "us-east-1"
}

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 3.0"
    }
  }
}

resource "aws_network_acl" "negative1" {
  vpc_id = aws_vpc.main.id

  egress = [
    {
      protocol   = "tcp"
      rule_no    = 200
      action     = "allow"
      cidr_block = "10.3.0.0/18"
      from_port  = 443
      to_port    = 443
    }
  ]

  ingress = [
    {
      protocol   = "tcp"
      rule_no    = 100
      action     = "allow"
      cidr_block = "10.3.0.0/18"
      from_port   = 3389
      to_port     = 3389
    }
  ]

  tags = {
    Name = "main"
  }
}

Non-Compliant Code Examples

provider "aws" {
  region = "us-east-1"
}

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 3.0"
    }
  }
}

resource "aws_network_acl" "positive2" {
  vpc_id = aws_vpc.main.id

  tags = {
    Name = "main"
  }
}

resource "aws_network_acl_rule" "postive2" {
  network_acl_id = aws_network_acl.positive2.id
  rule_number    = 100
  egress         = false
  protocol       = "tcp"
  rule_action    = "allow"
  from_port      = 3389
  to_port        = 3389
  cidr_block     = "0.0.0.0/0"
}

provider "aws" {
  region = "us-east-1"
}

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "<= 3.52.0"
    }
  }
}

resource "aws_network_acl" "positive3" {
  vpc_id = aws_vpc.main.id

  egress {
      protocol   = "tcp"
      rule_no    = 200
      action     = "allow"
      cidr_block = "10.3.0.0/18"
      from_port  = 443
      to_port    = 443
  }

  ingress {
      protocol   = "tcp"
      rule_no    = 100
      action     = "allow"
      cidr_block = "0.0.0.0/0"
      from_port   = 3389
      to_port     = 3389
  }

  tags = {
    Name = "main"
  }
}

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "3.7.0"

  name = "my-vpc"
  cidr = "10.0.0.0/16"

  azs             = ["eu-west-1a", "eu-west-1b", "eu-west-1c"]
  private_subnets = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]
  public_subnets  = ["10.0.101.0/24", "10.0.102.0/24", "10.0.103.0/24"]

  default_network_acl_ingress = [
    {
      "action" : "allow",
      "cidr_block" : "0.0.0.0/0",
      "from_port" : 0,
      "protocol" : "tcp",
      "rule_no" : 3389,
      "to_port" : 0
    }
  ]

  enable_nat_gateway = true
  enable_vpn_gateway = true

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