EC2 Network ACL Duplicate Rule

• Query id: 045ddb54-cfc5-4abb-9e05-e427b2bc96fe
• Query name: EC2 Network ACL Duplicate Rule
• Platform: CloudFormation
• Severity: Low
• Category: Networking and Firewall
• URL: Github

Description

A Network ACL's rule numbers cannot be repeated unless one is egress and the other is ingress
Documentation

Code samples

Code samples with security vulnerabilities

Positive test num. 1 - yaml file

AWSTemplateFormatVersion: '2010-09-09'
Resources:
  MyNACL:
    Type: AWS::EC2::NetworkAcl
    Properties:
       VpcId: vpc-1122334455aabbccd
  InboundRule:
    Type: AWS::EC2::NetworkAclEntry
    Properties:
       NetworkAclId:
         Ref: MyNACL
       RuleNumber: 100
       Protocol: 6
       Egress: true
       RuleAction: allow
       CidrBlock: 172.16.0.0/24
       PortRange:
         From: 22
         To: 22
  OutboundRule:
    Type: AWS::EC2::NetworkAclEntry
    Properties:
       NetworkAclId:
         Ref: MyNACL
       RuleNumber: 100
       Protocol: -1
       Egress: true
       RuleAction: allow
       CidrBlock: 0.0.0.0/0
  MyNACL2:
    Type: AWS::EC2::NetworkAcl
    Properties:
       VpcId: vpc-1122334455aabbccdd
  InboundRule2:
    Type: AWS::EC2::NetworkAclEntry
    Properties:
       NetworkAclId:
         Ref: MyNACL2
       RuleNumber: "112"
       Protocol: 6
       Ingress: true
       RuleAction: allow
       CidrBlock: 172.16.0.0/24
       PortRange:
         From: 22
         To: 22
  OutboundRule2:
    Type: AWS::EC2::NetworkAclEntry
    Properties:
       NetworkAclId:
         Ref: MyNACL2
       RuleNumber: "112"
       Protocol: -1
       Ingress: true
       RuleAction: allow
       CidrBlock: 0.0.0.0/0

Positive test num. 2 - json file

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Resources": {
    "MyNACL": {
      "Properties": {
        "VpcId": "vpc-1122334455aabbccd"
      },
      "Type": "AWS::EC2::NetworkAcl"
    },
    "InboundRule": {
      "Type": "AWS::EC2::NetworkAclEntry",
      "Properties": {
        "Egress": true,
        "RuleAction": "allow",
        "CidrBlock": "172.16.0.0/24",
        "PortRange": {
          "From": 22,
          "To": 22
        },
        "NetworkAclId": {
          "Ref": "MyNACL"
        },
        "RuleNumber": 100,
        "Protocol": 6
      }
    },
    "OutboundRule": {
      "Type": "AWS::EC2::NetworkAclEntry",
      "Properties": {
        "NetworkAclId": {
          "Ref": "MyNACL"
        },
        "RuleNumber": 100,
        "Protocol": -1,
        "Egress": true,
        "RuleAction": "allow",
        "CidrBlock": "0.0.0.0/0"
      }
    },
    "MyNACL2": {
      "Type": "AWS::EC2::NetworkAcl",
      "Properties": {
        "VpcId": "vpc-1122334455aabbccdd"
      }
    },
    "InboundRule2": {
      "Type": "AWS::EC2::NetworkAclEntry",
      "Properties": {
        "CidrBlock": "172.16.0.0/24",
        "PortRange": {
          "From": 22,
          "To": 22
        },
        "NetworkAclId": {
          "Ref": "MyNACL2"
        },
        "RuleNumber": "112",
        "Protocol": 6,
        "Ingress": true,
        "RuleAction": "allow"
      }
    },
    "OutboundRule2": {
      "Properties": {
        "Ingress": true,
        "RuleAction": "allow",
        "CidrBlock": "0.0.0.0/0",
        "NetworkAclId": {
          "Ref": "MyNACL2"
        },
        "RuleNumber": "112",
        "Protocol": -1
      },
      "Type": "AWS::EC2::NetworkAclEntry"
    }
  }
}

Code samples without security vulnerabilities

Negative test num. 1 - yaml file

AWSTemplateFormatVersion: '2010-09-09'
Resources:
  MyNACL:
    Type: AWS::EC2::NetworkAcl
    Properties:
       VpcId: vpc-1122334455aabbccd
  InboundRule:
    Type: AWS::EC2::NetworkAclEntry
    Properties:
       NetworkAclId:
         Ref: MyNACL
       RuleNumber: 100
       Protocol: 6
       RuleAction: allow
       CidrBlock: 172.16.0.0/24
       PortRange:
         From: 22
         To: 22
  OutboundRule:
    Type: AWS::EC2::NetworkAclEntry
    Properties:
       NetworkAclId:
         Ref: MyNACL
       RuleNumber: 100
       Protocol: -1
       Egress: true
       RuleAction: allow
       CidrBlock: 0.0.0.0/0

Negative test num. 2 - json file

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Resources": {
    "MyNACL": {
      "Type": "AWS::EC2::NetworkAcl",
      "Properties": {
        "VpcId": "vpc-1122334455aabbccd"
      }
    },
    "InboundRule": {
      "Properties": {
        "NetworkAclId": {
          "Ref": "MyNACL"
        },
        "RuleNumber": 100,
        "Protocol": 6,
        "RuleAction": "allow",
        "CidrBlock": "172.16.0.0/24",
        "PortRange": {
          "From": 22,
          "To": 22
        }
      },
      "Type": "AWS::EC2::NetworkAclEntry"
    },
    "OutboundRule": {
      "Type": "AWS::EC2::NetworkAclEntry",
      "Properties": {
        "NetworkAclId": {
          "Ref": "MyNACL"
        },
        "RuleNumber": 100,
        "Protocol": -1,
        "Egress": true,
        "RuleAction": "allow",
        "CidrBlock": "0.0.0.0/0"
      }
    }
  }
}