For AI agents: A markdown version of this page is available at https://docs.datadoghq.com/security/code_security/iac_security/iac_rules/cloudformation-aws-security-groups-without-vpc-attached.md. A documentation index is available at /llms.txt.

Security groups without VPC attached

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Metadata

Id: cloudformation-aws-security-groups-without-vpc-attached

Provider: AWS

Platform: CloudFormation

Severity: Low

Category: Networking and Firewall

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Description

Security groups must be associated with a specific VPC to ensure network isolation and to allow enforcement of VPC-level controls. A security group without a VpcId may end up outside the intended VPC (or require EC2-Classic), which prevents proper subnet scoping, VPC flow logging, and other VPC security controls and can lead to unintended exposure.

For AWS::EC2::SecurityGroup resources, Properties.VpcId must be defined (for example, !Ref MyVPC or an explicit VPC ID). This rule flags security groups where Properties.VpcId is undefined and the group is not the default group (GroupName != "default").

Secure example with VpcId set:

MySecurityGroup:
  Type: AWS::EC2::SecurityGroup
  Properties:
    GroupDescription: Allow HTTP
    VpcId: !Ref MyVPC
    SecurityGroupIngress:
      - IpProtocol: tcp
        FromPort: 80
        ToPort: 80
        CidrIp: 10.0.0.0/16

Compliant Code Examples

Parameters:
  KeyName:
    Description: The EC2 Key Pair to allow SSH access to the instance
    Type: 'AWS::EC2::KeyPair::KeyName'
Resources:
  Ec2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      SecurityGroups:
        - !Ref InstanceSecurityGroup
        - MyExistingSecurityGroup
      KeyName: !Ref KeyName
      ImageId: ami-7a11e213
  InstanceSecurityGroup:
    Type: 'AWS::EC2::SecurityGroup'
    Properties:
      GroupName: My Group Name
      GroupDescription: Enable SSH access via port 22
      VpcId:
        Ref: myVPC
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: '22'
          ToPort: '22'
          CidrIp: 0.0.0.0/0

{
  "Resources": {
    "Ec2Instance": {
      "Type": "AWS::EC2::Instance",
      "Properties": {
        "SecurityGroups": [
          "InstanceSecurityGroup",
          "MyExistingSecurityGroup"
        ],
        "KeyName": "KeyName",
        "ImageId": "ami-7a11e213"
      }
    },
    "InstanceSecurityGroup": {
      "Type": "AWS::EC2::SecurityGroup",
      "Properties": {
        "SecurityGroupIngress": [
          {
            "IpProtocol": "tcp",
            "FromPort": "22",
            "ToPort": "22",
            "CidrIp": "0.0.0.0/0"
          }
        ],
        "GroupName": "My Group Name",
        "GroupDescription": "Enable SSH access via port 22",
        "VpcId": {
          "Ref": "myVPC"
        }
      }
    }
  },
  "Parameters": {
    "KeyName": {
      "Description": "The EC2 Key Pair to allow SSH access to the instance",
      "Type": "AWS::EC2::KeyPair::KeyName"
    }
  }
}

Non-Compliant Code Examples

Parameters:
  KeyName:
    Description: The EC2 Key Pair to allow SSH access to the instance
    Type: 'AWS::EC2::KeyPair::KeyName'
Resources:
  Ec2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      SecurityGroups:
        - !Ref InstanceSecurityGroup
        - MyExistingSecurityGroup
      KeyName: !Ref KeyName
      ImageId: ami-7a11e213
  InstanceSecurityGroup:
    Type: 'AWS::EC2::SecurityGroup'
    Properties:
      GroupName: My Group Name
      GroupDescription: Enable SSH access via port 22
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: '22'
          ToPort: '22'
          CidrIp: 0.0.0.0/0

{
  "Parameters": {
    "KeyName": {
      "Description": "The EC2 Key Pair to allow SSH access to the instance",
      "Type": "AWS::EC2::KeyPair::KeyName"
    }
  },
  "Resources": {
    "Ec2Instance": {
      "Properties": {
        "SecurityGroups": [
          "InstanceSecurityGroup",
          "MyExistingSecurityGroup"
        ],
        "KeyName": "KeyName",
        "ImageId": "ami-7a11e213"
      },
      "Type": "AWS::EC2::Instance"
    },
    "InstanceSecurityGroup": {
      "Type": "AWS::EC2::SecurityGroup",
      "Properties": {
        "GroupName": "My Group Name",
        "GroupDescription": "Enable SSH access via port 22",
        "SecurityGroupIngress": [
          {
            "IpProtocol": "tcp",
            "FromPort": "22",
            "ToPort": "22",
            "CidrIp": "0.0.0.0/0"
          }
        ]
      }
    }
  }
}