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Security groups allows unrestricted outbound traffic

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Metadata

Id: cloudformation-aws-security-groups-allows-unrestricted-outbound-traffic

Cloud Provider: AWS

Platform: CloudFormation

Severity: Medium

Category: Networking and Firewall

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Description

Security groups must not allow unrestricted outbound traffic by combining IpProtocol: "ALL" with CidrIp: "0.0.0.0/0". This permits any protocol to any internet destination and can enable data exfiltration, malware callbacks, and make detection or containment harder.

In CloudFormation, check AWS::EC2::SecurityGroup resources and flag any SecurityGroupEgress entry where IpProtocol is ALL and CidrIp is 0.0.0.0/0. Entries that include IpProtocol: "ALL" together with CidrIp: "0.0.0.0/0" will be flagged. Instead, define explicit protocols and ports and restrict destination CIDR blocks to the minimum required. If broad internet egress is needed, prefer specific ports (for example, TCP/443) or centralize outbound access through NAT gateways, proxies, or VPC endpoints.

Secure configuration example with explicit ports and restricted destinations:

MySecurityGroup:
  Type: AWS::EC2::SecurityGroup
  Properties:
    GroupDescription: Example secure egress rules
    VpcId: vpc-123456
    SecurityGroupEgress:
      - IpProtocol: tcp
        FromPort: 443
        ToPort: 443
        CidrIp: 0.0.0.0/0
      - 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:
      GroupDescription: Enable SSH access via port 22
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: '22'
          ToPort: '22'
          CidrIp: 0.0.0.0/0
      SecurityGroupEgress:
        - 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": {
      "Type": "AWS::EC2::Instance",
      "Properties": {
        "SecurityGroups": [
          "InstanceSecurityGroup",
          "MyExistingSecurityGroup"
        ],
        "KeyName": "KeyName",
        "ImageId": "ami-7a11e213"
      }
    },
    "InstanceSecurityGroup": {
      "Type": "AWS::EC2::SecurityGroup",
      "Properties": {
        "GroupDescription": "Enable SSH access via port 22",
        "SecurityGroupIngress": [
          {
            "IpProtocol": "tcp",
            "FromPort": "22",
            "ToPort": "22",
            "CidrIp": "0.0.0.0/0"
          }
        ],
        "SecurityGroupEgress": [
          {
            "CidrIp": "0.0.0.0/0",
            "IpProtocol": "tcp",
            "FromPort": "22",
            "ToPort": "22"
          }
        ]
      }
    }
  }
}

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": {
        "ImageId": "ami-7a11e213",
        "SecurityGroups": [
          "InstanceSecurityGroup",
          "MyExistingSecurityGroup"
        ],
        "KeyName": "KeyName"
      }
    },
    "InstanceSecurityGroup": {
      "Properties": {
        "SecurityGroupIngress": [
          {
            "CidrIp": "0.0.0.0/0",
            "IpProtocol": "tcp",
            "FromPort": "22",
            "ToPort": "22"
          }
        ],
        "SecurityGroupEgress": [
          {
            "IpProtocol": "ALL",
            "FromPort": "22",
            "ToPort": "22",
            "CidrIp": "0.0.0.0/0"
          }
        ],
        "GroupDescription": "Enable SSH access via port 22"
      },
      "Type": "AWS::EC2::SecurityGroup"
    }
  }
}

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:
      GroupDescription: Enable SSH access via port 22
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: '22'
          ToPort: '22'
          CidrIp: 0.0.0.0/0
      SecurityGroupEgress:
        - IpProtocol: ALL
          FromPort: '22'
          ToPort: '22'
          CidrIp: 0.0.0.0/0