VPC Flow Logs disabled

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Metadata

Id: cloudformation-aws-vpc-flowlogs-disabled

Provider: AWS

Platform: CloudFormation

Severity: Medium

Category: Observability

Learn More

Description

VPCs must have VPC Flow Logs enabled so network traffic metadata is recorded for detecting suspicious activity and supporting incident investigation and compliance audits.

For each AWS::EC2::VPC resource, ensure there is an AWS::EC2::FlowLog resource whose Properties.ResourceId references that VPC (for example, Ref: MyVPC) so traffic for that VPC is captured. Resources missing a flow log, or where the flow log’s ResourceId does not reference the VPC, will be flagged. The flow log should also specify a valid destination (CloudWatch Logs or S3) via LogDestination/LogDestinationType.

Secure example (CloudFormation YAML):

MyVPC:
  Type: AWS::EC2::VPC
  Properties:
    CidrBlock: 10.0.0.0/16

FlowLogGroup:
  Type: AWS::Logs::LogGroup

VPCFlowLog:
  Type: AWS::EC2::FlowLog
  Properties:
    ResourceType: VPC
    ResourceId: !Ref MyVPC
    TrafficType: ALL
    LogDestinationType: cloud-watch-logs
    LogDestination: !GetAtt FlowLogGroup.Arn

Compliant Code Examples

AWSTemplateFormatVersion: '2010-09-09'
Description: 'VPC: public and private subnets in two availability zones, a cloudonaut.io template'
Parameters:
  ClassB:
    Description: 'Class B of VPC (10.XXX.0.0/16)'
    Type: Number
    Default: 0
    ConstraintDescription: 'Must be in the range [0-255]'
    MinValue: 0
    MaxValue: 255
Resources:
  Role:
    Type: 'AWS::IAM::Role'
    Properties:
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
        - Effect: Allow
          Principal:
            Service: 'vpc-flow-logs.amazonaws.com'
          Action: 'sts:AssumeRole'
      Policies:
      - PolicyName: 'flowlogs-policy'
        PolicyDocument:
          Version: '2012-10-17'
          Statement:
          - Effect: Allow
            Action:
            - 'logs:CreateLogStream'
            - 'logs:PutLogEvents'
            - 'logs:DescribeLogGroups'
            - 'logs:DescribeLogStreams'
            Resource: !GetAtt 'LogGroup.Arn'
  MyVPC:
    Type: 'AWS::EC2::VPC'
    Properties:
      CidrBlock: !Sub '10.${ClassB}.0.0/16'
      EnableDnsSupport: true
      EnableDnsHostnames: true
      InstanceTenancy: default
      Tags:
      - Key: Name
        Value: !Sub '10.${ClassB}.0.0/16'
  LogGroup:
    Type: 'AWS::Logs::LogGroup'
    Properties:
      RetentionInDays: 14
  FlowLog:
    Type: 'AWS::EC2::FlowLog'
    Properties:
      DeliverLogsPermissionArn: !GetAtt 'Role.Arn'
      LogGroupName: !Ref LogGroup
      ResourceId: !Ref MyVPC
      ResourceType: 'VPC'
      TrafficType: ACCEPT

AWSTemplateFormatVersion: '2010-09-09'
Description: VPC whose flow log references the VPC through a short-form Ref intrinsic
Resources:
  IsolatedVPC:
    Type: 'AWS::EC2::VPC'
    Properties:
      CidrBlock: 10.0.0.0/16
  IsolatedLogGroup:
    Type: 'AWS::Logs::LogGroup'
    Properties:
      RetentionInDays: 14
  IsolatedFlowLog:
    Type: 'AWS::EC2::FlowLog'
    Properties:
      LogGroupName: !Ref IsolatedLogGroup
      ResourceId: !Ref IsolatedVPC
      ResourceType: 'VPC'
      TrafficType: ALL

Non-Compliant Code Examples

AWSTemplateFormatVersion: '2010-09-09'
Description: 'VPC: public and private subnets in two availability zones, a cloudonaut.io template'
Parameters:
  ClassB:
    Description: 'Class B of VPC (10.XXX.0.0/16)'
    Type: Number
    Default: 0
    ConstraintDescription: 'Must be in the range [0-255]'
    MinValue: 0
    MaxValue: 255
Resources:
  Role:
    Type: 'AWS::IAM::Role'
    Properties:
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
        - Effect: Allow
          Principal:
            Service: 'vpc-flow-logs.amazonaws.com'
          Action: 'sts:AssumeRole'
      Policies:
      - PolicyName: 'flowlogs-policy'
        PolicyDocument:
          Version: '2012-10-17'
          Statement:
          - Effect: Allow
            Action:
            - 'logs:CreateLogStream'
            - 'logs:PutLogEvents'
            - 'logs:DescribeLogGroups'
            - 'logs:DescribeLogStreams'
            Resource: !GetAtt 'LogGroup.Arn'
  MyVPC:
    Type: 'AWS::EC2::VPC'
    Properties:
      CidrBlock: !Sub '10.${ClassB}.0.0/16'
      EnableDnsSupport: true
      EnableDnsHostnames: true
      InstanceTenancy: default
      Tags:
      - Key: Name
        Value: !Sub '10.${ClassB}.0.0/16'
  LogGroup:
    Type: 'AWS::Logs::LogGroup'
    Properties:
      RetentionInDays: 14
  FlowLog:
    Type: 'AWS::EC2::FlowLog'
    Properties:
      DeliverLogsPermissionArn: !GetAtt 'Role.Arn'
      LogGroupName: !Ref LogGroup
      ResourceId: !Ref MyVPC1
      ResourceType: 'VPC'
      TrafficType: ACCEPT