Lambda functions with full privileges

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Metadata

Id: cloudformation-aws-lambda-functions-with-full-privileges

Provider: AWS

Platform: CloudFormation

Severity: High

Category: Access Control

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Description

Lambda functions must not be assigned IAM roles whose inline policies grant full administrative privileges, because a compromised function with full permissions can exfiltrate data, modify or delete resources, or escalate privileges across your account. This rule inspects AWS::Lambda::Function resources’ Properties.Role reference (when the role is defined in the same template) and examines the referenced role’s Resources.<role>.Properties.Policies[].PolicyDocument.Statement[]. A statement will be flagged when Effect is Allow and both Action and Resource are * (including arrays that contain *). To remediate, follow least privilege. Replace wildcard actions and resources with explicit actions and scoped ARNs, or attach narrowly scoped managed policies. Inline policies must not simultaneously allow Action: '*' and Resource: '*'.

Secure example (inline role policy scoped to a single S3 bucket):

MyLambdaRole:
  Type: AWS::IAM::Role
  Properties:
    AssumeRolePolicyDocument:
      Statement:
        - Effect: Allow
          Principal:
            Service: lambda.amazonaws.com
          Action: sts:AssumeRole
    Policies:
      - PolicyName: LambdaS3ReadOnly
        PolicyDocument:
          Statement:
            - Effect: Allow
              Action:
                - s3:GetObject
              Resource: arn:aws:s3:::example-bucket/*

Compliant Code Examples

AWSTemplateFormatVersion: '2010-09-09'
Parameters:
  ExistingSecurityGroups:
    Type: List<AWS::EC2::SecurityGroup::Id>
  ExistingVPC:
    Type: AWS::EC2::VPC::Id
    Description: The VPC ID that includes the security groups in the ExistingSecurityGroups
      parameter.
  InstanceType:
    Type: String
    Default: t2.micro
    AllowedValues:
    - t2.micro
    - m1.small
Resources:
  SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Allow HTTP traffic to the host
      VpcId:
        Ref: ExistingVPC
      SecurityGroupIngress:
      - IpProtocol: tcp
        FromPort: '80'
        ToPort: '80'
        CidrIp: 0.0.0.0/0
      SecurityGroupEgress:
      - IpProtocol: tcp
        FromPort: '80'
        ToPort: '80'
        CidrIp: 0.0.0.0/0
  AllSecurityGroups:
    Type: Custom::Split
    Properties:
      ServiceToken: !GetAtt AppendItemToListFunction.Arn
      List:
        Ref: ExistingSecurityGroups
      AppendedItem:
        Ref: SecurityGroup
  AppendItemToListFunction:
    Type: AWS::Lambda::Function
    Properties:
      Handler: index.handler
      Role: !GetAtt LambdaExecutionRole.Arn
      Code:
        ZipFile: |
          var response = require('cfn-response');
          exports.handler = function(event, context) {
             var responseData = {Value: event.ResourceProperties.List};
             responseData.Value.push(event.ResourceProperties.AppendedItem);
             response.send(event, context, response.SUCCESS, responseData);
          };
      Runtime: nodejs8.10
  MyEC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: ami-0ff8a91507f77f867
      SecurityGroupIds: !GetAtt AllSecurityGroups.Value
      InstanceType:
        Ref: InstanceType
  LambdaExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
        - Effect: Allow
          Principal:
            Service:
            - lambda.amazonaws.com
          Action:
          - sts:AssumeRole
      Path: "/"
      Policies:
      - PolicyName: root
        PolicyDocument:
          Version: '2012-10-17'
          Statement:
          - Effect: Allow
            Action:
            - iam:ChangePassword
            Resource: arn:aws:iam::account-ID-without-hyphens:user/Bob

Non-Compliant Code Examples

AWSTemplateFormatVersion: '2010-09-09'
Parameters:
  ExistingSecurityGroups:
    Type: List<AWS::EC2::SecurityGroup::Id>
  ExistingVPC:
    Type: AWS::EC2::VPC::Id
    Description: The VPC ID that includes the security groups in the ExistingSecurityGroups
      parameter.
  InstanceType:
    Type: String
    Default: t2.micro
    AllowedValues:
    - t2.micro
    - m1.small
Resources:
  SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Allow HTTP traffic to the host
      VpcId:
        Ref: ExistingVPC
      SecurityGroupIngress:
      - IpProtocol: tcp
        FromPort: '80'
        ToPort: '80'
        CidrIp: 0.0.0.0/0
      SecurityGroupEgress:
      - IpProtocol: tcp
        FromPort: '80'
        ToPort: '80'
        CidrIp: 0.0.0.0/0
  AllSecurityGroups:
    Type: Custom::Split
    Properties:
      ServiceToken: !GetAtt AppendItemToListFunction.Arn
      List:
        Ref: ExistingSecurityGroups
      AppendedItem:
        Ref: SecurityGroup
  AppendItemToListFunction:
    Type: AWS::Lambda::Function
    Properties:
      Handler: index.handler
      Role: !GetAtt LambdaExecutionRole.Arn
      Code:
        ZipFile: |
          var response = require('cfn-response');
          exports.handler = function(event, context) {
             var responseData = {Value: event.ResourceProperties.List};
             responseData.Value.push(event.ResourceProperties.AppendedItem);
             response.send(event, context, response.SUCCESS, responseData);
          };
      Runtime: nodejs8.10
  MyEC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: ami-0ff8a91507f77f867
      SecurityGroupIds: !GetAtt AllSecurityGroups.Value
      InstanceType:
        Ref: InstanceType
  LambdaExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
        - Effect: Allow
          Principal:
            Service:
            - lambda.amazonaws.com
          Action:
          - sts:AssumeRole
      Path: "/"
      Policies:
      - PolicyName: root
        PolicyDocument:
          Version: '2012-10-17'
          Statement:
          - Effect: Allow
            Action:
            - "*"
            Resource: arn:aws:logs:*:*:*