# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   lambda_function_public_access_prohibited_check
# 
# Description:
#   This control checks whether an AWS Lambda function resource-based policy prohibits public access.
# 
# Reports on:
#   AWS::Lambda::Permission
# 
# Evaluates:
#   AWS CloudFormation, AWS CloudFormation hook
# 
# Rule Parameters:
#   None
# 
# Scenarios:
#   Scenario: 1
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document does not contain any Lambda permission resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a Lambda permission resource
#       And: 'FunctionUrlAuthType' has been provided with a value of 'NONE'
#      Then: FAIL
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a Lambda permission resource
#       And: 'Principal' has been provided with a wildcard value ('*')
#       And: 'SourceAccount' has not been provided or provided with an empty string value
#       And: 'SourceArn' has not been provided or provided with an empty string value or non-valid local reference
#       And: 'PrincipalOrgID' has not been provided or provided with an empty string value
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a Lambda permission resource
#       And: 'Principal' has been provided with value that does not match an AWS Account ID, AWS IAM ARN or
#            wildcard value ('*')
#       And: 'SourceAccount' has not been provided or provided with an empty string value
#       And: 'SourceArn' has not been provided or provided with an empty string value or non-valid local reference
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a Lambda permission resource
#       And: 'Principal' has been provided with an AWS Account ID or AWS IAM ARN value
#      Then: PASS
#   Scenario: 6
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a Lambda permission resource
#       And: 'Principal' has been provided with a wildcard value ('*')
#       And: At least one of 'SourceAccount', 'SourceArn' or 'PrincipalOrgID' have been provided with non-empty string
#            values (or a valid local reference for 'SourceArn')
#      Then: PASS
#   Scenario: 7
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a Lambda permission resource
#       And: 'Principal' has been provided with value that does not match an AWS Account ID or AWS IAM ARN
#       And: At least one of 'SourceAccount', 'SourceArn' have been provided with non-empty string values (or a valid
#            local reference for 'SourceArn')
#      Then: PASS

#
# Constants
#
let LAMBDA_PERMISSION_TYPE = "AWS::Lambda::Permission"
let AWS_ACCOUNT_ID_PATTERN = /\d{12}/
let AWS_IAM_PRINCIPAL_PATTERN = /^arn:aws[a-z0-9\-]*:iam::\d{12}:.+/

let INPUT_DOCUMENT = this

#
# Assignments
#
let lambda_permissions = Resources.*[ Type == %LAMBDA_PERMISSION_TYPE ]

#
# Primary Rules
#
rule lambda_function_public_access_prohibited_check when is_cfn_template(%INPUT_DOCUMENT)
                                                         %lambda_permissions not empty {
    check(%lambda_permissions.Properties)
        <<
        [CT.LAMBDA.PR.2]: Require AWS Lambda function policies to prohibit public access
        [FIX]: When setting 'Principal' to '*', provide one of 'SourceAccount', 'SourceArn', or 'PrincipalOrgID'. When setting 'Principal' to a service principal (for example, s3.amazonaws.com), provide one of 'SourceAccount' or 'SourceArn'.
        >>
}

rule lambda_function_public_access_prohibited_check when is_cfn_hook(%INPUT_DOCUMENT, %LAMBDA_PERMISSION_TYPE) {
    check(%INPUT_DOCUMENT.%LAMBDA_PERMISSION_TYPE.resourceProperties)
        <<
        [CT.LAMBDA.PR.2]: Require AWS Lambda function policies to prohibit public access
        [FIX]: When setting 'Principal' to '*', provide one of 'SourceAccount', 'SourceArn', or 'PrincipalOrgID'. When setting 'Principal' to a service principal (for example, s3.amazonaws.com), provide one of 'SourceAccount' or 'SourceArn'.
        >>
}

#
# Parameterized Rules
#
rule check(lambda_permission) {
    %lambda_permission {
        # Scenario 2 and 5
        FunctionUrlAuthType not exists or
        FunctionUrlAuthType != "NONE"
    }

    %lambda_permission [
        Principal exists
        Principal == "*"
    ] {
        # Scenario 3 and 6
        SourceAccount exists or
        SourceArn exists or
        PrincipalOrgID exists

        check_is_string_and_not_empty(SourceAccount) or
        check_is_string_or_local_reference(SourceArn) or
        check_is_string_and_not_empty(PrincipalOrgID)
    }

    %lambda_permission [
        Principal exists
        Principal != "*"
        Principal != %AWS_ACCOUNT_ID_PATTERN
        Principal != %AWS_IAM_PRINCIPAL_PATTERN
    ] {
        # Scenario 4 and 7
        SourceAccount exists or
        SourceArn exists

        check_is_string_and_not_empty(SourceAccount) or
        check_is_string_or_local_reference(SourceArn)
    }
}

rule check_is_string_or_local_reference(value) {
    %value {
        check_is_string_and_not_empty(this) or
        check_local_references(%INPUT_DOCUMENT, this)
    }
}

rule check_local_references(doc, reference_properties) {
    %reference_properties {
        'Fn::GetAtt' {
            query_for_resource(%doc, this[0])
                <<Local Stack reference was invalid>>
        } or Ref {
            query_for_resource(%doc, this)
                <<Local Stack reference was invalid>>
        }
    }
}

rule query_for_resource(doc, resource_key) {
    let referenced_resource = %doc.Resources[ keys == %resource_key ]
    %referenced_resource not empty
}

#
# Utility Rules
#
rule is_cfn_template(doc) {
    %doc {
        AWSTemplateFormatVersion exists or
        Resources exists
    }
}

rule is_cfn_hook(doc, RESOURCE_TYPE) {
    %doc.%RESOURCE_TYPE.resourceProperties exists
}

rule check_is_string_and_not_empty(value) {
    %value {
        this is_string
        this != /\A\s*\z/
    }
}