# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   ec2_transit_gateway_auto_vpc_attach_disabled_check
# 
# Description:
#   This control checks whether Amazon EC2 transit gateways are configured to accept Amazon VPC attachment requests automatically.
# 
# Reports on:
#   AWS::EC2::TransitGateway
# 
# 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 EC2 transit gateway resources
#      Then: SKIP
#   Scenario: 2
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an EC2 transit gateway resource
#        And: 'AutoAcceptSharedAttachments' configuration has been provided and is set to a value other than 'disable'
#       Then: FAIL
#   Scenario: 3
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an EC2 transit gateway resource
#        And: 'AutoAcceptSharedAttachments' configuration has not been provided
#       Then: PASS
#   Scenario: 4
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an EC2 transit gateway resource
#        And: 'AutoAcceptSharedAttachments' configuration has been provided and set to 'disable'
#       Then: PASS

#
# Constants
#
let EC2_TRANSIT_GATEWAY_TYPE = "AWS::EC2::TransitGateway"
let INPUT_DOCUMENT = this

#
# Assignments
#
let ec2_transit_gateway = Resources.*[ Type == %EC2_TRANSIT_GATEWAY_TYPE ]

#
# Primary Rules
#
rule ec2_transit_gateway_auto_vpc_attach_disabled_check when is_cfn_template(%INPUT_DOCUMENT)
                                                             %ec2_transit_gateway not empty {
    check(%ec2_transit_gateway.Properties)
        <<
        [CT.EC2.PR.6]: Require that Amazon EC2 transit gateways refuse automatic Amazon VPC attachment requests
        [FIX]: Omit the 'AutoAcceptSharedAttachments' property or set the property to 'disable'.
        >>
}

rule ec2_transit_gateway_auto_vpc_attach_disabled_check when is_cfn_hook(%INPUT_DOCUMENT, %EC2_TRANSIT_GATEWAY_TYPE) {
    check(%INPUT_DOCUMENT.%EC2_TRANSIT_GATEWAY_TYPE.resourceProperties)
        <<
        [CT.EC2.PR.6]: Require that Amazon EC2 transit gateways refuse automatic Amazon VPC attachment requests
        [FIX]: Omit the 'AutoAcceptSharedAttachments' property or set the property to 'disable'.
        >>
}

#
# Parameterized Rules
#
rule check(ec2_transit_gateway) {
    %ec2_transit_gateway {
        # Scenario 3
        AutoAcceptSharedAttachments not exists or
        # Scenario 2 and 4
        AutoAcceptSharedAttachments == "disable"
    }
}

#
# 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
}