# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   cloudfront_sni_enabled_check
# 
# Description:
#   This control checks whether your Amazon CloudFront distributions are configured to use SNI to serve HTTPS requests.
# 
# Reports on:
#   AWS::CloudFront::Distribution
# 
# 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 CloudFront distribution resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'ViewerCertificate' is not present on the CloudFront distribution resource
#      Then: FAIL
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'ViewerCertificate' is present on the CloudFront distribution resource
#       And: 'CloudFrontDefaultCertificate' is set to bool(true)
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'ViewerCertificate' is present on the CloudFront distribution resource
#       And: 'AcmCertificateArn' or 'IamCertificateId' are provided in the 'ViewerCertificate' configuration
#       And: 'MinimumProtocolVersion' is provided in the 'ViewerCertificate' configuration with a protocol that does not
#            support SNI (SSLv3)
#       Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'ViewerCertificate' is present on the CloudFront distribution resource
#       And: 'AcmCertificateArn' or 'IamCertificateId' are provided in the 'ViewerCertificate' configuration
#       And: 'MinimumProtocolVersion' is provided in the 'ViewerCertificate' configuration with a protocol that supports
#            SNI (TLSv1 or greater)
#       And: 'SslSupportMethod' is set to 'vip'
#      Then: FAIL
#   Scenario: 6
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'ViewerCertificate' is present on the CloudFront distribution resource
#       And: 'AcmCertificateArn' or 'IamCertificateId' are provided in the 'ViewerCertificate' configuration
#       And: 'MinimumProtocolVersion' is provided in the 'ViewerCertificate' configuration with a protocol that supports
#            SNI (TLSv1 or greater)
#       And: 'SslSupportMethod' is set to 'sni-only'
#      Then: PASS

#
# Constants
#
let CLOUDFRONT_DISTRIBUTION_TYPE = "AWS::CloudFront::Distribution"
let UNSUPPORTED_PROTOCOLS_FOR_SNI = [ "SSLv3" ]
let INPUT_DOCUMENT = this

#
# Assignments
#
let cloudfront_distributions = Resources.*[ Type == %CLOUDFRONT_DISTRIBUTION_TYPE ]

#
# Primary Rules
#
rule cloudfront_sni_enabled_check when is_cfn_template(%INPUT_DOCUMENT)
                                       %cloudfront_distributions not empty {
    check(%cloudfront_distributions.Properties)
        <<
        [CT.CLOUDFRONT.PR.7]: Require an Amazon CloudFront distribution to use SNI to serve HTTPS requests
        [FIX]: Within 'ViewerCertificate', set 'SslSupportMethod' to 'sni-only', 'MinimumProtocolVersion' to a protocol that supports SNI ('TLSv1' or greater), and 'AcmCertificateArn' to the ARN of an AWS ACM certificate.
        >>
}

rule cloudfront_sni_enabled_check when is_cfn_hook(%INPUT_DOCUMENT, %CLOUDFRONT_DISTRIBUTION_TYPE) {
    check(%INPUT_DOCUMENT.%CLOUDFRONT_DISTRIBUTION_TYPE.resourceProperties)
        <<
        [CT.CLOUDFRONT.PR.7]: Require an Amazon CloudFront distribution to use SNI to serve HTTPS requests
        [FIX]: Within 'ViewerCertificate', set 'SslSupportMethod' to 'sni-only', 'MinimumProtocolVersion' to a protocol that supports SNI ('TLSv1' or greater), and 'AcmCertificateArn' to the ARN of an AWS ACM certificate.
        >>
}

#
# Parameterized Rules
#
rule check(cloudfront_distribution) {
    %cloudfront_distribution {
        DistributionConfig exists
        DistributionConfig is_struct

        DistributionConfig {
            ViewerCertificate exists
            ViewerCertificate is_struct

            ViewerCertificate {
                CloudFrontDefaultCertificate not exists or
                CloudFrontDefaultCertificate == false

                check_custom_acm_certificate_provided(AcmCertificateArn, "AWS::CertificateManager::Certificate") or
                check_custom_iam_certificate_provided(IamCertificateId)

                MinimumProtocolVersion exists
                MinimumProtocolVersion not in %UNSUPPORTED_PROTOCOLS_FOR_SNI

                SslSupportMethod exists
                SslSupportMethod == "sni-only"
            }
        }
    }
}

rule check_custom_acm_certificate_provided(certificate, cfn_type) {
    %certificate {
        this exists
        check_is_string_and_not_empty(this) or
        check_local_references(%INPUT_DOCUMENT, this, %cfn_type)
    }
}

rule check_custom_iam_certificate_provided(certificate) {
    %certificate {
        this exists
        check_is_string_and_not_empty(this)
    }
}

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

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

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