# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   alb_http_drop_invalid_header_enabled_check
# 
# Description:
#   This control checks whether application load balancers are configured to drop non-valid HTTP headers.
# 
# Reports on:
#   AWS::ElasticLoadBalancingV2::LoadBalancer
# 
# 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 ELBv2 load balancer resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ELBv2 load balancer resource
#       And: 'Type' is set to a value other than 'application'
#      Then: SKIP
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ELBv2 load balancer resource
#       And: 'Type' is set to 'application' for the ELBv2 load balancer resource
#       And: 'LoadBalancerAttributes' have not been specified on the ELBv2 load balancer resource
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ELBv2 load balancer resource
#       And: 'Type' is set to 'application' for the ELBv2 load balancer resource
#       And: 'LoadBalancerAttributes' have been specified on the ELBv2 load balancer resource
#       And: 'routing.http.drop_invalid_header_fields.enabled' has not been provided as a 'LoadBalancerAttribute'
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ELBv2 load balancer resource
#       And: 'Type' is set to 'application' for the ELBv2 load balancer resource
#       And: 'LoadBalancerAttributes' have been specified on the ELBv2 load balancer resource
#       And: The 'LoadBalancerAttribute' 'routing.http.drop_invalid_header_fields.enabled' has been provided
#            and is set to bool(false) or string(false)
#      Then: FAIL
#   Scenario: 6
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ELBv2 load balancer resource
#       And: 'Type' is set to 'application' for the ELBv2 load balancer resource
#       And: 'LoadBalancerAttributes' have been specified on the ELBv2 load balancer resource
#       And: The 'LoadBalancerAttribute' 'routing.http.drop_invalid_header_fields.enabled' has been provided and
#            is set to bool(true) or string(true)
#      Then: PASS

#
# Constants
#
let ELASTIC_LOAD_BALANCER_V2_TYPE = "AWS::ElasticLoadBalancingV2::LoadBalancer"
let INPUT_DOCUMENT = this

#
# Assignments
#
let elastic_load_balancers = Resources.*[ Type == %ELASTIC_LOAD_BALANCER_V2_TYPE ]

#
# Primary Rules
#
rule alb_http_drop_invalid_header_enabled_check when is_cfn_template(%INPUT_DOCUMENT)
                                                     %elastic_load_balancers not empty {

    check(%elastic_load_balancers.Properties)
        <<
        [CT.ELASTICLOADBALANCING.PR.4]: Require that any application load balancer must be configured to drop HTTP headers
        [FIX]: Set the load balancer attribute 'routing.http.drop_invalid_header_fields.enabled' to 'true'.
        >>
}

rule alb_http_drop_invalid_header_enabled_check when is_cfn_hook(%INPUT_DOCUMENT, %ELASTIC_LOAD_BALANCER_V2_TYPE) {

    check(%INPUT_DOCUMENT.%ELASTIC_LOAD_BALANCER_V2_TYPE.resourceProperties)
        <<
        [CT.ELASTICLOADBALANCING.PR.4]: Require that any application load balancer must be configured to drop HTTP headers
        [FIX]: Set the load balancer attribute 'routing.http.drop_invalid_header_fields.enabled' to 'true'.
        >>
}

#
# Parameterized Rules
#
rule check(elastic_load_balancer) {
    %elastic_load_balancer[ Type == "application" ] {
        # Scenario 2
        LoadBalancerAttributes exists
        LoadBalancerAttributes is_list
        LoadBalancerAttributes not empty

        # Scenario 3, 4 and 5
        some LoadBalancerAttributes[*] {
            Key exists
            Value exists

            Key == "routing.http.drop_invalid_header_fields.enabled"
            Value in [ true, "true" ]
        }
    }
}

#
# 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
}