# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   alb_desync_mode_check
# 
# Description:
#   This control checks to ensure that an application load balancer is configured with 'defensive' or 'strictest' desync mitigation mode.
# 
# 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 been specified on the ELBv2 load balancer resource
#       And: The 'LoadBalancerAttribute' 'routing.http.desync_mitigation_mode' has been provided
#            and is not one of 'defensive' or 'strictest'
#      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 not been specified on the ELBv2 load balancer resource or specified
#            as an empty list
#      Then: PASS
#   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: 'routing.http.desync_mitigation_mode' has not been provided as a 'LoadBalancerAttribute'
#      Then: PASS
#   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.desync_mitigation_mode' has been provided
#            and is one of 'defensive' or 'strictest'
#      Then: PASS

#
# Constants
#
let ELASTIC_LOAD_BALANCER_V2_TYPE = "AWS::ElasticLoadBalancingV2::LoadBalancer"
let ALLOWED_DESYNC_MODES = [ "defensive", "strictest" ]
let INPUT_DOCUMENT = this

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

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

    check(%elastic_load_balancers.Properties)
        <<
        [CT.ELASTICLOADBALANCING.PR.3]: Require any application load balancer to have defensive or strictest desync mitigation mode activated
        [FIX]: Omit the load balancer attribute 'routing.http.desync_mitigation_mode' or set the attribute to one of 'defensive' or 'strictest'.
        >>

}

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

    check(%INPUT_DOCUMENT.%ELASTIC_LOAD_BALANCER_V2_TYPE.resourceProperties)
        <<
        [CT.ELASTICLOADBALANCING.PR.3]: Require any application load balancer to have defensive or strictest desync mitigation mode activated
        [FIX]: Omit the load balancer attribute 'routing.http.desync_mitigation_mode' or set the attribute to one of 'defensive' or 'strictest'.
        >>
}

#
# Parameterized Rules
#
rule check(elastic_load_balancer) {
    %elastic_load_balancer[
        # Scenario 2
        Type == "application"
    ] {
        # Scenario 4
        LoadBalancerAttributes not exists or
        check_application_load_balancer_attributes(this)
    }
}

rule check_application_load_balancer_attributes(application_load_balancer) {
    %application_load_balancer {
        LoadBalancerAttributes is_list
        LoadBalancerAttributes[
            # Scenario 5
            Key exists
            Key == "routing.http.desync_mitigation_mode"
        ] {
            # Scenarios 3 and 6
            Value exists
            Value in %ALLOWED_DESYNC_MODES
        }

    }
}

#
# 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
}