# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   ecs_task_definition_nonroot_user_check
# 
# Description:
#   This control checks whether Amazon Elastic Container Service (ECS) task definitions run as a non-root user within Amazon ECS containers.
# 
# Reports on:
#   AWS::ECS::TaskDefinition
# 
# Evaluates:
#   AWS CloudFormation, AWS CloudFormation hook
# 
# Rule Parameters:
#   None
# 
# Scenarios:
#   Scenario: 1
#     Given: The input document is an AWS CloudFormation document or CloudFormation hook document
#       And: The input document does not contain an ECS task definition resource
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation document or CloudFormation hook document
#       And: The input document contains an ECS task definition resource
#       And: 'ContainerDefinitions' is not present or is an empty list
#      Then: SKIP
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation document or CloudFormation hook document
#       And: The input document contains an ECS task definition resource
#       And: 'ContainerDefinitions' property is present and is not an empty list
#       And: One or more containers defined in 'ContainerDefinitions' do not provide a 'User' property
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation document or CloudFormation hook document
#       And: The input document contains an ECS task definition resource
#       And: 'ContainerDefinitions' is present and is not an empty list
#       And: One or more containers defined in 'ContainerDefinitions' has a 'User' property set to a root user
#            value (0, 'root', '0:<group>', 'root:<group>')
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation document or CloudFormation hook document
#       And: The input document contains an ECS task definition resource
#       And: 'ContainerDefinitions' is present and is not an empty list
#       And: All containers defined in 'ContainerDefinitions' do not have a 'User' property set to a root user
#            value (0, 'root', '0:<group>', 'root:<group>')
#      Then: PASS

#
# Constants
#
let ECS_TASK_DEFINITION_TYPE = "AWS::ECS::TaskDefinition"
let INPUT_DOCUMENT = this
let ROOT_USER_PATTERNS = [ 0 , "0" , "root" , /^0:.*$/ , /^root:.*$/ ]

#
# Assignments
#
let ecs_task_definitions = Resources.*[ Type == %ECS_TASK_DEFINITION_TYPE ]

#
# Primary Rules
#
rule ecs_task_definition_nonroot_user_check when is_cfn_template(%INPUT_DOCUMENT)
                                                 %ecs_task_definitions not empty {
    check(%ecs_task_definitions.Properties)
        <<
        [CT.ECS.PR.3]: Require any Amazon ECS task definition to specify a user that is not the root
        [FIX]: Set the 'User' property to a non-root user.
        >>
}

rule ecs_task_definition_nonroot_user_check when is_cfn_hook(%INPUT_DOCUMENT, %ECS_TASK_DEFINITION_TYPE) {
    check(%INPUT_DOCUMENT.%ECS_TASK_DEFINITION_TYPE.resourceProperties)
        <<
        [CT.ECS.PR.3]: Require any Amazon ECS task definition to specify a user that is not the root
        [FIX]: Set the 'User' property to a non-root user.
        >>
}

#
# Parameterized Rules
#
rule check(ecs_task_definition) {
    %ecs_task_definition [
        filter_container_definitions_is_present(this)
    ]{
        ContainerDefinitions[*] {
            # Scenario 3
            User exists

            # Scenario 4 and 5
            User not in %ROOT_USER_PATTERNS
        }
    }
}

rule filter_container_definitions_is_present(ecs_task_definition) {
    %ecs_task_definition {
        # Scenario 2
        ContainerDefinitions exists
        ContainerDefinitions is_list
        ContainerDefinitions 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
}