# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   nacl_no_unrestricted_ssh_rdp_check
# 
# Description:
#   This control checks whether the Amazon EC2 network ACL inbound entry allows unrestricted incoming traffic ('0.0.0.0/0' or '::/0') for SSH or RDP.
# 
# Reports on:
#   AWS::EC2::NetworkAclEntry
# 
# 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 network ACL entry resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a EC2 network ACL entry resource
#       And: EC2 network ACL entry resource has no CIDR block allowing inbound traffic
#            from source '0.0.0.0/0' or '::/0'
#      Then: SKIP
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a EC2 network ACL entry resource
#       And: EC2 network ACL entry resource allows inbound traffic
#            from source '0.0.0.0/0' or '::/0'
#       And: EC2 network ACL entry resource allows all traffic
#            ('IpProtocol' is set to '-1')
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a EC2 network ACL entry resource
#       And: EC2 network ACL entry resource allows inbound traffic
#            from source '0.0.0.0/0' or '::/0'
#       And: EC2 network ACL entry resource allows TCP (protocol 6) traffic
#       And: EC2 network ACL entry resource allows traffic from a PortRange that includes 22
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a EC2 network ACL entry resource
#       And: EC2 network ACL entry resource allows inbound traffic
#            from source '0.0.0.0/0' or '::/0'
#       And: EC2 network ACL entry resource allows TCP (protocol 6) or UDP (protocol 17) traffic
#       And: EC2 network ACL entry resource allows traffic from a PortRange that includes 3389
#      Then: FAIL
#   Scenario: 6
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a EC2 network ACL entry resource
#       And: EC2 network ACL entry resource allows inbound traffic
#            from source '0.0.0.0/0' or '::/0'
#       And: EC2 network ACL entry resource allows TCP (protocol 6) traffic
#       And: EC2 network ACL entry resource allows traffic from a PortRange that excludes 22
#      Then: PASS
# Scenario: 7
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a EC2 network ACL entry resource
#       And: EC2 network ACL entry resource allows inbound traffic
#            from source '0.0.0.0/0' or '::/0'
#       And: EC2 network ACL entry resource allows TCP (protocol 6) or UDP (protocol 17) traffic
#       And: EC2 network ACL entry resource allows traffic from a PortRange that excludes 3389
#      Then: PASS

#
# Constants
#
let NETWORK_ACL_TYPE = "AWS::EC2::NetworkAclEntry"
let INPUT_DOCUMENT = this
let ALL_TRAFFIC_PROTOCOL = [-1, "-1"]
let TCP_PROTOCOL = [6, "6"]
let UDP_PROTOCOL = [17, "17"]
let UNRESTRICTED_IPV4_RANGES = ["0.0.0.0/0"]
let UNRESTRICTED_IPV6_RANGES = ["::/0"]
let SSH_PORT = 22
let RDP_PORT = 3389

#
# Assignments
#
let nacl_entries = Resources.*[ Type == %NETWORK_ACL_TYPE ]

#
# Primary Rules
#
rule nacl_no_unrestricted_ssh_rdp_check when is_cfn_template(%INPUT_DOCUMENT)
                                             %nacl_entries not empty {
    check(%nacl_entries.Properties)
        <<
        [CT.EC2.PR.5]: Require any Amazon EC2 network ACL to prevent ingress from 0.0.0.0/0 to port 22 or port 3389
        [FIX]: For Amazon EC2 network ACL entries that allow inbound connectivity on port 22 or port 3389, provide a CIDR range in 'CidrBlock' or 'Ipv6CidrBlock' that does not allow traffic from all sources.
        >>
}

rule nacl_no_unrestricted_ssh_rdp_check when is_cfn_hook(%INPUT_DOCUMENT, %NETWORK_ACL_TYPE) {
    check(%INPUT_DOCUMENT.%NETWORK_ACL_TYPE.resourceProperties)
        <<
        [CT.EC2.PR.5]: Require any Amazon EC2 network ACL to prevent ingress from 0.0.0.0/0 to port 22 or port 3389
        [FIX]: For Amazon EC2 network ACL entries that allow inbound connectivity on port 22 or port 3389, provide a CIDR range in 'CidrBlock' or 'Ipv6CidrBlock' that does not allow traffic from all sources.
        >>
}

#
# Parameterized Rules
#
rule check(nacl_entry) {
    %nacl_entry [
        # Scenario 2
        filter_allow_unrestricted_ingress(this)
    ] {
        # Scenario 3
        Protocol exists
        Protocol not in %ALL_TRAFFIC_PROTOCOL

        # Scenario 4, 6
        check_for_open_ssh(this)

        # Scenario 5, 7
        check_for_open_rdp(this)
    }
}

rule filter_allow_unrestricted_ingress(nacl_entry) {
    Egress not exists or
    Egress != true

    CidrBlock in %UNRESTRICTED_IPV4_RANGES or
    Ipv6CidrBlock in %UNRESTRICTED_IPV6_RANGES

    RuleAction == "allow"
}

rule check_for_open_ssh(nacl_entry) {
    %nacl_entry [
        Protocol in %TCP_PROTOCOL
    ] {
        check_port_range_exists(this)
        check_ports(%SSH_PORT, PortRange.From, PortRange.To)
   }
}

rule check_for_open_rdp(nacl_entry) {
    %nacl_entry [
           Protocol in %TCP_PROTOCOL or
           Protocol in %UDP_PROTOCOL
    ] {
        check_port_range_exists(this)
        check_ports(%RDP_PORT, PortRange.From, PortRange.To)
   }
}

rule check_port_range_exists(nacl_entry) {
    PortRange exists
    PortRange is_struct
    PortRange {
        From exists
        To exists
    }
}

rule check_ports(port, nacl_from_port, nacl_to_port) {
    %nacl_from_port > %port or
    %nacl_to_port < %port
}

#
# Utility Rules
#
rule is_cfn_template(doc) {
    %doc {
        AWSTemplateFormatVersion exists  or
        Resources exists
    }
}

rule is_cfn_hook(doc, NETWORK_ACL_TYPE) {
    %doc.%NETWORK_ACL_TYPE.resourceProperties exists
}