AWS API Call Outside Of Organization

/*
 * Copyright 2023 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

rule aws_api_call_outside_of_organization {

  meta:
    author = "Google Cloud Security"
    description = "Detects API Calls from AWS accounts that are not part of the organization."
    rule_id = "mr_205120b7-6654-410a-ab6d-e40484e2fafb"
    rule_name = "AWS API Call Outside Of Organization"
    mitre_attack_tactic = "Defense Evasion, Persistence, Privilege Escalation, Initial Access"
    mitre_attack_technique = "Valid Accounts: Cloud Accounts"
    mitre_attack_url = "https://attack.mitre.org/techniques/T1078/004/"
    mitre_attack_version = "v13.1"
    type = "Alert"
    data_source = "AWS CloudTrail"
    platform = "AWS"
    severity = "High"
    priority = "High"

  events:
    $cloudtrail.metadata.vendor_name = "AMAZON"
    $cloudtrail.metadata.product_name = "AWS CloudTrail"
    // Requires a Reference List named aws_accounts with a list of approved and known AWS Account IDs
    not $cloudtrail.principal.user.group_identifiers in %aws_accounts
    not $cloudtrail.target.user.group_identifiers in %aws_accounts
    $cloudtrail.additional.fields["recipientAccountId"] = $accountId

  match:
    $accountId over 1h

  outcome:
    $risk_score = max(75)
    $mitre_attack_tactic = "Defense Evasion, Persistence, Privilege Escalation, Initial Access"
    $mitre_attack_technique = "Valid Accounts: Cloud Accounts"
    $mitre_attack_technique_id = "T1078.004"
    $event_count = count_distinct($cloudtrail.metadata.id)
    $network_http_user_agent = array_distinct($cloudtrail.network.http.user_agent)
    $principal_ip = array_distinct($cloudtrail.principal.ip)
    $principal_ip_country = array_distinct($cloudtrail.principal.ip_geo_artifact.location.country_or_region)
    $principal_ip_state = array_distinct($cloudtrail.principal.ip_geo_artifact.location.state)
    $principal_user_display_name = array_distinct($cloudtrail.principal.user.user_display_name)
    $recipient_aws_account_id = array_distinct($cloudtrail.additional.fields["recipientAccountId"])
    $aws_region = array_distinct($cloudtrail.principal.location.name)
    $product_event_type = array_distinct($cloudtrail.metadata.product_event_type)
    $target_resource_name = array_distinct($cloudtrail.target.resource.name)
    $target_resource_product_object_id = array_distinct($cloudtrail.target.resource.product_object_id)

  condition:
    $cloudtrail
}