GCTI Tor Exit Nodes

/*
 * 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 gcti_tor_exit_nodes {

  meta:
    author = "Google Cloud Security"
    description = "Alert traffic destined for known Tor exit nodes"
    rule_id = "mr_1d31f0e4-8cd9-4fc2-ae87-e91a435fe34f"
    rule_name = "GCTI Tor Exit Nodes"
    tactic = "TA0011"
    technique = "T1090.003"
    type = "alert"
    tags = "tor"
    data_source = "crowdstrike, microsoft sysmon, microsoft defender atp"
    severity = "High"
    priority = "High"

  events:
    $network.metadata.event_type = "NETWORK_CONNECTION"
    $network.security_result.action = "ALLOW"
    $network.target.ip = $ip

    // Tor IP listing provided by GCTI Feed
    $gcti_feed.graph.entity.artifact.ip = $ip
    $gcti_feed.graph.metadata.entity_type = "IP_ADDRESS"
    $gcti_feed.graph.metadata.threat.threat_feed_name = "Tor Exit Nodes"
    $gcti_feed.graph.metadata.product_name = "GCTI Feed"
    $gcti_feed.graph.metadata.source_type = "GLOBAL_CONTEXT"

  match:
    $ip over 1h

  outcome:
    $risk_score = 85
    $event_count = count_distinct($network.metadata.id)
    $tor_geoip_country = array_distinct($network.target.ip_geo_artifact.location.country_or_region)
    $tor_geoip_state = array_distinct($network.target.ip_geo_artifact.location.state)
    // added to populate alert graph with additional context
    $principal_ip = array_distinct($network.principal.ip)
    // Commented out target.ip because it is already represented in graph as match variable. If match changes, can uncomment to add to results
    //$target_ip = array_distinct($network.target.ip)
    $principal_process_pid = array_distinct($network.principal.process.pid)
    $principal_process_command_line = array_distinct($network.principal.process.command_line)
    $principal_process_file_sha256 = array_distinct($network.principal.process.file.sha256)
    $principal_process_file_full_path = array_distinct($network.principal.process.file.full_path)
    $principal_process_product_specific_process_id = array_distinct($network.principal.process.product_specific_process_id)
    $principal_process_parent_process_product_specific_process_id = array_distinct($network.principal.process.parent_process.product_specific_process_id)
    $target_process_pid = array_distinct($network.target.process.pid)
    $target_process_command_line = array_distinct($network.target.process.command_line)
    $target_process_file_sha256 = array_distinct($network.target.process.file.sha256)
    $target_process_file_full_path = array_distinct($network.target.process.file.full_path)
    //$target_process_product_specific_process_id = array_distinct($network.target.process.product_specific_process_id)
    //$target_process_parent_process_product_specific_process_id = array_distinct($network.target.process.parent_process.product_specific_process_id)
    $principal_user_userid = array_distinct($network.principal.user.userid)
    $target_user_userid = array_distinct($network.target.user.userid)

  condition:
    $network and $gcti_feed
}