Network HTTP Low Prevalence Domain Access

/*
 * 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 network_http_low_prevalence_domain_access {

  meta:
    author = "Google Cloud Security"
    description = "Detects network web access to a low prevalence domain"
    rule_id = "mr_c376478e-1c93-4d27-8bea-a56c3c0052f5"
    rule_name = "Network HTTP Low Prevalence Domain Access"
    type = "alert"
    tags = "prevalence"
    data_source = "zscalar"
    severity = "Critical"
    priority = "Critical"

  events:
    $http.metadata.event_type = "NETWORK_HTTP"
    $http.principal.ip = $ip
    // filter out URLs with RFC 1918 IP addresses, i.e., internal assets
    not re.regex($http.target.hostname, `(127(?:\.(25[0-5]
| 2[0-4][0-9]
| [01]?[0-9][0-9]?)){3}$)
| (10(?:\.(25[0-5]
| 2[0-4][0-9]
| [01]?[0-9][0-9]?)){3}$)
| (192\.168(?:\.(25[0-5]
| 2[0-4][0-9]
| [01]?[0-9][0-9]?)){2}$)
| (172\.(?:1[6-9]
| 2\d
| 3[0-1])(?:\.(25[0-5]
| 2[0-4][0-9]
| [01]?[0-9][0-9]?)){2})`)
    // only match valid FQDN, filter out background non-routable noise
    re.regex($http.target.hostname, `(?:[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?\.)+[a-z0-9][a-z0-9-]{0,61}[a-z0-9]`)
    $http.target.hostname = $domain_name

    $prevalence.graph.entity.domain.name = $domain_name
    // derived from events ingested by Google SecOps
    $prevalence.graph.metadata.entity_type = "DOMAIN_NAME"
    $prevalence.graph.metadata.source_type = "DERIVED_CONTEXT"
    $prevalence.graph.entity.domain.prevalence.day_count = 10
    // tune prevalence as fits your results
    $prevalence.graph.entity.domain.prevalence.rolling_max > 0
    $prevalence.graph.entity.domain.prevalence.rolling_max <= 3

  match:
    $ip over 1h

  outcome:
    $risk_score = max(
      // increment risk score based upon rolling_max prevalence
      if ( $prevalence.graph.entity.domain.prevalence.rolling_max >= 10, 10) +
      if ( $prevalence.graph.entity.domain.prevalence.rolling_max >= 2 and $prevalence.graph.entity.domain.prevalence.rolling_max <= 9 , 20) +
      if ( $prevalence.graph.entity.domain.prevalence.rolling_max = 1, 30)
    )
    $event_count = count_distinct($http.metadata.id)
    $domain_list = array_distinct($domain_name)
    $domain_count = count_distinct($domain_name)
    // added to populate alert graph with additional context
    $principal_hostname = array_distinct($http.principal.hostname)
    $target_hostname = array_distinct($http.target.hostname)
    $principal_user_userid = array_distinct($http.principal.user.userid)
    $target_url = array_distinct($http.target.url)


  condition:
    $http and $prevalence
}