logins_from_terminated_employees

/*
 * 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 logins_from_terminated_employees {

  meta:
    author = "Google Cloud Security"
    description = "Allowed Logins from Terminated Employees"
    rule_id = "mr_69178541-285b-45cb-b723-e2b5d88f22d3"
    rule_name = "Logins From Terminated Employees"
    assumptions = "This rule requires a context data source such as AzureAD AD Context or Workday to demonstrate the correlation of context logs with event logs"
    type = "alert"
    tags = "user enrichment"
    data_source = "microsoft windows events"
    tactic = "TA0001"
    technique = "T1078"
    severity = "Medium"
    priority = "Medium"

  events:
    $login.metadata.event_type = "USER_LOGIN"
    $login.security_result.action = "ALLOW"
    $login.target.user.userid = $user
    $login.target.user.termination_date.seconds > 0
    $login.target.user.termination_date.seconds < $login.metadata.event_timestamp.seconds

  match:
    $user over 1d

  outcome:
    $risk_score = 65
    $event_count = count_distinct($login.metadata.id)
    $employee_last_login = max($login.target.user.termination_date.seconds)
    $total_distinct_ips = count_distinct($login.principal.ip)
    // added to populate alert graph with additional context
    $principal_hostname = array_distinct($login.principal.hostname)
    $target_hostname = array_distinct($login.target.hostname)
    $principal_user_userid = array_distinct($login.principal.user.userid)
    // Commented out target.user.userid because it is already represented in graph as match variable. If match changes, can uncomment to add to results
    //$target_user_userid = array_distinct($login.target.user.userid)
    $principal_resource_name = array_distinct($login.principal.resource.name)
    $target_resource_name = array_distinct($login.target.resource.name)
    $target_url = array_distinct($login.target.url)

  condition:
    $login
}