Windows Event Log Cleared

/*
 * 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 windows_event_log_cleared {

  meta:
    author = "Google Cloud Security"
    description = "Detects the clearing of event logs within the Windows Event Viewer. "
    rule_id = "mr_067f3ee9-9bc6-400d-9ba5-bebe7253482f"
    rule_name = "Windows Event Log Cleared"
    type = "alert"
    data_source = "microsoft sysmon, microsoft windows events"
    platform = "Windows"
    tactic = "TA0005"
    technique = "T1070.001"
    severity = "Medium"
    priority = "Medium"

  events:
    (
        $process.metadata.event_type = "PROCESS_LAUNCH" and
        // currently the command line is focused on the three primary log sets, could expand this further if desired
        re.regex($process.target.process.command_line, `wevtutil.*cl.*(system
| application
| security)`) nocase
    )
    or
    (
        $process.metadata.event_type = "SYSTEM_AUDIT_LOG_WIPE" and
        $process.metadata.vendor_name = "Microsoft"
    )
    or
    (
        $process.metadata.event_type = "STATUS_UPDATE" and
        $process.metadata.product_name = "PowerShell" and
        $process.metadata.vendor_name = "Microsoft" and
        (
            re.regex($process.security_result.description, `Remove-EventLog`) nocase or
            re.regex($process.security_result.description, `Clear-EventLog`) nocase
        )
    )
    $process.principal.hostname = $hostname

  match:
    $hostname over 5m

  outcome:
    $risk_score = 65
    $event_count = count_distinct($process.metadata.id)
    $process_description = array_distinct($process.metadata.description)
    // added to populate alert graph with additional context
    // Commented out principal.hostname because it is already represented in graph as match variable. If match changes, can uncomment to add to results
    //$principal_hostname = array_distinct($process.principal.hostname)
    $principal_process_pid = array_distinct($process.principal.process.pid)
    $principal_process_command_line = array_distinct($process.principal.process.command_line)
    $principal_process_file_sha256 = array_distinct($process.principal.process.file.sha256)
    $principal_process_file_full_path = array_distinct($process.principal.process.file.full_path)
    $principal_process_product_specific_process_id = array_distinct($process.principal.process.product_specific_process_id)
    $principal_process_parent_process_product_specific_process_id = array_distinct($process.principal.process.parent_process.product_specific_process_id)
    $target_process_pid = array_distinct($process.target.process.pid)
    $target_process_command_line = array_distinct($process.target.process.command_line)
    $target_process_file_sha256 = array_distinct($process.target.process.file.sha256)
    $target_process_file_full_path = array_distinct($process.target.process.file.full_path)
    $target_process_product_specific_process_id = array_distinct($process.target.process.product_specific_process_id)
    $principal_user_userid = array_distinct($process.principal.user.userid)

  condition:
    $process
}