Suspicious Download Via Certutil.EXE

/*
 * Copyright 2025 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 suspicious_download_via_certutil_exe {

  meta:
    author = "Florian Roth (Nextron Systems), Jonhnathan Ribeiro, oscd.community, Nasreddine Bencherchali (Nextron Systems)"
    description = "Detects the execution of certutil with certain flags that allow the utility to download files"
    reference = "https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_certutil_download.yml"
    license = "https://github.com/SigmaHQ/Detection-Rule-License/blob/main/LICENSE.Detection.Rules.md"
    rule_name = "Suspicious Download Via Certutil.EXE"
    sigma_uuid = "19b08b1c-861d-4e75-a1ef-ea0c1baf202b"
    sigma_status = "test"
    le_id = "mr_e9ec6964-4883-47b8-a6b3-2ece9962a813"
    tactic = "TA0005"
    technique = "T1027"
    type = "Detection"
    data_source = "Sysmon"
    platform = "Windows"
    severity = "Medium"
    priority = "Medium"
    false_positives = "Unknown"

  events:
    $process.metadata.event_type = "PROCESS_LAUNCH"
    (
        re.regex($process.target.process.file.full_path, `\\certutil\.exe$`) nocase or
        $process.src.process.file.full_path = "certutil.exe" nocase
    )
    (
        strings.contains(strings.to_lower($process.target.process.command_line), "urlcache ") or
        strings.contains(strings.to_lower($process.target.process.command_line), "verifyctl ")
    )
    strings.contains(strings.to_lower($process.target.process.command_line), "http")
    $process.principal.hostname = $hostname

  match:
    $hostname over 5m

  outcome:
      $risk_score = max(if($process.principal.user.userid = "user" and $process.principal.hostname = "hostname", 0, 15))
      $principal_process_pid = array_distinct($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)
      $principal_process_product_specfic_process_id = array_distinct($process.principal.process.product_specific_process_id)
      $principal_process_parent_process_product_specfic_process_id = array_distinct($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)
      $target_process_product_specfic_process_id = array_distinct($process.target.process.product_specific_process_id)
      $principal_user_userid = array_distinct($process.principal.user.userid)
      $log_type = array_distinct(strings.concat($process.metadata.log_type,"/",$process.metadata.product_event_type))

  condition:
    $process
}