Modify User Shell Folders Startup Value

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 * 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 modify_user_shell_folders_startup_value {

  meta:
    author = "frack113"
    description = "Detect modification of the startup key to a path where a payload could be stored to be launched during startup"
    reference = "https://github.com/SigmaHQ/sigma/blob/master/rules/windows/registry/registry_set/registry_set_susp_user_shell_folders.yml"
    license = "https://github.com/SigmaHQ/Detection-Rule-License/blob/main/LICENSE.Detection.Rules.md"
    rule_name = "Modify User Shell Folders Startup Value"
    rule_id = "mr_bf813e89-427f-46e4-a671-bb3eea037725"
    sigma_uuid = "9c226817-8dc9-46c2-a58d-66655aafd7dc"
    sigma_status = "test"
    tactic = "TA0003"
    //tactic = "TA0004"
    technique = "T1547.001"
    type = "Detection"
    data_source = "Sysmon Windows Registry"
    platform = "Windows"
    severity = "High"
    priority = "High"
    false_positives = "Unknown"

  events:
    $reg.metadata.event_type = "REGISTRY_MODIFICATION"
    strings.contains(strings.to_lower($reg.target.registry.registry_key), "software\\microsoft\\windows\\currentversion\\explorer\\user shell folders")
    re.regex($reg.target.registry.registry_key, `Startup$`) nocase

    $reg.principal.hostname = $hostname

  match:
    $hostname over 5m

  outcome:
    //example usage of specifying test user and hostname to adjust risk score
    $risk_score = max(if($reg.principal.user.userid = "user" and $reg.principal.hostname = "hostname", 0, 15))
    $principal_hostname = array_distinct($reg.principal.hostname)
    $principal_process_pid = array_distinct($reg.principal.process.pid)
    $principal_process_file_full_path = array_distinct($reg.principal.process.file.full_path)
    $principal_process_product_specific_process_id = array_distinct($reg.principal.process.product_specific_process_id)
    $principal_user_userid = array_distinct($reg.principal.user.userid)
    $target_registry_key = array_distinct($reg.target.registry.registry_key)
    $target_registry_value_data = array_distinct($reg.target.registry.registry_value_data)
    $log_type = array_distinct(strings.concat($reg.metadata.log_type,"/",$reg.metadata.product_event_type))

  condition:
    $reg
}