RDP Sensitive Settings Changed to Zero

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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 rdp_sensitive_settings_changed_to_zero {

  meta:
    author = "Samir Bousseaden, David ANDRE, Roberto Rodriguez @Cyb3rWard0g, Nasreddine Bencherchali"
    description = "Detects tampering of RDP Terminal Service/Server sensitive settings. Such as allowing unauthorized users access to a system via the 'fAllowUnsolicited' or enabling RDP via 'fDenyTSConnections', etc."
    reference = "https://github.com/SigmaHQ/sigma/blob/master/rules/windows/registry/registry_set/registry_set_terminal_server_suspicious.yml"
    license = "https://github.com/SigmaHQ/Detection-Rule-License/blob/main/LICENSE.Detection.Rules.md"
    rule_name = "RDP Sensitive Settings Changed to Zero"
    rule_id = "mr_553a08a3-f1b6-4962-9393-151b0fecdf55"
    sigma_uuid = "a2863fbc-d5cb-48d5-83fb-d976d4b1743b"
    sigma_status = "test"
    tactic = "TA0005"
    //tactic = "TA0003"
    technique = "T1112"
    type = "Detection"
    data_source = "Sysmon"
    platform = "Windows"
    severity = "Medium"
    priority = "Medium"
    false_positives = "Some of the keys mentioned here could be modified by an administrator while setting group policy"

  events:
    $reg.metadata.event_type = "REGISTRY_MODIFICATION"
    (
        re.regex($reg.target.registry.registry_key, `\\fDenyTSConnections$`) nocase or
        re.regex($reg.target.registry.registry_key, `\\fSingleSessionPerUser$`) nocase or
        re.regex($reg.target.registry.registry_key, `\\UserAuthentication$`) nocase
    )
    $reg.target.registry.registry_value_data = "DWORD (0x00000000)" 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
}