MITRE ATT&CK T1140 Encoded Powershell Command

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 * Copyright 2023 Google LLC
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 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
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 *     https://www.apache.org/licenses/LICENSE-2.0
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 * 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.
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rule mitre_attack_T1140_encoded_powershell_command {

  meta:
    author = "Google Cloud Security"
    description = "Detects encoded powershell commands"
    rule_id = "mr_c01d7d92-db49-4044-a2c5-5a16c74a141d"
    rule_name = "MITRE ATT&CK T1140 Encoded Powershell Command"
    tactic = "TA0005"
    technique = "T1140"
    type = "hunt"
    platform = "Windows"
    data_source = "microsoft sysmon, microsoft windows events"
    severity = "Info"
    priority = "Info"

  events:
    $process.metadata.event_type = "PROCESS_LAUNCH"
    $process.principal.hostname = $hostname
    re.regex($process.target.process.file.full_path, `(system32|syswow64)\\WindowsPowerShell\\v1\.0\\powershell(|\_ise)\.exe`) nocase
    re.regex($process.target.process.command_line, `(?i)(?:-enc|-ec|-en)\s*\S*`)
    $encoded_value = re.capture($process.target.process.command_line, `(?i)(?:-enc|-ec|-en)\s*(\S*)`)
    $decoded_value = re.replace(strings.base64_decode(re.capture($process.target.process.command_line, `(?i)(?:-enc|-ec|-en)\s*(\S*)`)),`\0`, "")

  match:
    $hostname over 5m

  outcome:
    $risk_score = 10
    $event_count = count_distinct($process.metadata.id)
    $encoded_powershell = array_distinct($encoded_value)
    $decoded_powershell = array_distinct($decoded_value)
    // 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
}