Base64 Encoded PowerShell Command Detected

// Copyright 2025 Google LLC. Licensed under Apache-2.0.

rule base64_encoded_powershell_command_detected {

  meta:
    author = "Florian Roth (Nextron Systems)"
    description = "Detects usage of the \"FromBase64String\" function in the commandline which is used to decode a base64 encoded string"
    reference = "https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_powershell_frombase64string.yml"
    license = "https://github.com/SigmaHQ/Detection-Rule-License/blob/main/LICENSE.Detection.Rules.md"
    rule_name = "Base64 Encoded PowerShell Command Detected"
    sigma_uuid = "e32d4572-9826-4738-b651-95fa63747e8a"
    sigma_status = "test"
    rule_id = "mr_a3603334-35e3-4050-b94e-f79aeccb899a"
    tactic = "TA0002"
    technique = "T1059.001"
    type = "Detection"
    data_source = "Sysmon"
    platform = "Windows"
    severity = "High"
    priority = "High"
    false_positives = "Administrative script libraries"

 events:
   $process.metadata.event_type = "PROCESS_LAUNCH"
   strings.contains(strings.to_lower($process.target.process.command_line), "::frombase64string(")

   $process.principal.hostname = $hostname

 match:
   $hostname over 5m

 outcome:
    //example usage of specifying test user and hostname to adjust risk score
    $risk_score = max(if($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
}