Detecção de Ataques Fileless em Endpoints

Quando Usar

Use esta habilidade quando:

• Estiver criando regras de detecção para malware fileless que opera inteiramente na memória
• Estiver caçando ataques baseados em PowerShell, injeção de DLL reflexiva e abuso de WMI
• Estiver configurando telemetria de endpoint (Sysmon, AMSI, logs do PowerShell) para capturar indicadores fileless
• Estiver investigando incidentes onde o antivírus tradicional não encontrou arquivos maliciosos

Não use para detectar malware baseado em arquivos ou para engenharia reversa de malware.

Pré-requisitos

• Sysmon com criação de processos e registro de eventos WMI habilitados
• Registro de Blocos de Script do PowerShell e Registro de Módulos habilitados
• AMSI (Antimalware Scan Interface) habilitado para inspeção de conteúdo de scripts
• EDR com capacidades de detecção comportamental (MDE, CrowdStrike, SentinelOne)

Fluxo de Trabalho

Passo 1: Habilitar Telemetria Necessária

# Enable PowerShell Script Block Logging (GPO or registry)
New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ScriptBlockLogging" `
  -Name EnableScriptBlockLogging -Value 1 -PropertyType DWORD -Force

# Enable PowerShell Module Logging
New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ModuleLogging" `
  -Name EnableModuleLogging -Value 1 -PropertyType DWORD -Force

# Enable PowerShell Transcription
New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows\PowerShell\Transcription" `
  -Name EnableTranscripting -Value 1 -PropertyType DWORD -Force

# Sysmon config for fileless detection (key events):
# Event ID 1: Process creation (captures CommandLine)
# Event ID 7: Image loaded (DLL loading)
# Event ID 8: CreateRemoteThread (injection)
# Event ID 10: Process access (LSASS access)
# Event ID 19/20/21: WMI events

Passo 2: Detectar Ataques Baseados em PowerShell

# Indicators of malicious PowerShell:

# Encoded command execution
EventID: 1
CommandLine contains: "powershell" AND ("-enc" OR "-e " OR "-encodedcommand" OR "FromBase64String")

# Download cradle patterns
CommandLine contains: "IEX" AND ("Net.WebClient" OR "DownloadString" OR "Invoke-WebRequest")
CommandLine contains: "Invoke-Expression" AND "New-Object"

# AMSI bypass attempts (Event ID 4104 - Script Block)
ScriptBlock contains: ("Amsi"+"Utils") OR ("amsi"+"InitFailed") OR "SetValue.*amsi"

# Splunk query for suspicious PowerShell:
index=windows source="WinEventLog:Microsoft-Windows-PowerShell/Operational" EventCode=4104
| where match(ScriptBlockText, "(?i)(iex|invoke-expression|downloadstring|net\.webclient|frombase64|bypass|amsi.utils)")
| table _time host ScriptBlockText

Passo 3: Detectar Técnicas de Injeção de Processos

# Reflective DLL injection - loads DLL from memory without touching disk
# Detection: Sysmon Event 7 (ImageLoaded) where image path is unusual
EventID: 7
ImageLoaded NOT starts with: "C:\Windows\" AND NOT starts with: "C:\Program Files"

# Process hollowing - creates process in suspended state, replaces memory
# Detection: Process creation followed by immediate memory write
EventID: 1 + 10 correlation
# Process created then accessed with PROCESS_VM_WRITE

# APC injection - queues code to thread's async procedure call queue
# Detection: Sysmon CreateRemoteThread from non-system process
EventID: 8
SourceImage NOT IN (known_legitimate_sources)

# MDE KQL:
DeviceEvents
| where ActionType in ("CreateRemoteThreadApiCall", "NtAllocateVirtualMemoryApiCall")
| where InitiatingProcessFileName !in ("MsMpEng.exe", "svchost.exe")
| project Timestamp, DeviceName, ActionType, InitiatingProcessFileName,
    InitiatingProcessCommandLine, FileName

Passo 4: Detectar Persistência Baseada em WMI

# Sysmon Event IDs 19/20/21 for WMI events
EventID: 19  # WmiEventFilter activity detected
EventID: 20  # WmiEventConsumer activity detected
EventID: 21  # WmiEventConsumerToFilter activity detected

# Any WMI event subscription creation is suspicious unless expected
# Common malicious WMI persistence:
Consumer contains: "CommandLineEventConsumer" OR "ActiveScriptEventConsumer"

# Query for WMI subscriptions via osquery or PowerShell:
Get-WMIObject -Namespace root\Subscription -Class __EventFilter
Get-WMIObject -Namespace root\Subscription -Class __EventConsumer
Get-WMIObject -Namespace root\Subscription -Class __FilterToConsumerBinding

Passo 5: Detectar Execução Baseada em Registro

# Malware armazenado em valores de registro e executado via PowerShell
# Sysmon Event 13 - Valor de registro definido com conteúdo codificado
EventID: 13

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