# 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
Step 3: Detect Process Injection Techniques
# 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
Step 4: Detect WMI-Based Persistence
# 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
Step 5: Detect Registry-Based Execution
# Malware stored in registry values and executed via PowerShell
# Sysmon Event 13 - Registry value set with encoded content
EventID: 13
TargetObject contains: "CurrentVersion\Run"
Details: unusually long value or Base64-encoded content
# Detection query:
index=sysmon EventCode=13
| where match(Details, "[A-Za-z0-9+/=]{100,}")
| table _time host TargetObject Details Image
Key Concepts
Term
Definition
Fileless Malware
Malware that operates entirely in memory without writing executable files to disk
AMSI
Antimalware Scan Interface; Windows API allowing security products to inspect script content before execution
Reflective DLL Injection
Loading a DLL from memory rather than disk, avoiding file-based detection
Process Hollowing
Creating a legitimate process in suspended state and replacing its memory with malicious code
Script Block Logging
PowerShell logging feature that captures deobfuscated script content (Event ID 4104)
Tools & Systems
Sysmon: Kernel-level process, DLL, and WMI monitoring
AMSI: Windows script content inspection API
PowerShell Logging: Script Block, Module, and Transcription logging
Microsoft Defender for Endpoint: Behavioral detection for fileless techniques
Volatility 3: Memory forensics for post-incident fileless malware analysis
Common Pitfalls
Relying on file-based AV: Traditional AV that scans files on disk will miss fileless attacks entirely. Behavioral detection and AMSI are required.
Disabled PowerShell logging: Without Script Block Logging, deobfuscated PowerShell commands are invisible to defenders.
AMSI bypass not detected: Sophisticated attackers bypass AMSI before executing payloads. Detect AMSI bypass attempts as a high-priority alert.
Not monitoring WMI events: WMI persistence is a favored technique of APT groups. Sysmon events 19-21 must be enabled.
