Anthropic-Cybersecurity-Skills/skills/detecting-fileless-attacks-on-endpoints/SKILL.md

---
name: detecting-fileless-attacks-on-endpoints
description: >
  Detects fileless malware and in-memory attacks that execute entirely in RAM without writing
  persistent files to disk, evading traditional antivirus. Use when building detections for
  PowerShell-based attacks, reflective DLL injection, WMI persistence, and registry-resident
  malware. Activates for requests involving fileless malware detection, in-memory attacks,
  PowerShell exploitation, or living-off-the-land techniques.
domain: cybersecurity
subdomain: endpoint-security
tags: [endpoint, fileless-malware, memory-attacks, PowerShell, detection-engineering]
version: 1.0.0
author: mahipal
license: MIT
---
# Detecting Fileless Attacks on Endpoints

## When to Use

Use this skill when:
- Building detection rules for fileless malware that operates entirely in memory
- Hunting for PowerShell-based attacks, reflective DLL injection, and WMI abuse
- Configuring endpoint telemetry (Sysmon, AMSI, PowerShell logging) to capture fileless indicators
- Investigating incidents where traditional AV found no malicious files

**Do not use** for detecting file-based malware or for malware reverse engineering.

## Prerequisites

- Sysmon with process creation and WMI event logging enabled
- PowerShell Script Block Logging and Module Logging enabled
- AMSI (Antimalware Scan Interface) enabled for script content inspection
- EDR with behavioral detection capabilities (MDE, CrowdStrike, SentinelOne)

## Workflow

### Step 1: Enable Required Telemetry

```powershell
# 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
```

### Step 2: Detect PowerShell-Based Attacks

```
# 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: "AmsiUtils" OR "amsiInitFailed" 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|amsiutils)")
| 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.