Trawler - PowerShell Script To Help Incident Responders Discover Adversary Persistence Mechanisms

What is it?

Trawler is a PowerShell script designed to help Incident Responders discover potential indicators of compromise on Windows hosts, primarily focused on persistence mechanisms including Scheduled Tasks, Services, Registry Modifications, Startup Items, Binary Modifications and more.

Currently, trawler can detect most of the persistence techniques specifically called out by MITRE and Atomic Red Team with more detections being added on a regular basis.

Main Features

Scanning Windows OS for a variety of persistence techniques (Listed below)
CSV Output with MITRE Technique and Investigation Jumpstart Metadata
Analysis and Remediation Guidance Documentation (https://github.com/joeavanzato/Trawler/wiki/Analysis-and-Remediation-Guidance)
Dynamic Risk Assignment for each detection
Built-in Allow Lists for common Windows configurations spanning Windows 10/Server 2012|2016|2019|2022 to reduce noise
Capture persistence metadata from 'golden' enterprise image for use as a dynamic allow-list at runtime
Analyze mounted disk images via drive re-targeting

How do I use it?

Just download and run trawler.ps1 from an Administrative PowerShell/cmd prompt - any detections will be displayed in the console as well as written to a CSV ('detections.csv') in the current working directory. The generated CSV will contain Detection Name, Source, Risk, Metadata and the relevant MITRE Technique.

Or use this one-liner from an Administrative PowerShell terminal:

iex ((New-Object System.Net.WebClient).DownloadString('https://raw.githubusercontent.com/joeavanzato/Trawler/main/trawler.ps1'))

Certain detections have allow-lists built-in to help remove noise from default Windows configurations (10/2016/2019/2022) - expected Scheduled Tasks, Services, etc. Of course, it is always possible for attackers to hijack these directly and masquerade with great detail as a default OS process - take care to use multiple forms of analysis and detection when dealing with skillful adversaries.

If you have examples or ideas for additional detections, please feel free to submit an Issue or PR with relevant technical details/references - the code-base is a little messy right now and will be cleaned up over time.

Additionally, if you identify obvious false positives, please let me know by opening an issue or PR on GitHub! The obvious culprits for this will be non-standard COMs, Services or Tasks.

CLI Parameters

-scanoptions : Tab-through possible detections and select a sub-set using comma-delimited terms (eg. .\trawler.ps1 -scanoptions Services,Processes)
-hide : Suppress Detection output to console
-snapshot : Capture a "persistence snapshot" of the current system, defaulting to "$PSScriptRoot\snapshot.csv"
-snapshotpath : Define a custom file-path for saving snapshot output to.
-outpath : Define a custom file-path for saving detection output to (defaults to "$PSScriptRoot\detections.csv")
-loadsnapshot : Define the path for an existing snapshot file to load as an allow-list reference
-drivetarget : Define the variable for a mounted target drive (eg. .\trawler.ps1 -targetdrive "D:") - using this alone leads to an 'assumed homedrive' variable of C: for analysis purposes

What separates this from PersistenceSniper?

PersistenceSniper is an awesome tool - I've used it heavily in the past - but there are a few key points that differentiate these utilities

trawler is (currently) a local utility - it would be pretty straight-forward to wrap it in a loop and use WinRM/PowerShell Sessions to execute it on remote hosts though
trawler implements allow-listing for many 'noisy' detections to help remove expected detections from default configurations of Windows (10/2016/2019/2022) and these are constantly being updated
- PersistenceSniper (for the most part) does not contain any type of allow-listing - therefore, there is more noise generated when considering items such as Services, Scheduled Tasks, general COM DLL scanning, etc.
trawler's output is much more simplified - Name, Risk, Source, MITRE Technique and Metadata are the only items provided for each detection to help analysts jump-start their persistence hunting efforts
Regex is used in many checks to help detect 'suspicious' keywords or patterns in various critical areas including scanned file contents, registry values, etc.
trawler supports 'snapshotting' a system (for example, an enterprise golden image) then using the generated snapshot as an allow-list to reduce noise.
trawler supports 'drive-retargeting' to check dead-boxes mounted to an analysis machine.

Overall, these tools are extremely similar but approach the problem from slightly different angles - PersistenceSniper provides all information back to the analyst for review while Trawler tries to limit what is returned to only results that are likely to be potential adversary persistence mechanisms. As such, there is a possibility for false-negatives with trawler if an adversary completely mimics an allow-listed item.

Tuning to your environment

Trawler supports loading an allow-list from a 'snapshot' - to do this requires two steps.

Run '.\trawler.ps1 -snapshot' on a "Golden Image" representing the servers in your environment - once complete, in addition to the standard 'detections.csv' a file named 'snapshots.csv' will be generated
This file can then be used as input to trawler when running on other hosts and the data will be loaded dynamically as an allow-list for each appropriate detection
1. '.\trawler.ps1' -loadsnapshot "path\to\snapshot.csv"

That's it - all relevant detections will then draw from the snapshot file as an allow-list to reduce noise and identify any potential changes to the base image that may have occurred.

(Allow-listing is implemented for most of the checks but not all - still being actively implemented)

Drive ReTargeting

Often during an investigation, analysts may end up mounting a new drive that represents an imaged Windows device - Trawler now partially supports scanning these mounted drives through the use of the '-drivetarget' parameter.

At runtime, Trawler will re-target temporary script-level variables for use in checking file-based artifacts and also will attempt to load relevant Registry Hives (HKLM\SOFTWARE, HKLM\SYSTEM, NTUSER.DATs, USRCLASS.DATs) underneath HKLM/HKU and prefixed by 'ANALYSIS_'. Trawler will also attempt to unload these temporarily loaded hives upon script completion.

As an example, if you have an image mounted at a location such as 'F:\Test' which contains the NTFS file system ('F:\Test\Windows', 'F:\Test\User', etc) then you can invoke trawler like below;

.\trawler.ps1 -drivetarget "F:\Test"

Please note that since trawler attempts to load the registry hive files from the drive in question, mapping a UNC path to a live remote device will NOT work as those files will not be accessible due to system locks. I am working on an approach which will handle live remote devices, stay tuned.

What is not inspected when drive retargeting?

Running Processes
Network Connections
'Phantom' DLLs
WMI Consumers (Being worked on)
BITS Jobs (Being worked on)
Certificate Parsing (Being worked on)

Most other checks will function fine because they are based entirely on reading registry hives or file-based artifacts (or can be converted to do so, such as directly reading Task XML as opposed to using built-in command-lets.)

Any limitations in checks when doing drive-retargeting will be discussed more fully in the GitHub Wiki.

Example Images

What is inspected?

Scheduled Tasks
Users
Services
Running Processes
Network Connections
WMI Event Consumers (CommandLine/Script)
Startup Item Discovery
BITS Jobs Discovery
Windows Accessibility Feature Modifications
PowerShell Profile Existence
Office Addins from Trusted Locations
SilentProcessExit Monitoring
Winlogon Helper DLL Hijacking
Image File Execution Option Hijacking
RDP Shadowing
UAC Setting for Remote Sessions
Print Monitor DLLs
LSA Security and Authentication Package Hijacking
Time Provider DLLs
Print Processor DLLs
Boot/Logon Active Setup
User Initialization Logon Script Hijacking
ScreenSaver Executable Hijacking
Netsh DLLs
AppCert DLLs
AppInit DLLs
Application Shimming
COM Object Hijacking
LSA Notification Hijacking
'Office test' Usage
Office GlobalDotName Usage
Terminal Services DLL Hijacking
Autodial DLL Hijacking
Command AutoRun Processor Abuse
Outlook OTM Hijacking
Trust Provider Hijacking
LNK Target Scanning (Suspicious Terms, Multiple Extensions, Multiple EXEs)
'Phantom' Windows DLL Names loaded into running process (eg. un-signed WptsExtensions.dll)
Scanning Critical OS Directories for Unsigned EXEs/DLLs
Un-Quoted Service Path Hijacking
PATH Binary Hijacking
Common File Association Hijacks and Suspicious Keywords
Suspicious Certificate Hunting
GPO Script Discovery/Scanning
NLP Development Platform DLL Overrides
AeDebug/.NET/Script/Process/WER Debug Replacements
Explorer 'Load'
Windows Terminal startOnUserLogin Hijacks
App Path Mismatches
Service DLL/ImagePath Mismatches
GPO Extension DLLs
Potential COM Hijacks
Non-Standard LSA Extensions
DNSServerLevelPluginDll Presence
Explorer\MyComputer Utility Hijack
Terminal Services InitialProgram Check
RDP Startup Programs
Microsoft Telemetry Commands
Non-Standard AMSI Providers
Internet Settings LUI Error DLL
PeerDist\Extension DLL
ErrorHandler.CMD Checks
Built-In Diagnostics DLL
MiniDumpAuxiliary DLLs
KnownManagedDebugger DLLs
WOW64 Compatibility Layer DLLs
EventViewer MSC Hijack
Uninstall Strings Scan
PolicyManager DLLs
SEMgr Wallet DLL
WER Runtime Exception Handlers
HTML Help (.CHM)
Remote Access Tool Artifacts (Files, Directories, Registry Keys)
ContextMenuHandler DLL Checks
Office AI.exe Presence
Notepad++ Plugins
MSDTC Registry Hijacks
Narrator DLL Hijack (MSTTSLocEnUS.DLL)
Suspicious File Location Checks

TODO

Add Analysis/Remediation Guidance to each detection in the GitHub Wiki (In-Progress)
Browser Extension Analysis (?)
RID Hijacking [https://www.ired.team/offensive-security/persistence/rid-hijacking][https://pentestlab.blog/2020/02/12/persistence-rid-hijacking/]
PowerAutomate Checks
ShadowPad Indicators [https://www.secureworks.com/research/shadowpad-malware-analysis, https://www.hexacorn.com/blog/2023/02/25/beyond-good-ol-run-key-part-141/]
OBS Startup Script Scanning [https://www.hexacorn.com/blog/2023/04/14/beyond-good-ol-run-key-part-142/]
SQL Server Management Addins [https://www.hexacorn.com/blog/2019/09/28/beyond-good-ol-run-key-part-117/]
AutoPlay Handler Inspection [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\AutoplayHandlers\Handlers]
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup\Pending\SPReviewEnabler
OCSetup [https://www.hexacorn.com/blog/2019/11/09/beyond-good-ol-run-key-part-122/]
Review https://hijacklibs.net/# for additional opportunities
Review https://www.wietzebeukema.nl/blog/hijacking-dlls-in-windows for additional opportunities
Review https://silentrunners.org/launchpoints.html for additional opportunities

MITRE Techniques Evaluated

Please be aware that some of these are (of course) more detected than others - for example, we are not detecting all possible registry modifications but rather inspecting certain keys for obvious changes and using the generic MITRE technique "Modify Registry" where no other technique is applicable. For other items such as COM hijacking, we are inspecting all entries in the relevant registry section, checking against 'known-good' patterns and bubbling up unknown or mismatched values, resulting in a much more complete detection surface for that particular technique.

T1037: Boot or Logon Initialization Scripts
T1037.001: Boot or Logon Initialization Scripts: Logon Script (Windows)
T1037.005: Boot or Logon Initialization Scripts: Startup Items
T1055.001: Process Injection: Dynamic-link Library Injection
T1059: Command and Scripting Interpreter
T1071: Application Layer Protocol
T1098: Account Manipulation
T1112: Modify Registry
T1053: Scheduled Task/Job
T1136: Create Account
T1137.001: Office Application Office Template Macros
T1137.002: Office Application Startup: Office Test
T1137.006: Office Application Startup: Add-ins
T1197: BITS Jobs
T1505.005: Server Software Component: Terminal Services DLL
T1543.003: Create or Modify System Process: Windows Service
T1546: Event Triggered Execution
T1546.001: Event Triggered Execution: Change Default File Association
T1546.002: Event Triggered Execution: Screensaver
T1546.003: Event Triggered Execution: Windows Management Instrumentation Event Subscription
T1546.007: Event Triggered Execution: Netsh Helper DLL
T1546.008: Event Triggered Execution: Accessibility Features
T1546.009: Event Triggered Execution: AppCert DLLs
T1546.010: Event Triggered Execution: AppInit DLLs
T1546.011: Event Triggered Execution: Application Shimming
T1546.012: Event Triggered Execution: Image File Execution Options Injection
T1546.013: Event Triggered Execution: PowerShell Profile
T1546.015: Event Triggered Execution: Component Object Model Hijacking
T1547.002: Boot or Logon Autostart Execution: Authentication Packages
T1547.003: Boot or Logon Autostart Execution: Time Providers
T1547.004: Boot or Logon Autostart Execution: Winlogon Helper DLL
T1547.005: Boot or Logon Autostart Execution: Security Support Provider
T1547.009: Boot or Logon Autostart Execution: Shortcut Modification
T1547.012: Boot or Logon Autostart Execution: Print Processors
T1547.014: Boot or Logon Autostart Execution: Active Setup
T1553: Subvert Trust Controls
T1553.004: Subvert Trust Controls: Install Root Certificate
T1556.002: Modify Authentication Process: Password Filter DLL
T1574: Hijack Execution Flow
T1574.007: Hijack Execution Flow: Path Interception by PATH Environment Variable
T1574.009: Hijack Execution Flow: Path Interception by Unquoted Path

References

This tool would not exist without the amazing InfoSec community - the most notable references I used are provided below.