THOR Lite – Free YARA and IOC Scanner

We are proud to announce the release of THOR Lite. It is a trimmed-down version of THOR v10 with a reduced feature set and the open source signature base used in LOKI and the now obsolete scanner SPARK Core.

It uses the completely rewritten code base of THOR v10 “Fusion” and is therefore faster, more thorough and stable than SPARK.

 

As you can see in the table below, we’ve come a long way since 2012. We’ve phased out the old THOR version based on Python and SPARK in 2019. Today, we’re replacing the community version of SPARK named SPARK Core with a community version of THOR v10, named THOR Lite. 

There are two main differences between THOR Lite and THOR: 

  1. Reduced feature set
  2. Open source signature base

Apart from that, you’ll get a fully maintained and tested scanner pre-compiled for the Windows, Linux and macOS platform. A limited support is available via the issues section on the github page for auxiliary scripts.

Upgrading from SPARK Core

There is no direct upgrade path from SPARK Core, since SPARK Core and THOR Lite are completely different products and have different upgrade paths.

New users have to subscribe to the newsletter to get download links and a free license. You can subscribe and download THOR Lite using the link on the product page

SPARK Core users that already have a valid license can use the following download links to download THOR Lite:

THOR Lite for Windows
THOR Lite for Linux
THOR Lite for macOS

Important: These download packages do not include a license. You need to subscribe on the product page to receive a valid license OR use your existing SPARK Core license with THOR Lite. 

Issues

Please report problems in the issues section of THOR Lite’s helper scripts github page

Upcoming ASGARD Version 2

The last five months we’ve been working on a shiny new version of our ASGARD platform that overcomes previous limitations and includes exciting new features.

ASGARD 2 is a completely rewritten management platform, featuring a new interface, load balancing options, a new lightweight agent, custom response playbooks and greatly improved IOC management.

 

Fundamental Changes

  • Easy to use GUI and API for response functions (replaces GRR as underlying framework)
  • Rewritten agents consume much less memory
  • New dynamic agent load control allows to connect up to 25,000 endpoints
  • Predefined and custom playbooks
  • IOC management support for MISP
  • Remote consoles

IOC Management

The new IOC management allows to interface with a MISP instance and create rule sets based on filters.

For example, you can search for and select all MISP events containing the keyword “Emotet”, create a new rule set from them and then select this rule set to be used in a new THOR scan. 

Playbooks

The so-called playbooks allow you to define a set of steps that the agent executes on an end system. 

Each playbook can have up to 16 independant steps of the types “Run Command Line”, “Download File” or “Upload File”.

It is easy to set up new playbooks that e.g. download a certain tool to the endpoints, run it and collect the generated output. 

Each or all results of playbook executions can be collected via GUI or API. Playbooks can be triggered via API to allow the integration into security orchestration, automation and response (SOAR) solutions. 

ASGARD v2 ships with a set of predefined playbooks including: 

  • Collect system memory
  • Collect file or folders
  • Quarantine endpoint
  • Collect triage package
  • Collect process tree 

Remote Console

The remote console allows you to open up a web based command line window on any attached end system. This greatly facilitates the analysis of suspicious events. Analysts can browse the remote system, review or change settings and issue commands.

During the session, you can select files for collection or define certain playbooks to be executed after disconnecting the command line session.

Every session gets recorded for complete traceability.

Time Schedule

Beta customers will test drive ASGARD v2 in March and April. We expect a first release in June.

An upgrade guide for ASGARD v1 customers will be provided. 

Automated Citrix Netscaler Forensic Analysis with THOR

In this blog post I’d like to outline an idea on how to perform an automated compromise assessment on Citrix Netscaler / Citrix ADC appliances.

 

I you haven’t heard about that vulnerability yet, you should read my tweets over the past weekend or try to get a full picture with the help of this Reddit

This blog post is about a subsequent forensic analysis, not detection, not protection and contains no marketing blah-blah.

The described solution should work with the free / open scanners as well. They just don’t have that huge signature set as it is included in THOR.  

 

The reason for this is that although you’ve implemented the workaround on Monday 13th of January, you can’t trust your gateway anymore.

The vulnerability is known since December 17th and the public exploit code available since Friday the 10th of January. Nation state actors have most likely used Christmas holiday season to analyze Netscaler systems in their lab and produce an exploit, just like many offensive researchers did. Since Friday the 10th, even script kiddies can exploit this vulnerbility.

Strictly speaking, you can’t trust these gateways anymore. 

But how can we regain trust? 

A. Wait for a patched version and reinstall the gateways from the scratch

B. Perform a manual forensic analysis 

C. Perform an automated forensic analysis 

Automated Analysis with THOR

We don’t have and plan a THOR (SPARK Core or LOKI) version for FreeBSD 8.4, but we could try to mount the remote filesystem of the Netscaler gateway with SSHFS. 

DigitalOcean has a good tutorial on how to enable it on your workstation.  

mkdir ~/netscaler-mount
sudo sshfs -o allow_other,defer_permissions nsroot@citrix-gateway.nextron:/netscaler ~/netscaler-mount/

We can then use one of our scanners on that mounted volume using the `–fsonly` parameter (which activates forensic lab mode and scans directories regardless of their type). 

./thor64 --fsonly -p ~/netscaler-mount

With this method, we can detect dropped malicious templates. 

You can use the public YARA rule, that I’Ve published in LOKI’s and SPARK Core’s signature base. (note that a SPARK Core version that includes that rule hasn’t been released yet) 

Important note: this only works for templates that have been dropped since the last reboot. Users reported that dropped malicious templates get removed during a reboot. As the workaround described by Citrix requires a reboot, scanning with this single YARA rule alone won’t be a reliable method.

I recommend scanning with the full signatures set in all scanners, as they also include rules for web shells and other suspicious content in files. I’ll also extend the YARA rules on github and add further indicators. 

Additional Indicators

We can use the following YARA rule to check for suspicious strings in log files. But we don’t want to use that rule on any Linux/FreeBSD system. Therefore I won’t integrate that YARA rule in the public signature base. You can just copy it from this blog post and place it in the ‘custom-signatures/yara’ sub folder (‘/signatures’ for LOKI). 

This rule for the forensic artefacts is based on remarks made in this article by TrustedSec and shared as gist. 

First, we mount the ‘/var/log’ directory from our remote Netscaler system. 

mkdir ~/netscaler-logs
sudo sshfs -o allow_other,defer_permissions nsroot@citrix-gateway.nextron:/var/log ~/netscaler-logs/
We then start a scan on the mounted log directory.

As you can see, the scanner noticed a an issued ‘whoami’ command in the ‘bash.log’ and ‘notice.log’ files, which is a good starting point for further investigations. 

Final Thoughts

I think we’ll develop more rules for post-exploitation payloads over the coming days. Stay tuned and follow my twitter account for updates.

THOR Integration into Microsoft Defender ATP

Why Integrate THOR into Microsoft Defender ATP

While Microsoft Defender ATP fully plays off its strength in detecting live attacks, suspicious process starts and network connections, THOR shines as a live forensic scanner that scans the local filesystem, registry, logs and other elements for traces of hacking activity.

While Microsoft Defender ATP features a forensic package collection that retrieves elements from a remote system, THOR scans these elements on the remote system, applying more than 10,000 hand-written YARA rules and thousands of filename, C2, hash, mutex and named pipe IOCs to them. This live forensic scan reduces the work of your forensic analysts to a minimum and generates results as fast as possible for you to react in a timely manner. 

THOR extends Microsoft Defender ATP’s real-time monitoring by intense local scans to allow a full on-demand compromise assessment.

Deployment Options

Due to the fact that both Microsoft Defender ATP and THOR are very flexible and open products, the integration is no one-lane road with a single possible solution. Depending on the network size, segmentation and available 3rd party solutions like a SIEM the integration allows and requires different setups.

This blog post starts with an example use case and then outlines many of these setup options.

Live Response Scripts

The Microsoft Defender Security Center allows us to upload PowerShell scripts into a so called “live response library”, which is available on the endpoint during “live response” sessions.

These scripts allow us to facilitate the download and execution of THOR on the endpoint.

There are two ways to implement different scan modes and parameters. THOR has numerous command line options, which can be passed either as parameters of the PowerShell scripts or predefined in YAML config files.

Example: Turla Malware

We’ll use a simple demo script that contains a path to a file share providing the THOR package. 

It uses a config file named “rootkit-check.yml”, which is located in the program folder on the file share. It activates 3 rootkit related modules, sets the path for all output files as rebase-dir and deactivates some features. 

We upload that script into a live response session to investigate suspicious behaviour of a workstation that showed several alerts regarding a malware and the use of a “living-off-the-land” binary to run malicious code. 

The details reveal that the use of certutil.exe triggered the alert.

We can see other commands like tasklist, net and netstat, which are often used in reconnaissance scripts, executed in the context of a user named “admin”. 

We start a “Live Response Session” for further live forensic investigations with the help of THOR. 

Since this is our first investigation with that specific script, we have to upload it to the live response library. 

We can then verify the upload using the “library” command and run the script from the command line. 

It takes about a minute to complete the Rootkit check.

THOR recognized a malicious mutex used by Turla malware and gives further information on the related process and process binary, which can be used for additional verification of the threat. 

The HTML report and text log file have been saved back to the file share.

Other Setup Options

Scanner Provisioning

In this chapter we describe different methods to provide a THOR package to an end system during live response investigations.

Option A: File Share

The complete THOR package including binaries and signatures can be provided on a network share. This network share should be read-only to avoid that attackers notice the activity and manipulate the program or signatures on the file share.

Advantages:

  • Quick setup
  • Only a file server is needed

Disadvantages:

  • Requires SMB/CIFS connection from end system to file share
  • Scanner / signature updates must be scripted (thor-util.exe)
  • Manual license generation (in Nextron’s customer portal) or expensive IR license (not host-based)

Option B: ASGARD Management Center

The central management platform ASGARD Management Center is hardened Debian-based soft appliance that serves as software repository and licensing server in our use case.

The PowerShell scripts in the script library can retrieve THOR packages via HTTPS from the ASGARD Management Center.

Advantages:

  • HTTPS download of THOR packages
  • Integrated licensing
  • Automatic scanner and signature updates

Disadvantages:

  • Additional server system (VM; maintenance)

Option C: THOR via Script Library as SFX

The complete THOR program folder can be packaged into a self-extracting & executing archive (SFX), which could then be uploaded into the “live response library”. It could then be executed right from the script library (run) or uploaded to the end system (put).

Advantages:

  • No servers needed
  • Microsoft Defender ATP native solution

Disadvantages:

  • Scanner / signature updates and SFX creation must be scripted on an analyst system (thor-util.exe)
  • Manual license generation (in Nextron’s customer portal) or expensive IR license (not host-based)

Output Options

The results of the scans can be stored and transmitted to different locations.

Option A: Log and Report on File Share

THOR writes a log file in real-time during the scan and renders an HTML report at the end of the scan. Users can set an output directory other than the working directory for all output files with the “–rebase-dir” parameter.

This output folder can be a file share, e.g. “\\server\share”.

Analysts can check the log file during the scan, which takes between minutes and hours to complete.

Advantages:

  • Only a file server required

Disadvantages:

  • Requires access to file share from the end system (SMB/CIFS)
  • File share must be writable (possible manipulation by the attackers)

Option B: SYSLOG, JSON or CEF to SIEM

THOR can send the logs via SYSLOG (UDP, TCP, TCP+SSL, CEF) or in JSON (UDP, TCP, TCP+SSL) to a remote SIEM or log management system.

Advantages:

  • Integrates into existing solution and processes

Disadvantages:

  • Requires SIEM system and some base-lining
  • Requires connection to port 514 from end system to SIEM system

Option C: SYSLOG, JSON or CEF to ASGARD Analysis Cockpit

ASGARD Analysis Cockpit is the optimized log analysis platform (soft appliance) to process, baseline and forward THOR logs.

It most relevant features in this use case are:

  • Base-lining and central false positive filtering
  • Event forwarding of filtered events

ASGARD Analysis Cockpit already has several options to create alerts for incoming logs.

Similar to the current “Webhook” output, Analysis Cockpit could add a feature to connect with Microsoft Defender Security Center and create Alerts as described in the official API documentation.

Advantages:

  • Optimal THOR log base-lining and forwarding of relevant events only

Disadvantages:

  • Additional server system (VM; maintenance)
  • Requires connection to port 514 from end system to Analysis Cockpit

Option D: Local Eventlog

THOR can be instructed to log to the local Windows Eventlog with the “—eventlog” command line parameter. Customers that already forward their Windows Application Eventlog to a central SIEM could then use the existing integration and analyze the THOR events in their SIEM.

Advantages:

  • Integrates into existing security monitoring
  • No additional open port needed

Disadvantages:

  • Requires SIEM system and some base-lining

Option E: Live Response – “getfile”

Local log files that were written to the working directory can be retrieved with the “getfile” command.

Advantages:

  • Integrates into analyst workflow
  • No additional open port needed

Disadvantages:

  • Files could be left on the end system
    (causing false positives in other products; in plain sight for attackers)

Future Integrations

This chapter contains an outlook on expected future integrations based on upcoming features and APIs. 

Sample Collection

The Microsoft Defender ATP API allows to fetch a certain file from a remote system. Similar to the alerting mechanisms via Webhooks in ASGARD Analysis Cockpit, users will be able to fetch any suspicious or malicious file reported by THOR with a given minimum threat score using the Microsoft Defender ATP API. 

THOR Cloud

The upcoming cloud based version of our licensing and download server, which is currently integrated into our customer portal, will be able to serve THOR packages that contain an integrated license for the host which is supposed to be scanned

This way, you will we be able to run a PowerShell script from the live response library that downloads an up-to-date THOR package with a valid license file right from the new online service and don’t need a local ASGARD server that provides the THOR packages and licenses.

Not All IOC Scanning Is The Same

People often tell us that EDR product X already does IOC scanning and that they don’t have to check for these indicators a second time using our scanners. Especially when it comes to network wide sweeps for traces of activity due to an ongoing incident I recommend scanning a second time with one of our scanners or a tool of similar quality.

This blog post explains why.

People usually spend a fair amount of time on selecting threat intel feeds and interesting indicators for their scans. However when it comes to the actual application of these indicators they seem to be satisfied with the simplest form of checks.

Especially when we look at C2 or Filename IOCs I can easily explain the difference between the „compulsory“ and „freestyle“ methods of IOC scanning.

A plain „compulsory“ filename IOC check would walk the disk or query a database looking for a certain filename, right?

 

However if you think about it for a second and ask yourself „where else could we check for that filename?“ you’ll realize that the following elements could also contain the malicious filename:

  • Eventlog entries (e.g. process starts, service installs with image path, access failures …)
  • Log files (local Antivirus log file, access to file in web root > web server access log, backup errors, PowerShell history …)
  • Registry (recently opened files, shell bags, service image path, other caches …)
  • MFT (deleted entries)
  • Archive content (packed in ZIP file)
  • WMI (scripts – e.g. see this PoC by Matt Graeber)
  • Crash dumps
  • Windows Error Report (WER – file names and content)
  • Free disk space (filename as content of batch files or other scripts, scheduled tasks …)

Actually we often see that during lateral movement attackers access systems, run their tools remotely, copy the output, delete the output files and leave no file system traces behind. Our scanners use the locations mentioned above and others to detect them although all the files have already been removed from disk. That’s the „freestyle“ method.

The same counts for the C2 IOCs. The „compulsory“ plain method would check the system’s network connections.

The „freestyle“ method also includes checking for these C2 IOCs in the following locations:

  • Process memory (C2 strings loaded and decrypted in process memory)
  • Log files (web server access logs, Windows firewall log file, AV module log file …)
  • Hosts file
  • Files (in backdoor config files on disk)
  • Registry (hard coded C2 server in registry key)

It is sad to see great indicators from expensive feeds used into tools that do „IOC scanning“ the „compulsory“ way missing so many interesting spots.

If all you have is a hammer, everything looks like a nail.

So – the next time when someone tells you that their tool checks for IOCs on the endpoint, your question should be „How and where do you check for these IOCs?“.