Sigma Scanning with THOR

Our compromise assessment scanner THOR is able to apply Sigma rules during the local Eventlog analysis. This can help any customer that has no central SIEM system or performs a live forensic analysis on a system group that does not report to central monitoring. 

By running THOR on these systems with activated Sigma feature, THOR becomes a kind of a distributed and portable SIEM.

Since the Sigma scan module isn’t active by default, we thought it a good idea to explain how to activate an use it in the best possible way. 

Open Source Rule Set

By default THOR uses the open-source Sigma rule set with more than 500+ rules provided by the Sigma project on their Github page

Since our head of research is also one of the project maintainers, it was reasonable to combine the detection capabilities of Sigma with THOR’s scanning functionality on the endpoint. 

We comply with Sigma’s DRL (Detection Rule License) by including the rule authors in the event data produced by these rules.  

Custom Sigma Rules

You can easily add you own Sigma rules by placing them in the “./custom-signatures/sigma” sub folder.

THOR’s Sigma Config

The THOR default configuration for Sigma can be found in the Sigma repository. 

This configuration shows you, which Windows Eventlogs and Linux/Unix log files get analyzed by the Sigma module in THOR.

 

Sigma Scanning

To activate the Sigma module simply use the “–sigma” flag (or “sigma: True” in a YML config file).

You can start a THOR scan that analyzes the local Eventlog and activates the Sigma feature with:

thor64.exe -a Eventlog --sigma

To run a Sigma scan on a single Eventlog e.g. Sysmon’s log, use the “-n” flag.

thor64.exe -a Eventlog --sigma -n "Microsoft-Windows-Sysmon/Operational"

To include the Sigma feature in a standard THOR scan and check only the last 3 days of the Windows Eventlogs to reduce the scan duration, use:

thor64.exe --sigma -lookback 3

Sigma Matches

Once a Sigma rule matches on a log entry, you’ll see it listed in one of the REASON’s that lead to the classification of an event. 

The following example shows the detection of a China Chopper (Caidao) ASP web shell. That web shell has been detected by multiple Sigma rules. 

  1. Webshell Detection With Command Line Keywords 
  2. Shells Spawned by Web Servers
  3. Whoami Execution

Getting Started

Since this feature isn’t available in THOR Lite, please contact us via the “Get Started” button in the upper right corner and get a free trial voucher. Most customers that use THOR with Sigma choose one of our THOR license packs, especially the SOC Toolkit Pack, which was geared to the needs of today’s SOC teams. 

New VALHALLA Web Features

The newest update of our popular YARA rule feed named VALHALLA adds new features to its web interface.

The most awaited new feature is a keyword search that allows you to query the database for certain keywords, rule names, reports, MITRE ATT&CK ids or tags.

The result page shows you if VALHALLA already has related rules in its database. 

 

Keyword Search

The search results show all rules in our database related to the search keyword.

You can see the rule name, description, the rule date, a reference URL and a set of links.

The new search function helps you to determine if VALHALLA and THOR already contain rules for a given report or threat. 

New Links

We have integrated new links that lead you to:

  1. the reference listed in the rule (report, source)
  2. a Virustotal lookup for that rule / sample
  3. a detailed info page for that specific rule

Rule Info Pages

The rule info page contains all the details to a certain rule. These include all metadata values liks score, tags, reference links, required YARA version and modules, the rule date and the average AV detection ratio.

Two additional tables include all antivirus verdicts for samples on which that rule has matched and a list of all observed samples with links to Virustotal. 

 

Community Rule Info

We’ve also added notes on the 2400+ rules that are available as open source in the signature-base repository on github, e.g. try SUSP_LNK_Big_Link_File.

Category Counts

A new table on the start page informs users about the rules per subscribable category. 

Also note that queries of any type to Valhalla are rate limited. Too many requests in a relatively short time frame will lead to complete blocks as well as a high amount of requests over a longe time period and other suspicious activity. Customers can get their source IP addresses whitelisted on request. 

The new version will be deployed in the coming days.

Web Proxy Event Analysis Cheat Sheet

The “Web Proxy Event Analysis Cheat Sheet” can help SOCs and security analysts classify proxy events (blocks, alerts) and is based on my ideas and many ideas from experts that helped me collect detection ideas for this document.

You can download version 1.0 here.

We also recommend checking Sigma’s “proxy” section for detection rules that can be used to detect threats in web proxy or similar logs as long as they contain web connection information (EDR, HIDS etc.).

 

Web Proxy Event Analysis Cheat Sheet