3.12. Sigma

THOR and Aurora are using Sigma in order to improve detections.

3.12.1. What is Sigma

From the project website:

Sigma is a generic and open signature format that allows you to describe relevant log events in a straightforward manner. The rule format is very flexible, easy to write and applicable to any type of log file. The main purpose of this project is to provide a structured form in which researchers or analysts can describe their once developed detection methods and make them shareable with others.

Sigma is for log files what Snort is for network traffic and YARA is for files.

3.12.2. Creating a Ruleset

Rulesets are used to group rules to manageable units. As an asset can only have one service configuration, rulesets are used to determine which rules are used in which service configuration. There exist default rulesets for high and critical Sigma rules. If you want to create a custom ruleset go to Service Control > Sigma > Rulesets > Create Ruleset.

Create a Ruleset

If you have chosen that new Sigma rules should be added automatically to the new ruleset, they will be added now. If you didn't set any Sigma levels to automatically add to this rule, you now need to add the desired rules manually by going to Service Control > Sigma > Rules. Choose the rules that should be added to this ruleset by selecting the checkboxes and then Add to Ruleset. A rule can be assigned to multiple rulesets.

Add a Rule to Rulesets

Note

You need to commit and push your changes after editing a ruleset. ASGARD has to restart the service controller to read new configurations. In order to prevent multiple restarts in the case of an admin performing several configuration changes in succession, the admin has to initiate the reloading of the new configuration by going to Service Control > Sigma > Rulesets and performing the Compile ruleset action (gear wheels). The need for compiling is indicated in the Uncompiled Changes column.

Uncompiled Changes Indicator

3.12.3. Choosing which Rules to activate

It is not advised to enable all available rules on an asset. We suggest to start with all "critical" and then advance to all "high" rules. We already provide a default ruleset for those two levels for you to use. "Medium" rules should not be enabled in bulk, and "low"/"informational" at all . Single medium rules, which increase an organization's detection coverage and do not trigger a bigger number of false positives, can be added to the active configuration, but should be tested rule by rule.

In order to easily add rules to a ruleset you can use the column filters to select the desired rules and add the bulk to a ruleset. As an example you can add all rules of level "critical" to a ruleset:

Add All Critical Rules to a Ruleset

Another great way to pivot the Sigma rule database is the usage of MITRE ATT&CK® IDs.

Search by MITRE ATT&CK® ID

Or you can just search the title or description field of the rules. You can also search the rule itself using the "Rule" column. (the "Rule" column is not shown by default and has to be added using the gear wheel button).

Search by Rule Title or Description

3.12.4. False Positive Tuning of Sigma Rules

Not every environment is the same. It is expected that some rules will trigger false positive matches in your environment. You have multiple options to tackle that issue.

1. If it is a general false positive, probably not only occurring in your environment, consider reporting it at as a Github issue or e-mail to us at rules@nextron-systems.com. We will take care of the tuning for you and your peers.

2. If the false positive is specific to your environment, you can tune single Sigma rules at Service Control > Sigma > Rules, filter for the rule in question and choose the "Edit false positive filters of this rule" action. Here you can do simple rule tunings on your own. By clicking the Add False Positive Filter button you can add single lines that filter the event for false positives (i.e. they are OR-connected meaning: "Do not match the event if any of those lines matches). They are applied on top of the rule logic and persist automatic rule updates.

   

   Example of the false positive tuning of a Sigma rule

   To see the resulting rule you can click the "Show Preview" button or look at the "Compiled Rule" row in the rule's drop down menu.

   If you want to review the tuned rules: To filter for all rules containing a custom false positive tuning, you have to add the "Filters" column to your view (gear wheels icon) and show all non-empty rows by using the NOT - column filter.

3. If the rule is adding too much noise and tuning is not sensible, you can remove the rule from the ruleset for a subset of your machines (maybe you need to define and use a separate ruleset for that use-case) or you can disable the rule altogether. This is done using the Disable this rule action of the rule. Disabling the rule affects the rule in all rulesets.

After tuning a rule, the rulesets using that rule have to be re-compiled at Service Control > Sigma > Rulesets.

3.12.5. Adding Custom Rules

Custom rules can be added using the sigma format complying with the specification. You can upload single files or a ZIP compressed archive. This can be done at Service Control > Sigma > Rules > Upload Rules.

Adding Custom Rules

3.12.6. Rule and Response Updates

If new rules or rule updates are provides by the Aurora signatures, the updates have to be applied by the user manually in order to be affecting Aurora agents managed by ASGARD. An indicator is shown in the WebUI and the rules changes can be reviewed and applied at Service Control > Sigma > Rule Updates.

Sigma Rule Updates for Aurora

Clicking on the Update button in the "Update Available" column opens a diff view in which the changes are shown and where the user can apply or discard the changes. If you do not need to review each single change, you can apply all changes using the Update All Rules button.

Analogous the updates of response actions can be viewed and applied at Service Control > Sigma > Response Updates.

3.12.7. How to activate Responses

As a fail safe and for administration purposes, responses are generally only simulated if not explicitly set to active. This has to be done on different levels:

• Service configuration level

• Ruleset configuration level (on updates)

• Ruleset rule level

If on one level a rule is simulated, it will not execute the response actions but only generate a log line that describes the action that would have been performed. You can see an overview of the state of all responses in the Service Control > Aurora > Configurations menu.

Aurora Configuration Response Action Overview

1. indicates whether responses are activated on configuration level. Edit the configuration to change it.

2. indicates how many rules are only simulated in that ruleset (or in sum).

3. indicates how many rules have active responses in that ruleset (or in sum)

To change the status of a response in the ruleset click the ruleset link. You can view all simulated or all active responses. Use the checkbox and the button in the upper right to switch the response status of the rules between active and simulated.

Response Configuration in Rulesets

The default response mode of a ruleset is important for the behavior of response updates. It can be seen at Service Control > Sigma > Rulesets in the Default Response Mode column.

Ruleset Default Response Mode

If "Simulation" is selected, response actions of new and updated rules will be put in simulation mode. If "Active" is selected, new rules will automatically be put in active mode and updated rules will not change their current response mode. We advise to leave the default response mode in "Simulation" mode.