10. Custom Signatures and IOCs

10.1. Management using ASGARD

ASGARD Management Center allows you to create rule sets of sigma rules and apply them to groups of end systems. It also features a "difference view" that shows you rules that have changed in the remote Sigma repository and allows you to accept or deny the changes. It also provides ways to filter false positives right at the source.

Signature updates and Aurora upgrades can be executed for all end points from the Management Center.

Asgard Rule Management

The rule management is described in more detail in this section of the ASGARD Management Center manual.

10.2. Manual signature management

Signatures can be specified when starting Aurora using the --rules-path and --ioc-path parameters. These parameters default to the built-in rules and IOCs at signatures\sigma-rules and signatures\iocs and the provided paths for custom signatures at custom-signatures\sigma-rules and custom-signatures\iocs respectively. Aurora traverses the directories that are specified with these parameters recursively and initializes all signature files it finds.

In order to add new sigma rules or IOCs, you can either:

  • Add them to the corresponding subfolder in custom-signatures

  • Specify the folder where they are located using --rules-path or --ioc-path

Warning

If you specifiy --rules-path or --ioc-path, if you want to use the Aurora built-in rules and IOCs, you need to add them manually as well. E.g.:

C:\Program Files\Aurora-Agent>aurora-agent.exe --install --rules-path .\signatures\sigma-rules --rules-path .\my-rules

If paths are configured, only the configured paths are used.

10.2.1. Signature format

IOCs follow the same format that THOR IOCs do; the full description can be found in the THOR manual.

Sigma rules must adhere to the specification found in the Sigma repository.

10.2.2. Encrypted signatures

Both IOCs and sigma rules can be encrypted using the encrypt function in Aurora Agent Util. Aurora will automatically decrypt encrypted signatures at startup. This functionality is only available in the full version of Aurora.

10.3. Signature Application

10.3.1. Sigma rules

Sigma rules must contain a logsource element which (indirectly) determines on which events the sigma rule is applied.

Aurora utilizes a number of log sources which map between these logsource elements and the actual sources. The log source definitions which can be found in the log-sources folder. Rules are applied on every log source which has a matching logsource definition.

Log sources may also utilize:
  • conditions to filter events from the given sources

  • fieldmappings to rename specific fields in the events that occur. This is useful to have all events with the same logsource appear to have the same fields, even if the underlying sources and field names differ.

  • rewrite to reference each other. rewrite is meant to be used in combination with the other elements: For example, Sysmon events are split into different categories using conditions and rewrite.

10.3.2. IOCs

10.3.2.1. Hashes

Hash IOCs are applied to:

  • Process creation events

  • Image load events

  • Driver load events

10.3.2.2. Filenames

Filename IOCs are applied to:

  • Process creation events

  • Image load events

  • File creation events

  • Handle events that reference files

  • Driver load events

10.3.2.3. C2

C2 IOCs are applied to:

  • DNS query events

  • TCP connection events

10.3.2.4. Named Pipe

Named Pipe IOCs are applied to handle events that reference named pipes.

10.3.2.5. Handle

Handle IOCs (which include mutex and event IOCs) are applied to handle events.