[New Rule][BBR] Potential PowerShell Obfuscation via High Special Character Proportion

[w0rk3r]

Issues

Part of https://github.com/elastic/ia-trade-team/issues/533

Summary

Identifies PowerShell scripts with an abnormally high proportion of non-alphanumeric characters, often resulting from
encoding, string mangling, or dynamic code generation.

High volume of matches in the telemetry, limited FPs with the use of special chars to formatting, which I excluded in the regex.

Pushing this as BBR as our telemetry doesn't contain enough benign data for us to truly see how much noise this rule will generate and adjust the threshold accordingly.

Additional information

From my testing, the | KEEP condition doesn’t need to specify any fields other than the metadata ones (_id and _index), as the engine appears to populate the alert using them. However, I’m keeping it as-is because it significantly improves performance in Discovery and makes the results more understandable if someone uses the query for hunting.

Sample Match

[github-actions]

Rule: New - Guidelines

These guidelines serve as a reminder set of considerations when proposing a new rule.

Documentation and Context

• Detailed description of the rule.
• List any new fields required in ECS/data sources.
• Link related issues or PRs.
• Include references.

Rule Metadata Checks

• creation_date matches the date of creation PR initially merged.
• min_stack_version should support the widest stack versions.
• name and description should be descriptive and not include typos.
• query should be inclusive, not overly exclusive, considering performance for diverse environments. Non ecs fields should be added to non-ecs-schema.json if not available in an integration.
• min_stack_comments and min_stack_version should be included if the rule is only compatible starting from a specific stack version.
• index pattern should be neither too specific nor too vague, ensuring it accurately matches the relevant data stream (e.g., use logs-endpoint.process-* for process data).
• integration should align with the index. If the integration is newly introduced, ensure the manifest, schemas, and new_rule.yaml template are updated.
• setup should include the necessary steps to configure the integration.
• note should include any additional information (e.g. Triage and analysis investigation guides, timeline templates).
• tags should be relevant to the threat and align/added to the EXPECTED_RULE_TAGS in the definitions.py file.
• threat, techniques, and subtechniques should map to ATT&CK always if possible.

New BBR Rules

• building_block_type should be included if the rule is a building block and the rule should be located in the rules_building_block folder.
• bypass_bbr_timing should be included if adding custom lookback timing to the rule.

Testing and Validation

• Provide evidence of testing and detecting the expected threat.
• Check for existence of coverage to prevent duplication.