[New Rule] Potential PowerShell Obfuscation via Reverse Keywords

[w0rk3r]

Issues

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

Summary

Identifies PowerShell scripts that use reversed strings as a form of obfuscation. These methods are designed to evade static analysis and bypass security protections such as the Antimalware Scan Interface (AMSI).

3 hits last 90d on telemetry, no FPs

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[bot]]

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.

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ Potential PowerShell Obfuscation via Reverse Keywords (esql)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ Potential PowerShell Obfuscation via Reverse Keywords (esql)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ Potential PowerShell Obfuscation via Reverse Keywords (esql)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ Potential PowerShell Obfuscation via Reverse Keywords (esql)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta