[New Rule] Potential PowerShell Obfuscation via Invalid Escape Sequences

[w0rk3r]

Issues

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

Summary

Identifies PowerShell scripts that use invalid escape sequences as a form of obfuscation. This technique introduces
backticks (`) between characters in a way that does not correspond to valid PowerShell escape sequences, breaking up
strings and bypassing pattern-based detections while preserving execution logic. This is designed to evade static
analysis and bypass security protections such as the Antimalware Scan Interface (AMSI).

83 hits last 90d on telemetry, FPs related to TSS scripts with malformed strings, excluded in the query

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: Tuning - Guidelines

These guidelines serve as a reminder set of considerations when tuning an existing rule.

Documentation and Context

• Detailed description of the suggested changes.
• Provide example JSON data or screenshots.
• Provide evidence of reducing benign events mistakenly identified as threats (False Positives).
• Provide evidence of enhancing detection of true threats that were previously missed (False Negatives).
• Provide evidence of optimizing resource consumption and execution time of detection rules (Performance).
• Provide evidence of specific environment factors influencing customized rule tuning (Contextual Tuning).
• Provide evidence of improvements made by modifying sensitivity by changing alert triggering thresholds (Threshold Adjustments).
• Provide evidence of refining rules to better detect deviations from typical behavior (Behavioral Tuning).
• Provide evidence of improvements of adjusting rules based on time-based patterns (Temporal Tuning).
• Provide reasoning of adjusting priority or severity levels of alerts (Severity Tuning).
• Provide evidence of improving quality integrity of our data used by detection rules (Data Quality).
• Ensure the tuning includes necessary updates to the release documentation and versioning.

Rule Metadata Checks

• updated_date matches the date of tuning PR 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. Review to ensure the original intent of the rule is maintained.

Testing and Validation

• Validate that the tuned rule's performance is satisfactory and does not negatively impact the stack.
• Ensure that the tuned rule has a low false positive rate.

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ Potential PowerShell Obfuscation via Invalid Escape Sequences (esql)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ Potential PowerShell Obfuscation via Invalid Escape Sequences (esql)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[terrancedejesus]

Any reason not to filter this up front before doing the evals and aggregations?

[w0rk3r]

No particular reason, but as it is not a significant pattern to exclude, it makes sense to do only at the end of the condition, just like we do on other rule types