[New Rule] Dynamic IEX Reconstruction via Method String Access

[w0rk3r]

Issues

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

Summary

Identifies PowerShell scripts that reconstruct the IEX (Invoke-Expression) command by accessing and indexing the string
representation of method references. This obfuscation technique uses constructs like ''.IndexOf.ToString() to expose
method metadata as a string, then extracts specific characters through indexed access and joins them to form IEX,
bypassing static keyword detection and evading defenses such as AMSI.

18 hits last 90d on telemetry all related to my testing, 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]

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

• ❌ Dynamic IEX Reconstruction via Method String Access (esql)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[netom4040]

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