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Potential Dynamic IEX Reconstruction via Environment Variables

Identifies PowerShell scripts that reconstruct the IEX (Invoke-Expression) command at runtime using indexed slices of environment variables. This technique leverages character access and join operations to build execution logic dynamically, bypassing static keyword detection and evading defenses such as AMSI.

Rule type: esql
Rule indices:

Rule Severity: low
Risk Score: 21
Runs every:
Searches indices from: now-9m
Maximum alerts per execution: ?
References:

Tags:

  • Domain: Endpoint
  • OS: Windows
  • Use Case: Threat Detection
  • Tactic: Defense Evasion
  • Data Source: PowerShell Logs

Version: ?
Rule authors:

  • Elastic

Rule license: Elastic License v2

The 'PowerShell Script Block Logging' logging policy must be enabled. Steps to implement the logging policy with Advanced Audit Configuration:

Computer Configuration >
Administrative Templates >
Windows PowerShell >
Turn on PowerShell Script Block Logging (Enable)

Steps to implement the logging policy via registry:

reg add "hklm\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ScriptBlockLogging" /v EnableScriptBlockLogging /t REG_DWORD /d 1

FROM logs-windows.powershell_operational* metadata _id, _version, _index
| WHERE event.code == "4104"

// Look for scripts with more than 500 chars that contain a related keyword
| EVAL script_len = LENGTH(powershell.file.script_block_text)
| WHERE script_len > 500

// Replace string format expressions with 🔥 to enable counting the occurrence of the patterns we are looking for
// The emoji is used because it's unlikely to appear in scripts and has a consistent character length of 1
| EVAL replaced_with_fire = REPLACE(powershell.file.script_block_text, """(?i)(\$(?:\w+|\w+\:\w+)\[\d++\]\+\$(?:\w+|\w+\:\w+)\[\d++\]\+['"]x['"]|\$(?:\w+\:\w+)\[\d++,\d++,\d++\]|\.name\[\d++,\d++,\d++\])""", "🔥")

// Count how many patterns were detected by calculating the number of 🔥 characters inserted
| EVAL count = LENGTH(replaced_with_fire) - LENGTH(REPLACE(replaced_with_fire, "🔥", ""))

// Keep the fields relevant to the query, although this is not needed as the alert is populated using _id
| KEEP count, replaced_with_fire, powershell.file.script_block_text, powershell.file.script_block_id, file.path, powershell.sequence, powershell.total, _id, _index, host.name, agent.id, user.id
| WHERE count >= 1

Framework: MITRE ATT&CK

Framework: MITRE ATT&CK