Potential PowerShell Obfuscation via Reverse Keywords

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Potential PowerShell Obfuscation via Reverse Keywords

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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).

Rule type: esql

Rule indices: None

Severity: low

Risk score: 21

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References: None

Tags:

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

Version: 1

Rule authors:

Elastic

Rule license: Elastic License v2

Setup

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Setup

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

Rule query

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

// Filter for scripts that contains these keywords using MATCH, boosts the query performance, match will ignore the | and look for the individual words
| WHERE powershell.file.script_block_text : "rahc|metsys|stekcos|tcejboimw|ecalper|ecnerferpe|noitcennoc|nioj|eman|vne|gnirts|tcejbo-wen|_23niw|noisserpxe|ekovni|daolnwod"

// 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)(rahc|metsys|stekcos|tcejboimw|ecalper|ecnerferpe|noitcennoc|nioj|eman\.|:vne|gnirts|tcejbo-wen|_23niw|noisserpxe|ekovni|daolnwod)""", "🔥")

// 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, agent.id
| WHERE count >= 2

Framework: MITRE ATT&CK^TM

Tactic:
- Name: Defense Evasion
- ID: TA0005
- Reference URL: https://attack.mitre.org/tactics/TA0005/
Technique:
- Name: Obfuscated Files or Information
- ID: T1027
- Reference URL: https://attack.mitre.org/techniques/T1027/
Technique:
- Name: Deobfuscate/Decode Files or Information
- ID: T1140
- Reference URL: https://attack.mitre.org/techniques/T1140/
Tactic:
- Name: Execution
- ID: TA0002
- Reference URL: https://attack.mitre.org/tactics/TA0002/
Technique:
- Name: Command and Scripting Interpreter
- ID: T1059
- Reference URL: https://attack.mitre.org/techniques/T1059/
Sub-technique:
- Name: PowerShell
- ID: T1059.001
- Reference URL: https://attack.mitre.org/techniques/T1059/001/

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