IMPORTANT: No additional bug fixes or documentation updates will be released for this version. For the latest information, see the current release documentation.

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Potential PowerShell Obfuscated Script

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Potential PowerShell Obfuscated Script

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Identifies scripts that contain patterns and known methods that obfuscate PowerShell code. Attackers can use obfuscation techniques to bypass PowerShell security protections such as Antimalware Scan Interface (AMSI).

Rule type: query

Rule indices:

winlogbeat-*
logs-windows.powershell*

Severity: medium

Risk score: 47

Runs every: 5m

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

Maximum alerts per execution: 100

References:

https://github.com/danielbohannon/Invoke-Obfuscation

Tags:

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

Version: 3

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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event.category:process and host.os.type:windows and
  powershell.file.script_block_text : (
    "[string]::join" or
    "-Join" or
    "[convert]::toint16" or
    "[char][int]$_" or
    ("ConvertTo-SecureString" and "PtrToStringAuto") or
    ".GetNetworkCredential().password" or
    "-BXor" or
    ("replace" and "char") or
    "[array]::reverse"
  ) and
  powershell.file.script_block_text : (
    ("$pSHoMe[" and "+$pSHoMe[") or
    ("$ShellId[" and "+$ShellId[") or
    ("$env:ComSpec[4" and "25]-Join") or
    (("Set-Variable" or "SV" or "Set-Item") and "OFS") or
    ("*MDR*" and "Name[3,11,2]") or
    ("$VerbosePreference" and "[1,3]+'X'-Join''") or
    ("rahc" or "ekovin" or "gnirts" or "ecnereferpesobrev" or "ecalper" or "cepsmoc" or "dillehs")
  )

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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