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

« Potential PowerShell Obfuscation via Backtick-Escaped Variable Expansion Potential PowerShell Obfuscation via Concatenated Dynamic Command Invocation »

› › ›

Potential PowerShell Obfuscation via Character Array Reconstruction

edit

IMPORTANT: This documentation is no longer updated. Refer to Elastic's version policy and the latest documentation.

Potential PowerShell Obfuscation via Character Array Reconstruction

edit

Identifies PowerShell scripts that use character arrays and runtime string reconstruction as a form of obfuscation. This technique breaks strings into individual characters, often using constructs like char[] with index-based access or joining logic. 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

edit

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

edit

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

// Filter for scripts that contain the "char" keyword using MATCH, boosts the query performance
| WHERE powershell.file.script_block_text : "char"

// 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, """(char\[\]\]\(\d+,\d+[^)]+|(\s?\(\[char\]\d+\s?\)\+){2,})""", "🔥")

// 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^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/

« Potential PowerShell Obfuscation via Backtick-Escaped Variable Expansion Potential PowerShell Obfuscation via Concatenated Dynamic Command Invocation »