rules/windows/defense_evasion_posh_obfuscation_char_arrays.toml

[metadata]
creation_date = "2025/04/14"
integration = ["windows"]
maturity = "production"
min_stack_comments = "The ES|QL MATCH operator was introduced in 8.17"
min_stack_version = "8.17.0"
updated_date = "2025/04/14"

[rule]
author = ["Elastic"]
description = """
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).
"""
from = "now-9m"
language = "esql"
license = "Elastic License v2"
name = "Potential PowerShell Obfuscation via Character Array Reconstruction"
risk_score = 21
rule_id = "85e2d45e-a3df-4acf-83d3-21805f564ff4"
setup = """## 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
```
"""
severity = "low"
tags = [
    "Domain: Endpoint",
    "OS: Windows",
    "Use Case: Threat Detection",
    "Tactic: Defense Evasion",
    "Data Source: PowerShell Logs",
]
timestamp_override = "event.ingested"
type = "esql"

query = '''
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
'''


[[rule.threat]]
framework = "MITRE ATT&CK"
[[rule.threat.technique]]
id = "T1027"
name = "Obfuscated Files or Information"
reference = "https://attack.mitre.org/techniques/T1027/"

[[rule.threat.technique]]
id = "T1140"
name = "Deobfuscate/Decode Files or Information"
reference = "https://attack.mitre.org/techniques/T1140/"


[rule.threat.tactic]
id = "TA0005"
name = "Defense Evasion"
reference = "https://attack.mitre.org/tactics/TA0005/"
[[rule.threat]]
framework = "MITRE ATT&CK"
[[rule.threat.technique]]
id = "T1059"
name = "Command and Scripting Interpreter"
reference = "https://attack.mitre.org/techniques/T1059/"
[[rule.threat.technique.subtechnique]]
id = "T1059.001"
name = "PowerShell"
reference = "https://attack.mitre.org/techniques/T1059/001/"



[rule.threat.tactic]
id = "TA0002"
name = "Execution"
reference = "https://attack.mitre.org/tactics/TA0002/"