rules/windows/defense_evasion_posh_obfuscation_whitespace_special_proportion.toml

[metadata]
creation_date = "2025/04/16"
integration = ["windows"]
maturity = "production"
updated_date = "2025/04/16"

[rule]
author = ["Elastic"]
description = """
Identifies PowerShell scripts with an unusually high proportion of whitespace and special characters, often indicative
of obfuscation. This behavior is commonly associated with techniques such as SecureString encoding, formatting
obfuscation, or character-level manipulation designed to bypass static analysis and AMSI inspection.
"""
from = "now-9m"
language = "esql"
license = "Elastic License v2"
name = "Potential PowerShell Obfuscation via Special Character Overuse"
risk_score = 21
rule_id = "6ddb6c33-00ce-4acd-832a-24b251512023"
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"

// Look for scripts with more than 1000 chars that contain a related keyword
| EVAL script_len = LENGTH(powershell.file.script_block_text)
| WHERE script_len > 1000

// 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, """[\s\$\{\}\+\@\=\(\)\^\\\"~\[\]\?\.]""", "🔥")

// Count the occurrence of numbers and their proportion to the total chars in the script
| EVAL special_count = script_len - LENGTH(REPLACE(replaced_with_fire, "🔥", ""))
| EVAL proportion = special_count::double / script_len::double

// Keep the fields relevant to the query, although this is not needed as the alert is populated using _id
| KEEP special_count, script_len, proportion, 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

// Filter for scripts with a 75%+ proportion of numbers
| WHERE proportion > 0.75
'''


[[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/"