[New Rule] Potential PowerShell Obfuscation via Special Character Overuse (#4632)

rules/windows/defense_evasion_posh_obfuscation_whitespace_special_proportion.toml

@@ -0,0 +1,107 @@
+[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/"
+