[New Rule] PowerShell Obfuscation via Negative Index String Reversal

rules/windows/defense_evasion_posh_obfuscation_index_reversal.toml

@@ -0,0 +1,108 @@
+[metadata]
+creation_date = "2025/04/14"
+integration = ["windows"]
+maturity = "production"
+updated_date = "2025/04/14"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies PowerShell scripts that use negative index ranges to reverse the contents of a string or array at runtime as
+a form of obfuscation. This technique avoids direct use of reversal functions by iterating through array elements in
+reverse order. 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 = "PowerShell Obfuscation via Negative Index String Reversal"
+risk_score = 21
+rule_id = "9edd1804-83c7-4e48-b97d-c776b4c97564"
+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 500 chars that contain a related keyword
+| EVAL script_len = LENGTH(powershell.file.script_block_text)
+| WHERE script_len > 500
+
+// 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, """\$\w+\[\-\s?1\.\.""", "🔥")
+
+// 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, replaced_with_fire, powershell.file.script_block_id, file.path, powershell.sequence, powershell.total, _id, _index, host.name, agent.id, user.id
+| WHERE count >= 1
+
+// FP Patterns
+| WHERE NOT powershell.file.script_block_text LIKE "*GENESIS-5654*"
+'''
+
+
+[[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/"
+