[New Rule][BBR] Potential PowerShell Obfuscation via High Special Character Proportion (#4629)

rules_building_block/defense_evasion_posh_obfuscation_proportion_special_chars.toml

@@ -0,0 +1,110 @@
+[metadata]
+bypass_bbr_timing = true
+creation_date = "2025/04/16"
+integration = ["windows"]
+maturity = "production"
+updated_date = "2025/04/16"
+
+[rule]
+author = ["Elastic"]
+building_block_type = "default"
+description = """
+Identifies PowerShell scripts with an abnormally high proportion of non-alphanumeric characters, often resulting from
+encoding, string mangling, or dynamic code generation.
+"""
+from = "now-9m"
+language = "esql"
+license = "Elastic License v2"
+name = "Potential PowerShell Obfuscation via High Special Character Proportion"
+risk_score = 21
+rule_id = "f9753455-8d55-4ad8-b70a-e07b6f18deea"
+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",
+    "Rule Type: BBR"
+]
+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
+// Excludes spaces, #, = and - as they are heavily used in scripts for formatting
+| EVAL replaced_with_fire = REPLACE(powershell.file.script_block_text, """[^0-9A-Za-z\s#=-]""", "🔥")
+
+// Count the occurrence of special chars 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 25%+ proportion of special chars
+| WHERE proportion > 0.25
+'''
+
+
+[[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/"
+