[New Rule] Modification of AmsiEnable Registry Key (#1248)

* Create defense_evasion_amsienable_key_mod.toml

rules/windows/defense_evasion_amsienable_key_mod.toml

@@ -0,0 +1,53 @@
+[metadata]
+creation_date = "2021/06/01"
+maturity = "production"
+updated_date = "2021/06/01"
+
+[rule]
+author = ["Elastic"]
+description = """
+Jscript tries to query the AmsiEnable registry key from the HKEY_USERS registry hive before initializing Antimalware
+Scan Interface (AMSI). If this key is set to 0, AMSI is not enabled for the Jscript process. An adversary can modify
+this key to disable AMSI protections.
+"""
+from = "now-9m"
+index = ["winlogbeat-*", "logs-endpoint.events.*", "logs-windows.*"]
+language = "eql"
+license = "Elastic License v2"
+name = "Modification of AmsiEnable Registry Key"
+references = [
+    "https://hackinparis.com/data/slides/2019/talks/HIP2019-Dominic_Chell-Cracking_The_Perimeter_With_Sharpshooter.pdf",
+    "https://docs.microsoft.com/en-us/windows/win32/amsi/antimalware-scan-interface-portal",
+]
+risk_score = 73
+rule_id = "f874315d-5188-4b4a-8521-d1c73093a7e4"
+severity = "high"
+tags = ["Elastic", "Host", "Windows", "Threat Detection", "Defense Evasion"]
+timestamp_override = "event.ingested"
+type = "eql"
+
+query = '''
+registry where event.type in ("creation", "change") and
+  registry.path: "HKEY_USERS\\*\\Software\\Microsoft\\Windows Script\\Settings\\AmsiEnable" and
+  registry.data.strings: "0"
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1562"
+name = "Impair Defenses"
+reference = "https://attack.mitre.org/techniques/T1562/"
+[[rule.threat.technique.subtechnique]]
+id = "T1562.001"
+name = "Disable or Modify Tools"
+reference = "https://attack.mitre.org/techniques/T1562/001/"
+
+
+
+[rule.threat.tactic]
+id = "TA0005"
+name = "Defense Evasion"
+reference = "https://attack.mitre.org/tactics/TA0005/"
+