[New Rule] O365 Exchange Malware Filter Rule Modification (#590)

* [New Rule] O365 Exchange Malware Filter Rule Modification

* update description

rules/o365/defense_evasion_o365_exchange_malware_filter_rule_mod.toml

@@ -0,0 +1,52 @@
+[metadata]
+creation_date = "2020/11/19"
+ecs_version = ["1.6.0"]
+maturity = "production"
+updated_date = "2020/11/19"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies when a malware filter rule has been deleted or disabled in Office 365. An adversary or insider threat may
+want to modify a malware filter rule to evade detection.
+"""
+false_positives = [
+    """
+    A malware filter rule may be deleted by a system or network administrator. Verify that the configuration change was
+    expected. Exceptions can be added to this rule to filter expected behavior.
+    """,
+]
+from = "now-30m"
+index = ["filebeat-*"]
+language = "kuery"
+license = "Elastic License"
+name = "O365 Exchange Malware Filter Rule Modification"
+note = "The O365 Fleet integration or Filebeat module must be enabled to use this rule."
+references = [
+    "https://docs.microsoft.com/en-us/powershell/module/exchange/remove-malwarefilterrule?view=exchange-ps",
+    "https://docs.microsoft.com/en-us/powershell/module/exchange/disable-malwarefilterrule?view=exchange-ps",
+]
+risk_score = 47
+rule_id = "ca79768e-40e1-4e45-a097-0e5fbc876ac2"
+severity = "medium"
+tags = ["Elastic", "Cloud", "Office 365", "Continuous Monitoring", "SecOps", "Configuration Audit"]
+type = "query"
+
+query = '''
+event.dataset:o365.audit and event.provider:Exchange and event.category:web and event.action:("Remove-MalwareFilterRule" or "Disable-MalwareFilterRule") and event.outcome:success
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1562"
+name = "Impair Defenses"
+reference = "https://attack.mitre.org/techniques/T1562/"
+
+
+[rule.threat.tactic]
+id = "TA0005"
+name = "Defense Evasion"
+reference = "https://attack.mitre.org/tactics/TA0005/"
+