[New Rule] Microsoft 365 Teams External Access Enabled

etc/non-ecs-schema.json

@@ -9,5 +9,8 @@
   },
   "winlogbeat-*": {
     "winlog.event_data.OriginalFileName": "keyword"
+  },
+  "filebeat-*": {
+    "o365.audit.Parameters.AllowFederatedUsers": "keyword"
   }
 }

rules/microsoft-365/persistence_microsoft_365_teams_external_access_enabled.toml

@@ -0,0 +1,51 @@
+[metadata]
+creation_date = "2020/11/30"
+maturity = "production"
+updated_date = "2020/11/30"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies when external access is enabled in Microsoft Teams. External access lets Teams and Skype for Business users
+communicate with other users that are outside their organization. An adversary may enable external access or add an
+allowed domain to exfiltrate data or maintain persistence in an environment.
+"""
+false_positives = [
+    """
+    Teams external access may be enabled 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 = "Microsoft 365 Teams External Access Enabled"
+note = "The Microsoft 365 Fleet integration or Filebeat module must be enabled to use this rule."
+references = ["https://docs.microsoft.com/en-us/microsoftteams/manage-external-access"]
+risk_score = 47
+rule_id = "27f7c15a-91f8-4c3d-8b9e-1f99cc030a51"
+severity = "medium"
+tags = ["Elastic", "Cloud", "Microsoft 365", "Continuous Monitoring", "SecOps", "Configuration Audit"]
+type = "query"
+
+query = '''
+event.dataset:o365.audit and event.provider:(SkypeForBusiness or MicrosoftTeams) and
+event.category:web and event.action:"Set-CsTenantFederationConfiguration" and
+o365.audit.Parameters.AllowFederatedUsers:True and event.outcome:success
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1098"
+name = "Account Manipulation"
+reference = "https://attack.mitre.org/techniques/T1098/"
+
+
+[rule.threat.tactic]
+id = "TA0003"
+name = "Persistence"
+reference = "https://attack.mitre.org/tactics/TA0003/"
+