[New Rule] Azure External Guest User Invitation (#231)

* Create initial_access_external_guest_user_invite.toml

* Update rules/azure/initial_access_external_guest_user_invite.toml

Co-authored-by: David French <[email protected]>

* update mitre metadata

* lint

Co-authored-by: David French <[email protected]>

Author: bm11100

rules/azure/initial_access_external_guest_user_invite.toml

@@ -0,0 +1,66 @@
+[metadata]
+creation_date = "2020/08/31"
+ecs_version = ["1.6.0"]
+maturity = "production"
+updated_date = "2020/08/31"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies an invitation to an external user in Azure Active Directory (AD). Azure AD is extended to include
+collaboration, allowing you to invite people from outside your organization to be guest users in your cloud account.
+Unless there is a business need to provision guest access, it is best practice avoid creating guest users. Guest users
+could potentially be overlooked indefinitely leading to a potential vulnerability.
+"""
+false_positives = [
+    """
+    Guest user invitations may be sent out by a system or network administrator. Verify whether the username, hostname,
+    and/or resource name should be making changes in your environment. Guest user invitations from unfamiliar users or
+    hosts should be investigated. If known behavior is causing false positives, it can be exempted from the rule.
+    """,
+]
+from = "now-25m"
+index = ["filebeat-*"]
+language = "kuery"
+license = "Elastic License"
+name = "Azure External Guest User Invitation"
+note = "The Azure Filebeat module must be enabled to use this rule."
+references = ["https://docs.microsoft.com/en-us/azure/governance/policy/samples/cis-azure-1-1-0"]
+risk_score = 21
+rule_id = "141e9b3a-ff37-4756-989d-05d7cbf35b0e"
+severity = "low"
+tags = ["Elastic", "Azure", "SecOps", "Continuous Monitoring", "Identity and Access"]
+type = "query"
+
+query = '''
+event.module:azure and event.dataset:azure.auditlogs and event.category:AuditLogs and azure.auditlogs.operation_name:"Invite external user" and azure.auditlogs.properties.target_resources.*.display_name:guest and event.outcome:Success
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1078"
+name = "Valid Accounts"
+reference = "https://attack.mitre.org/techniques/T1078/"
+
+
+[rule.threat.tactic]
+id = "TA0001"
+name = "Initial Access"
+reference = "https://attack.mitre.org/tactics/TA0001/"
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1078"
+name = "Valid Accounts"
+reference = "https://attack.mitre.org/techniques/T1078/"
+
+
+[rule.threat.tactic]
+id = "TA0003"
+name = "Persistence"
+reference = "https://attack.mitre.org/tactics/TA0003/"
+