[New Rule] Azure Active Directory PowerShell Sign-in (#718)

* Create initial_access_azure_active_directory_powershell_signon.toml

* Apply suggestions from code review

Co-authored-by: Justin Ibarra <brokensound77@users.noreply.github.com>

* Update initial_access_azure_active_directory_powershell_signin.toml

Co-authored-by: Justin Ibarra <brokensound77@users.noreply.github.com>

rules/azure/initial_access_azure_active_directory_powershell_signin.toml

@@ -0,0 +1,57 @@
+[metadata]
+creation_date = "2020/12/14"
+maturity = "production"
+updated_date = "2020/12/14"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies a sign-in using the Azure Active Directory PowerShell module. PowerShell for Azure Active Directory allows
+for managing settings from the command line, which is intended for users who are members of an admin role.
+"""
+false_positives = [
+    """
+    Sign-ins using PowerShell may be done by a system or network administrator. Verify whether the username, hostname,
+    and/or resource name should be signing into your environment. Sign-ins 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-*", "logs-azure.*"]
+language = "kuery"
+license = "Elastic License"
+name = "Azure Active Directory PowerShell Sign-in"
+note = "The Azure Fleet Integration or Filebeat module must be enabled to use this rule."
+references = [
+    "https://msrc-blog.microsoft.com/2020/12/13/customer-guidance-on-recent-nation-state-cyber-attacks/",
+    "https://docs.microsoft.com/en-us/microsoft-365/enterprise/connect-to-microsoft-365-powershell?view=o365-worldwide",
+]
+risk_score = 21
+rule_id = "a605c51a-73ad-406d-bf3a-f24cc41d5c97"
+severity = "low"
+tags = ["Elastic", "Cloud", "Azure", "Continuous Monitoring", "SecOps", "Identity and Access"]
+type = "query"
+
+query = '''
+event.dataset:azure.signinlogs and
+  azure.signinlogs.properties.app_display_name:"Azure Active Directory PowerShell" and
+  azure.signinlogs.properties.token_issuer_type:AzureAD and event.outcome:(success or Success)
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1078"
+name = "Valid Accounts"
+reference = "https://attack.mitre.org/techniques/T1078/"
+[[rule.threat.technique.subtechnique]]
+id = "T1078.004"
+name = "Cloud Accounts"
+reference = "https://attack.mitre.org/techniques/T1078/004/"
+
+
+[rule.threat.tactic]
+id = "TA0001"
+name = "Initial Access"
+reference = "https://attack.mitre.org/tactics/TA0001/"