[New Rule] O365 Exchange Safe Link Policy Disabled (#577)

* Create initial_access_o365_exchange_safelinks_disabled.toml

* Update initial_access_o365_exchange_safelinks_disabled.toml

* linting

* update description

* update tags

Co-authored-by: David French <56409778+threat-punter@users.noreply.github.com>

rules/o365/initial_access_o365_exchange_safelinks_disabled.toml

@@ -0,0 +1,52 @@
+[metadata]
+creation_date = "2020/11/18"
+ecs_version = ["1.6.0"]
+maturity = "production"
+updated_date = "2020/11/18"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies when a Safe Link policy is disabled in Office 365. Safe Link policies for Office applications extend phishing
+protection to documents that contain hyperlinks, even after they have been delivered to a user.
+"""
+false_positives = [
+    """
+    Disabling safe links may be done 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 Safe Link Policy Disabled"
+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/disable-safelinksrule?view=exchange-ps",
+    "https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/atp-safe-links?view=o365-worldwide",
+]
+risk_score = 47
+rule_id = "a989fa1b-9a11-4dd8-a3e9-f0de9c6eb5f2"
+severity = "medium"
+tags = ["Elastic", "Cloud", "Office 365", "Continuous Monioring", "SecOps", "Identity and Access"]
+type = "query"
+
+query = '''
+event.dataset:o365.audit and event.provider:Exchange and event.category:web and event.action:"Disable-SafeLinksRule" and event.outcome:success
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1566"
+name = "Phishing"
+reference = "https://attack.mitre.org/techniques/T1566/"
+
+
+[rule.threat.tactic]
+id = "TA0001"
+name = "Initial Access"
+reference = "https://attack.mitre.org/tactics/TA0001/"
+