[New Rule] Add rule for AWS UpdateAssumeRolePolicy

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

rules/aws/privilege_escalation_updateassumerolepolicy.toml

@@ -0,0 +1,50 @@
+[metadata]
+creation_date = "2020/07/06"
+ecs_version = ["1.5.0"]
+maturity = "production"
+updated_date = "2020/07/06"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies attempts to modify an AWS IAM Assume Role Policy. An adversary may attempt to modify the AssumeRolePolicy of
+a misconfigured role in order to gain the privileges of that role.
+"""
+false_positives = [
+    """
+    Verify whether the user identity, user agent, and/or hostname should be making changes in your environment. Policy
+    updates from unfamiliar users or hosts should be investigated. If known behavior is causing false positives, it can
+    be exempted from the rule.
+    """,
+]
+from = "now-20m"
+index = ["filebeat-*"]
+interval = "10m"
+language = "kuery"
+license = "Elastic License"
+name = "AWS IAM Assume Role Policy Update"
+references = ["https://labs.bishopfox.com/tech-blog/5-privesc-attack-vectors-in-aws"]
+risk_score = 21
+rule_id = "a60326d7-dca7-4fb7-93eb-1ca03a1febbd"
+severity = "low"
+tags = ["AWS", "Elastic"]
+type = "query"
+
+query = '''
+event.module:aws and event.dataset:aws.cloudtrail and event.provider:iam.amazonaws.com and event.action:UpdateAssumeRolePolicy 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 = "TA0004"
+name = "Privilege Escalation"
+reference = "https://attack.mitre.org/tactics/TA0004/"
+