[New Rule] Cobalt Strike Beacon (#21)

rules/network/command_and_control_cobalt_strike_beacon.toml

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+[metadata]
+creation_date = "2020/07/06"
+ecs_version = ["1.6.0"]
+maturity = "production"
+updated_date = "2020/07/06"
+
+[rule]
+author = ["Elastic"]
+description = """
+Cobalt Strike is a threat emulation platform commonly modified and used by adversaries to conduct network attack and
+exploitation campaigns. This rule detects a network activity algorithm leveraged by Cobalt Strike implant beacons for
+command and control.
+"""
+false_positives = [
+    """
+    This rule should be tailored to either exclude systems, as sources or destinations, in which this behavior is
+    expected.
+    """,
+]
+index = ["packetbeat-*"]
+language = "lucene"
+license = "Elastic License"
+name = "Cobalt Strike Command and Control Beacon"
+note = "This activity has been observed in FIN7 campaigns."
+references = [
+    "https://blog.morphisec.com/fin7-attacks-restaurant-industry",
+    "https://www.fireeye.com/blog/threat-research/2017/04/fin7-phishing-lnk.html",
+]
+risk_score = 73
+rule_id = "cf53f532-9cc9-445a-9ae7-fced307ec53c"
+severity = "high"
+tags = ["Elastic", "Network"]
+type = "query"
+
+query = '''
+event.category:(network OR network_traffic) AND type:(tls OR http) AND network.transport:tcp AND destination.domain:/[a-z]{3}.stage.[0-9]{8}\..*/
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1071"
+name = "Application Layer Protocol"
+reference = "https://attack.mitre.org/techniques/T1071/"
+
+[[rule.threat.technique]]
+id = "T1483"
+name = "Domain Generation Algorithms"
+reference = "https://attack.mitre.org/techniques/T1483/"
+
+
+[rule.threat.tactic]
+id = "TA0011"
+name = "Command and Control"
+reference = "https://attack.mitre.org/tactics/TA0011/"