[New Rule] Pot. Rev. Shell via Background Process (#3114)

rules/linux/execution_shell_via_background_process.toml

@@ -0,0 +1,66 @@
+[metadata]
+creation_date = "2023/09/20"
+integration = ["endpoint"]
+maturity = "production"
+min_stack_comments = "New fields added: required_fields, related_integrations, setup"
+min_stack_version = "8.3.0"
+updated_date = "2023/09/20"
+
+[rule]
+author = ["Elastic"]
+description = """
+Monitors for the execution of background processes with process arguments capable of opening a socket in the /dev/tcp 
+channel. This may indicate the creation of a backdoor reverse connection, and should be investigated further. 
+"""
+from = "now-9m"
+index = ["logs-endpoint.events.*"]
+language = "eql"
+license = "Elastic License v2"
+name = "Potential Reverse Shell via Background Process"
+risk_score = 47
+rule_id = "259be2d8-3b1a-4c2c-a0eb-0c8e77f35e39"
+severity = "medium"
+timestamp_override = "event.ingested"
+tags = ["Domain: Endpoint",
+        "OS: Linux",
+        "Use Case: Threat Detection",
+        "Tactic: Execution",
+        "Data Source: Elastic Defend"
+        ]
+type = "eql"
+query = '''
+process where host.os.type == "linux" and event.action == "exec" and event.type == "start" and 
+process.name in ("setsid", "nohup") and process.args : "*/dev/tcp/*0>&1*" and 
+process.parent.name in ("bash", "dash", "sh", "tcsh", "csh", "zsh", "ksh", "fish")
+'''
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+
+[rule.threat.tactic]
+name = "Execution"
+id = "TA0002"
+reference = "https://attack.mitre.org/tactics/TA0002/"
+
+[[rule.threat.technique]]
+id = "T1059"
+name = "Command and Scripting Interpreter"
+reference = "https://attack.mitre.org/techniques/T1059/"
+
+[[rule.threat.technique.subtechnique]]
+id = "T1059.004"
+name = "Unix Shell"
+reference = "https://attack.mitre.org/techniques/T1059/004/"
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+
+[rule.threat.tactic]
+name = "Command and Control"
+id = "TA0011"
+reference = "https://attack.mitre.org/tactics/TA0011/"
+
+[[rule.threat.technique]]
+name = "Application Layer Protocol"
+id = "T1071"
+reference = "https://attack.mitre.org/techniques/T1071/"