[New Rule] busybox shell evasion threat (#1842)

* busybox shell evasion threat

Co-authored-by: Colson Wilhoit <48036388+DefSecSentinel@users.noreply.github.com>

rules/linux/execution_busybox_binary.toml

@@ -0,0 +1,49 @@
+[metadata]
+creation_date = "2022/03/15"
+maturity = "production"
+updated_date = "2022/03/15"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies Linux binary busybox abuse to break out from restricted environments by spawning an interactive system
+shell.The busybox is software utility suite that provides several Unix utilities in a single executable file and the activity of spawing a shell is
+not a standard use of this binary by a user or system administrator. It indicates a potentially
+malicious actor attempting to improve the capabilities or stability of their access.
+"""
+from = "now-9m"
+index = ["logs-endpoint.events.*"]
+language = "eql"
+license = "Elastic License v2"
+name = "Linux Restricted Shell Breakout via busybox Shell Evasion"
+references = ["https://gtfobins.github.io/gtfobins/busybox/"]
+risk_score = 47
+rule_id = "e9b4a3c7-24fc-49fd-a00f-9c938031eef1"
+severity = "medium"
+tags = ["Elastic", "Host", "Linux", "Threat Detection", "Execution", "GTFOBins"]
+timestamp_override = "event.ingested"
+type = "eql"
+
+query = '''
+process where event.type == "start" and process.name : "busybox" and process.args_count == 2 and process.args : ("/bin/sh", "/bin/ash", "sh", "ash")
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[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.tactic]
+id = "TA0002"
+name = "Execution"
+reference = "https://attack.mitre.org/tactics/TA0002/"
+