[New Rule] Privilege Escalation via Windir Environment Variable (#638)

* [New Rule] Privilege Escalation via Windir Environment Variable

* added equiv envar

* eql syntax

* Update rules/windows/privilege_escalation_rogue_windir_environment_var.toml

Co-authored-by: Brent Murphy <56412096+bm11100@users.noreply.github.com>

* ecs_version

Co-authored-by: Brent Murphy <56412096+bm11100@users.noreply.github.com>

rules/windows/privilege_escalation_rogue_windir_environment_var.toml

@@ -0,0 +1,40 @@
+[metadata]
+creation_date = "2020/11/26"
+maturity = "production"
+updated_date = "2020/11/26"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies a privilege escalation attempt via a rogue Windows directory (Windir) environment variable. This is a known
+primitive that is often combined with other vulnerabilities to elevate privileges.
+"""
+index = ["winlogbeat-*", "logs-endpoint.events.*"]
+language = "eql"
+license = "Elastic License"
+name = "Privilege Escalation via Windir Environment Variable"
+references = ["https://www.tiraniddo.dev/2017/05/exploiting-environment-variables-in.html"]
+risk_score = 71
+rule_id = "d563aaba-2e72-462b-8658-3e5ea22db3a6"
+severity = "high"
+tags = ["Elastic", "Host", "Windows", "Threat Detection", "Privilege Escalation"]
+type = "eql"
+
+query = '''
+registry where registry.path : ("HKEY_USERS\\*\\Environment\\windir", "HKEY_USERS\\*\\Environment\\systemroot") and 
+ not registry.data.strings : ("C:\\windows", "%SystemRoot%")
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1034"
+name = "Path Interception"
+reference = "https://attack.mitre.org/techniques/T1034/"
+
+
+[rule.threat.tactic]
+id = "TA0004"
+name = "Privilege Escalation"
+reference = "https://attack.mitre.org/tactics/TA0004/"