Added Sigma Rules

rules/Sigma/win_powershell_downgrade_attack.yml

@@ -0,0 +1,46 @@
+title: PowerShell Downgrade Attack
+author: Harish Segar (rule)
+date: 2020/03/20
+description: Detects PowerShell downgrade attack by comparing the host versions with
+    the actually used engine version 2.0
+detection:
+    SELECTION_1:
+        EventID: 1
+    SELECTION_2:
+        CommandLine: '* -version 2 *'
+    SELECTION_3:
+        CommandLine: '* -versio 2 *'
+    SELECTION_4:
+        CommandLine: '* -versi 2 *'
+    SELECTION_5:
+        CommandLine: '* -vers 2 *'
+    SELECTION_6:
+        CommandLine: '* -ver 2 *'
+    SELECTION_7:
+        CommandLine: '* -ve 2 *'
+    SELECTION_8:
+        Image: '*\powershell.exe'
+    condition: (SELECTION_1 and (SELECTION_2 or SELECTION_3 or SELECTION_4 or SELECTION_5
+        or SELECTION_6 or SELECTION_7) and SELECTION_8)
+falsepositives:
+- Penetration Test
+- Unknown
+id: b3512211-c67e-4707-bedc-66efc7848863
+level: medium
+logsource:
+    category: process_creation
+    product: windows
+references:
+- http://www.leeholmes.com/blog/2017/03/17/detecting-and-preventing-powershell-downgrade-attacks/
+related:
+-   id: 6331d09b-4785-4c13-980f-f96661356249
+    type: derived
+status: experimental
+tags:
+- attack.defense_evasion
+- attack.execution
+- attack.t1086
+- attack.t1059.001
+yml_filename: win_powershell_downgrade_attack.yml
+yml_path: /Users/user/Documents/YamatoSecurity/sigma/rules/windows/process_creation
+