Add: sigma rules (#175)

rules/sigma/windows/process_creation/win_powershell_frombase64string.yml

@@ -0,0 +1,27 @@
+
+title: FromBase64String Command Line
+author: Florian Roth
+date: 2020/01/29
+description: Detects suspicious FromBase64String expressions in command line arguments
+detection:
+  SELECTION_1:
+    EventID: 1
+  SELECTION_2:
+    CommandLine: '*::FromBase64String(*'
+  condition: (SELECTION_1 and SELECTION_2)
+falsepositives:
+- Administrative script libraries
+id: e32d4572-9826-4738-b651-95fa63747e8a
+level: high
+logsource:
+  category: process_creation
+  product: windows
+modified: 2020/09/06
+references:
+- https://gist.github.com/Neo23x0/6af876ee72b51676c82a2db8d2cd3639
+status: experimental
+tags:
+- attack.t1027
+- attack.defense_evasion
+- attack.t1140
+- attack.t1059.001