Add: sigma rules (#175)

rules/sigma/windows/process_creation/win_encoded_frombase64string.yml

@@ -0,0 +1,32 @@
+
+title: Encoded FromBase64String
+author: Florian Roth
+date: 2019/08/24
+description: Detects a base64 encoded FromBase64String keyword in a process command
+  line
+detection:
+  SELECTION_1:
+    EventID: 1
+  SELECTION_2:
+    CommandLine:
+    - '*OjpGcm9tQmFzZTY0U3RyaW5n*'
+    - '*o6RnJvbUJhc2U2NFN0cmluZ*'
+    - '*6OkZyb21CYXNlNjRTdHJpbm*'
+  condition: (SELECTION_1 and SELECTION_2)
+falsepositives:
+- unknown
+fields:
+- CommandLine
+- ParentCommandLine
+id: fdb62a13-9a81-4e5c-a38f-ea93a16f6d7c
+level: critical
+logsource:
+  category: process_creation
+  product: windows
+status: experimental
+tags:
+- attack.defense_evasion
+- attack.t1140
+- attack.execution
+- attack.t1059.001
+- attack.t1086