It appears the Security class has been added to the system/core folder and is loaded automatically as well.
Using $this->load->library('security'); in controllers currently returns FALSE, yet you might not notice because the Security class is already loaded.
I discovered this because of a custom Loader class I was using returned an error when loading the Security class.diff --git a/user_guide_src/source/general/core_classes.rst b/user_guide_src/source/general/core_classes.rst
index ac41407..4aa6693 100644
--- a/user_guide_src/source/general/core_classes.rst
+++ b/user_guide_src/source/general/core_classes.rst
@@ -31,6 +31,7 @@
- Log
- Output
- Router
+- Security
- URI
- Utf8