Make `AC::Params#to_h` return Hash with safe keys

`ActionController::Parameters#to_h` now returns a `Hash` with
unpermitted keys removed. This change is to reflect on a security
concern where some method performed on an `ActionController::Parameters`
may yield a `Hash` object which does not maintain `permitted?` status.
If you would like to get a `Hash` with all the keys intact, duplicate
and mark it as permitted before calling `#to_h`.

    params = ActionController::Parameters.new(name: 'Senjougahara Hitagi')
    params.to_h # => {}

    unsafe_params = params.dup.permit!
    unsafe_params.to_h # => {"name"=>"Senjougahara Hitagi"}

    safe_params = params.permit(:name)
    safe_params.to_h # => {"name"=>"Senjougahara Hitagi"}

This change is consider a stopgap as we cannot chage the code to stop
`ActionController::Parameters` to inherit from
`HashWithIndifferentAccess` in the next minor release.

Also, adding a CHANGELOG entry to mention that
`ActionController::Parameters` will not inheriting from
`HashWithIndifferentAccess` in the next major version.

1 files changed, 19 insertions, 0 deletions

diff --git a/actionpack/lib/action_controller/metal/strong_parameters.rb b/actionpack/lib/action_controller/metal/strong_parameters.rb
index bc27ecaa20..764474ad4e 100644
--- a/actionpack/lib/action_controller/metal/strong_parameters.rb
+++ b/actionpack/lib/action_controller/metal/strong_parameters.rb
@@ -141,6 +141,25 @@ module ActionController
       @permitted = self.class.permit_all_parameters
     end
 
+    # Returns a safe +Hash+ representation of this parameter with all
+    # unpermitted keys removed.
+    #
+    #   params = ActionController::Parameters.new({
+    #     name: 'Senjougahara Hitagi',
+    #     oddity: 'Heavy stone crab'
+    #   })
+    #   params.to_h # => {}
+    #
+    #   safe_params = params.permit(:name)
+    #   safe_params.to_h # => {"name"=>"Senjougahara Hitagi"}
+    def to_h
+      if permitted?
+        super
+      else
+        slice(*self.class.always_permitted_parameters).permit!.to_h
+      end
+    end
+
     # Attribute that keeps track of converted arrays, if any, to avoid double
     # looping in the common use case permit + mass-assignment. Defined in a
     # method to instantiate it only if needed.