Merge branch 'fix/2fa-authentication-spoofing' into 'master'

Fix 2FA authentication spoofing

## Summary

This is security fix for vulnerability described at 
https://gitlab.com/gitlab-org/gitlab-ce/issues/14900.

Attacker was able to bypass password authentication of users that have 2FA enabled, and consequently sign is as a different user, without knowing his password, if he managed to guess 2FA One Time Password for that user.

It was also possible to enumerate users and check if they have 2FA enabled, because GitLab responded with different error for each case.

## Fix

This MR attempts to change default user search scope if `otp_user_id` session variable has been set. If it is present, it means that user has 2FA enabled, and has already been verified with login and password. In this case we should look for user with `otp_user_id` first, before picking it up by `login`.

Both, 2FA authentication spoofing and 2FA discovery have been covered by specs.

## Further work

Current 2FA code is a bit tricky, so it probably needs some refactoring.



See merge request !1947

1 files changed, 9 insertions, 6 deletions

diff --git a/app/controllers/sessions_controller.rb b/app/controllers/sessions_controller.rb
index 65677a3dd3c..c29f4609e93 100644
--- a/app/controllers/sessions_controller.rb
+++ b/app/controllers/sessions_controller.rb
@@ -5,7 +5,8 @@ class SessionsController < Devise::SessionsController
   skip_before_action :check_2fa_requirement, only: [:destroy]
 
   prepend_before_action :check_initial_setup, only: [:new]
-  prepend_before_action :authenticate_with_two_factor, only: [:create]
+  prepend_before_action :authenticate_with_two_factor,
+    if: :two_factor_enabled?, only: [:create]
   prepend_before_action :store_redirect_path, only: [:new]
 
   before_action :auto_sign_in_with_provider, only: [:new]
@@ -56,10 +57,10 @@ class SessionsController < Devise::SessionsController
   end
 
   def find_user
-    if user_params[:login]
-      User.by_login(user_params[:login])
-    elsif user_params[:otp_attempt] && session[:otp_user_id]
+    if session[:otp_user_id]
       User.find(session[:otp_user_id])
+    elsif user_params[:login]
+      User.by_login(user_params[:login])
     end
   end
 
@@ -83,11 +84,13 @@ class SessionsController < Devise::SessionsController
     end
   end
 
+  def two_factor_enabled?
+    find_user.try(:two_factor_enabled?)
+  end
+
   def authenticate_with_two_factor
     user = self.resource = find_user
 
-    return unless user && user.two_factor_enabled?
-
     if user_params[:otp_attempt].present? && session[:otp_user_id]
       if valid_otp_attempt?(user)
         # Remove any lingering user data from login