Add OMNIAUTH_ONLY environment variable to enforce externa log-in (#17288)

* Remove support for OAUTH_REDIRECT_AT_SIGN_IN

Fixes #15959

Introduced in #6540, OAUTH_REDIRECT_AT_SIGN_IN allowed skipping the log-in form
to instead redirect to the external OmniAuth login provider.

However, it did not prevent the log-in form on /about introduced by #10232 from
appearing, and completely broke with the introduction of #15228.

As I restoring that previous log-in flow without introducing a security
vulnerability may require extensive care and knowledge of how OmniAuth works,
this commit removes support for OAUTH_REDIRECT_AT_SIGN_IN instead for the time
being.

* Add OMNIAUTH_ONLY environment variable to enforce external log-in only

* Disable user registration when OMNIAUTH_ONLY is set to true

* Replace log-in links When OMNIAUTH_ONLY is set with exactly one OmniAuth provider

1 files changed, 5 insertions, 1 deletions

diff --git a/app/controllers/auth/registrations_controller.rb b/app/controllers/auth/registrations_controller.rb
index 3c1730f25..f37e906fd 100644
--- a/app/controllers/auth/registrations_controller.rb
+++ b/app/controllers/auth/registrations_controller.rb
@@ -81,13 +81,17 @@ class Auth::RegistrationsController < Devise::RegistrationsController
   end
 
   def check_enabled_registrations
-    redirect_to root_path if single_user_mode? || !allowed_registrations?
+    redirect_to root_path if single_user_mode? || omniauth_only? || !allowed_registrations?
   end
 
   def allowed_registrations?
     Setting.registrations_mode != 'none' || @invite&.valid_for_use?
   end
 
+  def omniauth_only?
+    ENV['OMNIAUTH_ONLY'] == 'true'
+  end
+
   def invite_code
     if params[:user]
       params[:user][:invite_code]