Avoid CSRF check on SAML failure endpoint

SAML and OAuth failures should cause a message to be presented, as well
as logging that an attempt was made. These were incorrectly prevented by
the CSRF check on POST endpoints such as SAML.

In addition we were using a NullSession forgery protection, which made
testing more difficult and could have allowed account linking to take
place if a CSRF was ever needed but not present.

app/controllers/omniauth_callbacks_controller.rb

@@ -4,7 +4,7 @@ class OmniauthCallbacksController < Devise::OmniauthCallbacksController
   include AuthenticatesWithTwoFactor
   include Devise::Controllers::Rememberable
 
-  protect_from_forgery except: [:kerberos, :saml, :cas3], prepend: true
+  protect_from_forgery except: [:kerberos, :saml, :cas3, :failure], with: :exception, prepend: true
 
   def handle_omniauth
     omniauth_flow(Gitlab::Auth::OAuth)

changelogs/unreleased/jej-avoid-csrf-check-on-saml-failure.yml

+---
+title: Display SAML failure messages instead of expecting CSRF token
+merge_request: 24509
+author:
+type: fixed

spec/controllers/omniauth_callbacks_controller_spec.rb

@@ -45,6 +45,29 @@ describe OmniauthCallbacksController, type: :controller do
       end
     end
 
+    context 'when sign in fails' do
+      include RoutesHelpers
+
+      let(:extern_uid) { 'my-uid' }
+      let(:provider) { :saml }
+
+      def stub_route_as(path)
+        allow(@routes).to receive(:generate_extras) { [path, []] }
+      end
+
+      it 'it calls through to the failure handler' do
+        request.env['omniauth.error'] = OneLogin::RubySaml::ValidationError.new("Fingerprint mismatch")
+        request.env['omniauth.error.strategy'] = OmniAuth::Strategies::SAML.new(nil)
+        stub_route_as('/users/auth/saml/callback')
+
+        ForgeryProtection.with_forgery_protection do
+          post :failure
+        end
+
+        expect(flash[:alert]).to match(/Fingerprint mismatch/)
+      end
+    end
+
     context 'when a redirect fragment is provided' do
       let(:provider) { :jwt }
       let(:extern_uid) { 'my-uid' }