Raise exception on CSRF failure

app/controllers/application_controller.rb

@@ -3,7 +3,7 @@ class ApplicationController < ActionController::Base
 
   before_action :authenticate_user!
 
-  protect_from_forgery
+  protect_from_forgery with: :exception
 
   def error_400
     error 400

app/controllers/deployments_controller.rb

@@ -1,6 +1,8 @@
 class DeploymentsController < ApplicationController
   class ApplicationConflictError < RuntimeError; end
 
+  skip_forgery_protection if: :api_request_to_create_deployment?
+
   rescue_from ApplicationConflictError do
     head :conflict
   end
@@ -73,4 +75,8 @@
       :environment_filter,
     )
   end
+
+  def api_request_to_create_deployment?
+    GDS::SSO::ApiAccess.api_call?(request.env) && action_name == "create"
+  end
 end

test/functional/deployments_controller_test.rb

@@ -37,6 +37,25 @@ class DeploymentsControllerTest < ActionController::TestCase
   end
 
   context "POST create" do
+    context "when forgery protection is enabled" do
+      setup do
+        @controller.allow_forgery_protection = true
+      end
+
+      should "enable forgery protection for non-API requests" do
+        assert_raises(ActionController::InvalidAuthenticityToken) do
+          post :create, params: { repo: "org/app", deployment: { version: "1", environment: "env" } }
+        end
+      end
+
+      should "skip forgery protection for API requests" do
+        request.headers["Authorization"] = "Bearer <token>"
+        post :create, params: { repo: "org/app", deployment: { version: "1", environment: "env" } }
+
+        assert_response :ok
+      end
+    end
+
     setup do
       stub_request(:get, "http://docs.publishing.service.gov.uk/apps.json").to_return(status: 200, body: "", headers: {})
     end