Validate content-type of internal calls

lib/better_errors/middleware.rb

@@ -156,6 +156,8 @@ def internal_call(env, opts)
       body = JSON.parse(request.body.read)
       return invalid_csrf_token_json_response unless request.cookies[CSRF_TOKEN_COOKIE_NAME] == body['csrfToken']
 
+      return not_acceptable_json_response unless request.content_type == 'application/json'
+
       response = @error_page.send("do_#{opts[:method]}", body)
       [200, { "Content-Type" => "application/json; charset=utf-8" }, [JSON.dump(response)]]
     end
@@ -200,5 +202,12 @@ def invalid_csrf_token_json_response
           "or something went wrong.",
       )]]
     end
+
+    def not_acceptable_json_response
+      [406, { "Content-Type" => "application/json; charset=utf-8" }, [JSON.dump(
+        error: "Request not acceptable",
+        explanation: "The internal request did not match an acceptable content type.",
+      )]]
+    end
   end
 end

spec/better_errors/middleware_spec.rb

@@ -356,11 +356,30 @@ def initialize(message, original_exception = nil)
               request_env["HTTP_COOKIE"] = "BetterErrors-CSRF-Token=csrfToken123"
             end
 
-            it 'returns the HTML content' do
-              expect(error_page).to receive(:do_variables).and_return(html: "<content>")
-              expect(json_body).to match(
-                'html' => '<content>',
-              )
+            context 'when the Content-Type of the request is application/json' do
+              before do
+                request_env['CONTENT_TYPE'] = 'application/json'
+              end
+
+              it 'returns JSON containing the HTML content' do
+                expect(error_page).to receive(:do_variables).and_return(html: "<content>")
+                expect(json_body).to match(
+                  'html' => '<content>',
+                )
+              end
+            end
+
+            context 'when the Content-Type of the request is application/json' do
+              before do
+                request_env['HTTP_CONTENT_TYPE'] = 'application/json'
+              end
+
+              it 'returns a JSON error' do
+                expect(json_body).to match(
+                  'error' => 'Request not acceptable',
+                  'explanation' => /did not match an acceptable content type/,
+                )
+              end
             end
           end