Controller actions are protected from Cross-Site Request Forgery (CSRF) attacks by including a token in the rendered HTML for your application. This token is stored as a random string in the session, to which an attacker does not have access. When a request reaches your application, Rails verifies the received token with the token in the session. All requests are checked except GET requests as these should be idempotent. Keep in mind that all session-oriented requests should be CSRF protected, including JavaScript and HTML requests.

Since HTML and JavaScript requests are typically made from the browser, we need to ensure to verify request authenticity for the web browser. We can use session-oriented authentication for these types of requests, by using the protect_from_forgery method in our controllers.

GET requests are not protected since they don’t have side effects like writing to the database and don’t leak sensitive information. JavaScript requests are an exception: a third-party site can use a <script> tag to reference a JavaScript URL on your site. When your JavaScript response loads on their site, it executes. With carefully crafted JavaScript on their end, sensitive data in your JavaScript response may be extracted. To prevent this, only XmlHttpRequest (known as XHR or Ajax) requests are allowed to make GET requests for JavaScript responses.

It’s important to remember that XML or JSON requests are also affected and if you’re building an API you should change forgery protection method in ApplicationController (by default: :exception):

class ApplicationController < ActionController::Base
  protect_from_forgery unless: -> { request.format.json? }

CSRF protection is turned on with the protect_from_forgery method. By default protect_from_forgery protects your session with :null_session method, which provides an empty session during request.

We may want to disable CSRF protection for APIs since they are typically designed to be state-less. That is, the request API client will handle the session for you instead of Rails.

The token parameter is named authenticity_token by default. The name and value of this token must be added to every layout that renders forms by including csrf_meta_tags in the HTML head.

Learn more about CSRF attacks and securing your application in the Ruby on Rails Security Guide.


NULL_ORIGIN_MESSAGE = <<-MSG.strip_heredoc The browser returned a 'null' origin for a request with origin-based forgery protection turned on. This usually means you have the 'no-referrer' Referrer-Policy header enabled, or that the request came from a site that refused to give its origin. This makes it impossible for Rails to verify the source of the requests. Likely the best solution is to change your referrer policy to something less strict like same-origin or strict-same-origin. If you cannot change the referrer policy, you can disable origin checking with the Rails.application.config.action_controller.forgery_protection_origin_check setting. MSG


CROSS_ORIGIN_JAVASCRIPT_WARNING = "Security warning: an embedded " \ "