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. Only HTML and JavaScript requests are checked, so this will not protect your XML API (presumably you'll have a different authentication scheme there anyway).

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'll need something like:

class ApplicationController < ActionController::Base
  skip_before_action :verify_authenticity_token, if: :json_request?


  def json_request?

CSRF protection is turned on with the protect_from_forgery method, which checks the token and resets the session if it doesn't match what was expected. A call to this method is generated for new Rails applications by default.

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.

Included Modules
Instance Protected methods
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 208
def handle_unverified_request

The actual before_action that is used to verify the CSRF token. Don't override this directly. Provide your own forgery protection strategy instead. If you override, you'll disable same-origin `<script>` verification.

Lean on the protect_from_forgery declaration to mark which actions are due for same-origin request verification. If protect_from_forgery is enabled on an action, this before_action flags its after_action to verify that JavaScript responses are for XHR requests, ensuring they follow the browser's same-origin policy.

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 197
def verify_authenticity_token

  if !verified_request?
    if logger && log_warning_on_csrf_failure
      logger.warn "Can't verify CSRF token authenticity"