protect_from_forgery with: :exception where?

Question

How does protect_from_forgery with: :exception work?

I'd like to edit the code to look at it and learn from it. However, I cannot find where it is placed as in a higher level of abstraction.

Answer 1

You can find it here on Github : https://github.com/rails/rails/blob/c60be72c5243c21303b067c9c5cc398111cf48c8/actionpack/lib/action_controller/metal/request_forgery_protection.rb#L88

def protect_from_forgery(options = {})
  self.forgery_protection_strategy = protection_method_class(options[:with] || :null_session)
  self.request_forgery_protection_token ||= :authenticity_token
  prepend_before_action :verify_authenticity_token, options
end

The with: :exception is passed to protection_method_class(:exception). Which does :

def protection_method_class(name)
  ActionController::RequestForgeryProtection::ProtectionMethods.const_get(name.to_s.classify)
  rescue NameError
  raise ArgumentError, 'Invalid request forgery protection method, use :null_session, :exception, or :reset_session'
end

Then this ActionController::RequestForgeryProtection::ProtectionMethods.const_get(name.to_s.classify). name.to_s.classify here will be Exception.

Then you can find:

module ProtectionMethods
  class Exception
    def initialize(controller)
      @controller = controller
    end

    def handle_unverified_request
      raise ActionController::InvalidAuthenticityToken
    end
  end
end

All of this sets the way the invalid authenticity will be handled. Then it sets a before_action: :verify_authenticity_token.

def verify_authenticity_token
  unless verified_request?
    logger.warn "Can't verify CSRF token authenticity" if logger
    handle_unverified_request
  end
end

Which uses the previously defined strategy:

def handle_unverified_request
  forgery_protection_strategy.new(self).handle_unverified_request
end

To raise the exception as defined in Exception.