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 are CSRF protected by default, 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 requests for JavaScript responses.

Subclasses of ActionController::Base are protected by default with the :exception strategy, which raises an ActionController::InvalidAuthenticityToken error on unverified requests.

APIs may want to disable this behavior since they are typically designed to be state-less: that is, the request API client handles the session instead of Rails. One way to achieve this is to use the :null_session strategy instead, which allows unverified requests to be handled, but with an empty session:

class ApplicationController < ActionController::Base
  protect_from_forgery with: :null_session
end

Note that API only applications don't include this module or a session middleware by default, and so don't require CSRF protection to be configured.

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.

Namespace

Module

Methods

Included Modules

Constants

AUTHENTICITY_TOKEN_LENGTH = 32
NULL_ORIGIN_MESSAGE = <<~MSG

Instance Private methods

any_authenticity_token_valid?()

Checks if any of the authenticity tokens from the request are valid.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 295
      def any_authenticity_token_valid? # :doc:
        request_authenticity_tokens.any? do |token|
          valid_authenticity_token?(session, token)
        end
      end
🔎 See on GitHub

compare_with_global_token(token, session)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 381
      def compare_with_global_token(token, session) # :doc:
        ActiveSupport::SecurityUtils.fixed_length_secure_compare(token, global_csrf_token(session))
      end
🔎 See on GitHub

compare_with_real_token(token, session)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 377
      def compare_with_real_token(token, session) # :doc:
        ActiveSupport::SecurityUtils.fixed_length_secure_compare(token, real_csrf_token(session))
      end
🔎 See on GitHub

csrf_token_hmac(session, identifier)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 415
      def csrf_token_hmac(session, identifier) # :doc:
        OpenSSL::HMAC.digest(
          OpenSSL::Digest::SHA256.new,
          real_csrf_token(session),
          identifier
        )
      end
🔎 See on GitHub

form_authenticity_param()

The form's authenticity parameter. Override to provide your own.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 435
      def form_authenticity_param # :doc:
        params[request_forgery_protection_token]
      end
🔎 See on GitHub

global_csrf_token(session)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 411
      def global_csrf_token(session) # :doc:
        csrf_token_hmac(session, GLOBAL_CSRF_TOKEN_IDENTIFIER)
      end
🔎 See on GitHub

handle_unverified_request()

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 242
      def handle_unverified_request # :doc:
        forgery_protection_strategy.new(self).handle_unverified_request
      end
🔎 See on GitHub

mark_for_same_origin_verification!()

GET requests are checked for cross-origin JavaScript after rendering.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 267
      def mark_for_same_origin_verification! # :doc:
        @marked_for_same_origin_verification = request.get?
      end
🔎 See on GitHub

marked_for_same_origin_verification?()

If the verify_authenticity_token before_action ran, verify that JavaScript responses are only served to same-origin GET requests.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 273
      def marked_for_same_origin_verification? # :doc:
        @marked_for_same_origin_verification ||= false
      end
🔎 See on GitHub

mask_token(raw_token)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 370
      def mask_token(raw_token) # :doc:
        one_time_pad = SecureRandom.random_bytes(AUTHENTICITY_TOKEN_LENGTH)
        encrypted_csrf_token = xor_byte_strings(one_time_pad, raw_token)
        masked_token = one_time_pad + encrypted_csrf_token
        encode_csrf_token(masked_token)
      end
🔎 See on GitHub

masked_authenticity_token(session, form_options: {})

Creates a masked version of the authenticity token that varies on each request. The masking is used to mitigate SSL attacks like BREACH.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 314
      def masked_authenticity_token(session, form_options: {}) # :doc:
        action, method = form_options.values_at(:action, :method)

        raw_token = if per_form_csrf_tokens && action && method
          action_path = normalize_action_path(action)
          per_form_csrf_token(session, action_path, method)
        else
          global_csrf_token(session)
        end

        mask_token(raw_token)
      end
🔎 See on GitHub

non_xhr_javascript_response?()

Check for cross-origin JavaScript responses.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 278
      def non_xhr_javascript_response? # :doc:
        %r(\A(?:text|application)/javascript).match?(media_type) && !request.xhr?
      end
🔎 See on GitHub

normalize_action_path(action_path)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 465
      def normalize_action_path(action_path) # :doc:
        uri = URI.parse(action_path)
        uri.path.chomp("/")
      end
🔎 See on GitHub

per_form_csrf_token(session, action_path, method)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 404
      def per_form_csrf_token(session, action_path, method) # :doc:
        csrf_token_hmac(session, [action_path, method.downcase].join("#"))
      end
🔎 See on GitHub

protect_against_forgery?()

Checks if the controller allows forgery protection.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 440
      def protect_against_forgery? # :doc:
        allow_forgery_protection
      end
🔎 See on GitHub

real_csrf_token(session)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 399
      def real_csrf_token(session) # :doc:
        session[:_csrf_token] ||= generate_csrf_token
        decode_csrf_token(session[:_csrf_token])
      end
🔎 See on GitHub

request_authenticity_tokens()

Possible authenticity tokens sent in the request.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 302
      def request_authenticity_tokens # :doc:
        [form_authenticity_param, request.x_csrf_token]
      end
🔎 See on GitHub

unmask_token(masked_token)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 362
      def unmask_token(masked_token) # :doc:
        # Split the token into the one-time pad and the encrypted
        # value and decrypt it.
        one_time_pad = masked_token[0...AUTHENTICITY_TOKEN_LENGTH]
        encrypted_csrf_token = masked_token[AUTHENTICITY_TOKEN_LENGTH..-1]
        xor_byte_strings(one_time_pad, encrypted_csrf_token)
      end
🔎 See on GitHub

valid_authenticity_token?(session, encoded_masked_token)

Checks the client's masked token to see if it matches the session token. Essentially the inverse of masked_authenticity_token.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 330
      def valid_authenticity_token?(session, encoded_masked_token) # :doc:
        if encoded_masked_token.nil? || encoded_masked_token.empty? || !encoded_masked_token.is_a?(String)
          return false
        end

        begin
          masked_token = decode_csrf_token(encoded_masked_token)
        rescue ArgumentError # encoded_masked_token is invalid Base64
          return false
        end

        # See if it's actually a masked token or not. In order to
        # deploy this code, we should be able to handle any unmasked
        # tokens that we've issued without error.

        if masked_token.length == AUTHENTICITY_TOKEN_LENGTH
          # This is actually an unmasked token. This is expected if
          # you have just upgraded to masked tokens, but should stop
          # happening shortly after installing this gem.
          compare_with_real_token masked_token, session

        elsif masked_token.length == AUTHENTICITY_TOKEN_LENGTH * 2
          csrf_token = unmask_token(masked_token)

          compare_with_global_token(csrf_token, session) ||
            compare_with_real_token(csrf_token, session) ||
            valid_per_form_csrf_token?(csrf_token, session)
        else
          false # Token is malformed.
        end
      end
🔎 See on GitHub

valid_per_form_csrf_token?(token, session)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 385
      def valid_per_form_csrf_token?(token, session) # :doc:
        if per_form_csrf_tokens
          correct_token = per_form_csrf_token(
            session,
            request.path.chomp("/"),
            request.request_method
          )

          ActiveSupport::SecurityUtils.fixed_length_secure_compare(token, correct_token)
        else
          false
        end
      end
🔎 See on GitHub

valid_request_origin?()

Checks if the request originated from the same origin by looking at the Origin header.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 455
      def valid_request_origin? # :doc:
        if forgery_protection_origin_check
          # We accept blank origin headers because some user agents don't send it.
          raise InvalidAuthenticityToken, NULL_ORIGIN_MESSAGE if request.origin == "null"
          request.origin.nil? || request.origin == request.base_url
        else
          true
        end
      end
🔎 See on GitHub

verified_request?()

Returns true or false if a request is verified. Checks:

  • Is it a GET or HEAD request? GETs should be safe and idempotent

  • Does the form_authenticity_token match the given token value from the params?

  • Does the X-CSRF-Token header match the form_authenticity_token?

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 289
      def verified_request? # :doc:
        !protect_against_forgery? || request.get? || request.head? ||
          (valid_request_origin? && any_authenticity_token_valid?)
      end
🔎 See on GitHub

verify_authenticity_token()

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.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 227
      def verify_authenticity_token # :doc:
        mark_for_same_origin_verification!

        if !verified_request?
          if logger && log_warning_on_csrf_failure
            if valid_request_origin?
              logger.warn "Can't verify CSRF token authenticity."
            else
              logger.warn "HTTP Origin header (#{request.origin}) didn't match request.base_url (#{request.base_url})"
            end
          end
          handle_unverified_request
        end
      end
🔎 See on GitHub

verify_same_origin_request()

If verify_authenticity_token was run (indicating that we have forgery protection enabled for this request) then also verify that we aren't serving an unauthorized cross-origin response.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 257
      def verify_same_origin_request # :doc:
        if marked_for_same_origin_verification? && non_xhr_javascript_response?
          if logger && log_warning_on_csrf_failure
            logger.warn CROSS_ORIGIN_JAVASCRIPT_WARNING
          end
          raise ActionController::InvalidCrossOriginRequest, CROSS_ORIGIN_JAVASCRIPT_WARNING
        end
      end
🔎 See on GitHub

xor_byte_strings(s1, s2)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 423
      def xor_byte_strings(s1, s2) # :doc:
        s2 = s2.dup
        size = s1.bytesize
        i = 0
        while i < size
          s2.setbyte(i, s1.getbyte(i) ^ s2.getbyte(i))
          i += 1
        end
        s2
      end
🔎 See on GitHub