Action Controller Request Forgery Protection

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

Class

Methods

Included Modules

Constants

AUTHENTICITY_TOKEN_LENGTH = 32
CSRF_TOKEN = "action_controller.csrf_token"
NULL_ORIGIN_MESSAGE = <<~MSG

Class Public methods

new(...)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 364
    def initialize(...)
      super
      @_marked_for_same_origin_verification = nil
    end
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Instance Public methods

commit_csrf_token(request)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 374
    def commit_csrf_token(request) # :doc:
      csrf_token = request.env[CSRF_TOKEN]
      csrf_token_storage_strategy.store(request, csrf_token) unless csrf_token.nil?
    end
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reset_csrf_token(request)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 369
    def reset_csrf_token(request) # :doc:
      request.env.delete(CSRF_TOKEN)
      csrf_token_storage_strategy.reset(request)
    end
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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 467
      def any_authenticity_token_valid? # :doc:
        request_authenticity_tokens.any? do |token|
          valid_authenticity_token?(session, token)
        end
      end
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compare_with_global_token(token, session = nil)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 548
      def compare_with_global_token(token, session = nil) # :doc:
        ActiveSupport::SecurityUtils.fixed_length_secure_compare(token, global_csrf_token(session))
      end
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compare_with_real_token(token, session = nil)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 544
      def compare_with_real_token(token, session = nil) # :doc:
        ActiveSupport::SecurityUtils.fixed_length_secure_compare(token, real_csrf_token(session))
      end
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csrf_token_hmac(session, identifier)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 585
      def csrf_token_hmac(session, identifier) # :doc:
        OpenSSL::HMAC.digest(
          OpenSSL::Digest::SHA256.new,
          real_csrf_token(session),
          identifier
        )
      end
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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 605
      def form_authenticity_param # :doc:
        params[request_forgery_protection_token]
      end
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form_authenticity_token(form_options: {})

Creates the authenticity token for the current request.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 479
      def form_authenticity_token(form_options: {}) # :doc:
        masked_authenticity_token(form_options: form_options)
      end
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global_csrf_token(session = nil)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 581
      def global_csrf_token(session = nil) # :doc:
        csrf_token_hmac(session, GLOBAL_CSRF_TOKEN_IDENTIFIER)
      end
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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 437
      def mark_for_same_origin_verification! # :doc:
        @_marked_for_same_origin_verification = request.get?
      end
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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 443
      def marked_for_same_origin_verification? # :doc:
        @_marked_for_same_origin_verification ||= false
      end
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mask_token(raw_token)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 537
      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
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non_xhr_javascript_response?()

Check for cross-origin JavaScript responses.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 448
      def non_xhr_javascript_response? # :doc:
        %r(\A(?:text|application)/javascript).match?(media_type) && !request.xhr?
      end
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normalize_action_path(action_path)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 635
      def normalize_action_path(action_path) # :doc:
        uri = URI.parse(action_path)

        if uri.relative? && (action_path.blank? || !action_path.start_with?("/"))
          normalize_relative_action_path(uri.path)
        else
          uri.path.chomp("/")
        end
      end
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normalize_relative_action_path(rel_action_path)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 645
      def normalize_relative_action_path(rel_action_path) # :doc:
        uri = URI.parse(request.path)
        # add the action path to the request.path
        uri.path += "/#{rel_action_path}"
        # relative path with "./path"
        uri.path.gsub!("/./", "/")

        uri.path.chomp("/")
      end
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per_form_csrf_token(session, action_path, method)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 574
      def per_form_csrf_token(session, action_path, method) # :doc:
        csrf_token_hmac(session, [action_path, method.downcase].join("#"))
      end
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protect_against_forgery?()

Checks if the controller allows forgery protection.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 610
      def protect_against_forgery? # :doc:
        allow_forgery_protection && (!session.respond_to?(:enabled?) || session.enabled?)
      end
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real_csrf_token(_session = nil)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 566
      def real_csrf_token(_session = nil) # :doc:
        csrf_token = request.env.fetch(CSRF_TOKEN) do
          request.env[CSRF_TOKEN] = csrf_token_storage_strategy.fetch(request) || generate_csrf_token
        end

        decode_csrf_token(csrf_token)
      end
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request_authenticity_tokens()

Possible authenticity tokens sent in the request.

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 474
      def request_authenticity_tokens # :doc:
        [form_authenticity_param, request.x_csrf_token]
      end
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unmask_token(masked_token)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 530
      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 500
      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

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

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

valid_per_form_csrf_token?(token, session = nil)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 552
      def valid_per_form_csrf_token?(token, session = nil) # :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
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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 625
      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
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verified_request?()

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

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 461
      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 389
      def verify_authenticity_token # :doc:
        mark_for_same_origin_verification!

        if !verified_request?
          logger.warn unverified_request_warning_message if logger && log_warning_on_csrf_failure

          handle_unverified_request
        end
      end
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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 427
      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
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xor_byte_strings(s1, s2)

📝 Source code 
# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 593
      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
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