request_forgery_protection.rb

# frozen_string_literal: true

require "rack/session/abstract/id"
require "action_controller/metal/exceptions"
require "active_support/security_utils"

module ActionController #:nodoc:
  class InvalidAuthenticityToken < ActionControllerError #:nodoc:
  end

  class InvalidCrossOriginRequest < ActionControllerError #:nodoc:
  end

  # 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 <tt>protect_from_forgery</tt> 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.
  #
  # It's important to remember that XML or JSON requests are also checked by default. If
  # you're building an API or an SPA you could change forgery protection method in
  # <tt>ApplicationController</tt> (by default: <tt>:exception</tt>):
  #
  #   class ApplicationController < ActionController::Base
  #     protect_from_forgery unless: -> { request.format.json? }
  #   end
  #
  # It is generally safe to exclude XHR requests from CSRF protection
  # (like the code snippet above does), because XHR requests can only be made from
  # the same origin. Note however that any cross-origin third party domain
  # allowed via {CORS}[https://en.wikipedia.org/wiki/Cross-origin_resource_sharing]
  # will also be able to create XHR requests. Be sure to check your
  # CORS configuration before disabling forgery protection for XHR.
  #
  # CSRF protection is turned on with the <tt>protect_from_forgery</tt> method.
  # By default <tt>protect_from_forgery</tt> protects your session with
  # <tt>:null_session</tt> 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 <tt>authenticity_token</tt> by default. The name and
  # value of this token must be added to every layout that renders forms by including
  # <tt>csrf_meta_tags</tt> in the HTML +head+.
  #
  # Learn more about CSRF attacks and securing your application in the
  # {Ruby on Rails Security Guide}[https://guides.rubyonrails.org/security.html].
  module RequestForgeryProtection
    extend ActiveSupport::Concern

    include AbstractController::Helpers
    include AbstractController::Callbacks

    included do
      # Sets the token parameter name for RequestForgery. Calling +protect_from_forgery+
      # sets it to <tt>:authenticity_token</tt> by default.
      config_accessor :request_forgery_protection_token
      self.request_forgery_protection_token ||= :authenticity_token

      # Holds the class which implements the request forgery protection.
      config_accessor :forgery_protection_strategy
      self.forgery_protection_strategy = nil

      # Controls whether request forgery protection is turned on or not. Turned off by default only in test mode.
      config_accessor :allow_forgery_protection
      self.allow_forgery_protection = true if allow_forgery_protection.nil?

      # Controls whether a CSRF failure logs a warning. On by default.
      config_accessor :log_warning_on_csrf_failure
      self.log_warning_on_csrf_failure = true

      # Controls whether the Origin header is checked in addition to the CSRF token.
      config_accessor :forgery_protection_origin_check
      self.forgery_protection_origin_check = false

      # Controls whether form-action/method specific CSRF tokens are used.
      config_accessor :per_form_csrf_tokens
      self.per_form_csrf_tokens = false

      # Controls whether forgery protection is enabled by default.
      config_accessor :default_protect_from_forgery
      self.default_protect_from_forgery = false

      helper_method :form_authenticity_token
      helper_method :protect_against_forgery?
    end

    module ClassMethods
      # Turn on request forgery protection. Bear in mind that GET and HEAD requests are not checked.
      #
      #   class ApplicationController < ActionController::Base
      #     protect_from_forgery
      #   end
      #
      #   class FooController < ApplicationController
      #     protect_from_forgery except: :index
      #   end
      #
      # You can disable forgery protection on controller by skipping the verification before_action:
      #
      #   skip_before_action :verify_authenticity_token
      #
      # Valid Options:
      #
      # * <tt>:only/:except</tt> - Only apply forgery protection to a subset of actions. For example <tt>only: [ :create, :create_all ]</tt>.
      # * <tt>:if/:unless</tt> - Turn off the forgery protection entirely depending on the passed Proc or method reference.
      # * <tt>:prepend</tt> - By default, the verification of the authentication token will be added at the position of the
      #   protect_from_forgery call in your application. This means any callbacks added before are run first. This is useful
      #   when you want your forgery protection to depend on other callbacks, like authentication methods (Oauth vs Cookie auth).
      #
      #   If you need to add verification to the beginning of the callback chain, use <tt>prepend: true</tt>.
      # * <tt>:with</tt> - Set the method to handle unverified request.
      #
      # Valid unverified request handling methods are:
      # * <tt>:exception</tt> - Raises ActionController::InvalidAuthenticityToken exception.
      # * <tt>:reset_session</tt> - Resets the session.
      # * <tt>:null_session</tt> - Provides an empty session during request but doesn't reset it completely. Used as default if <tt>:with</tt> option is not specified.
      def protect_from_forgery(options = {})
        options = options.reverse_merge(prepend: false)

        self.forgery_protection_strategy = protection_method_class(options[:with] || :null_session)
        self.request_forgery_protection_token ||= :authenticity_token
        before_action :verify_authenticity_token, options
        append_after_action :verify_same_origin_request
      end

      # Turn off request forgery protection. This is a wrapper for:
      #
      #   skip_before_action :verify_authenticity_token
      #
      # See +skip_before_action+ for allowed options.
      def skip_forgery_protection(options = {})
        skip_before_action :verify_authenticity_token, options
      end

      private
        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
    end

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

        # This is the method that defines the application behavior when a request is found to be unverified.
        def handle_unverified_request
          request = @controller.request
          request.session = NullSessionHash.new(request)
          request.flash = nil
          request.session_options = { skip: true }
          request.cookie_jar = NullCookieJar.build(request, {})
        end

        private
          class NullSessionHash < Rack::Session::Abstract::SessionHash #:nodoc:
            def initialize(req)
              super(nil, req)
              @data = {}
              @loaded = true
            end

            # no-op
            def destroy; end

            def exists?
              true
            end
          end

          class NullCookieJar < ActionDispatch::Cookies::CookieJar #:nodoc:
            def write(*)
              # nothing
            end
          end
      end

      class ResetSession
        def initialize(controller)
          @controller = controller
        end

        def handle_unverified_request
          @controller.reset_session
        end
      end

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

        def handle_unverified_request
          raise ActionController::InvalidAuthenticityToken
        end
      end
    end

    private
      # 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
      # <tt><script></tt> 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.
      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

      def handle_unverified_request # :doc:
        forgery_protection_strategy.new(self).handle_unverified_request
      end

      #:nodoc:
      CROSS_ORIGIN_JAVASCRIPT_WARNING = "Security warning: an embedded " \
        "<script> tag on another site requested protected JavaScript. " \
        "If you know what you're doing, go ahead and disable forgery " \
        "protection on this action to permit cross-origin JavaScript embedding."
      private_constant :CROSS_ORIGIN_JAVASCRIPT_WARNING
      # :startdoc:

      # 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.
      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

      # GET requests are checked for cross-origin JavaScript after rendering.
      def mark_for_same_origin_verification! # :doc:
        @marked_for_same_origin_verification = request.get?
      end

      # If the +verify_authenticity_token+ before_action ran, verify that
      # JavaScript responses are only served to same-origin GET requests.
      def marked_for_same_origin_verification? # :doc:
        @marked_for_same_origin_verification ||= false
      end

      # Check for cross-origin JavaScript responses.
      def non_xhr_javascript_response? # :doc:
        %r(\A(?:text|application)/javascript).match?(media_type) && !request.xhr?
      end

      AUTHENTICITY_TOKEN_LENGTH = 32

      # 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?
      def verified_request? # :doc:
        !protect_against_forgery? || request.get? || request.head? ||
          (valid_request_origin? && any_authenticity_token_valid?)
      end

      # Checks if any of the authenticity tokens from the request are valid.
      def any_authenticity_token_valid? # :doc:
        request_authenticity_tokens.any? do |token|
          valid_authenticity_token?(session, token)
        end
      end

      # Possible authenticity tokens sent in the request.
      def request_authenticity_tokens # :doc:
        [form_authenticity_param, request.x_csrf_token]
      end

      # Sets the token value for the current session.
      def form_authenticity_token(form_options: {})
        masked_authenticity_token(session, form_options: form_options)
      end

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

        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
        Base64.urlsafe_encode64(masked_token, padding: false)
      end

      # Checks the client's masked token to see if it matches the
      # session token. Essentially the inverse of
      # +masked_authenticity_token+.
      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 = Base64.urlsafe_decode64(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_real_token(csrf_token, session) ||
            valid_per_form_csrf_token?(csrf_token, session)
        else
          false # Token is malformed.
        end
      end

      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

      def compare_with_real_token(token, session) # :doc:
        ActiveSupport::SecurityUtils.fixed_length_secure_compare(token, real_csrf_token(session))
      end

      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

      def real_csrf_token(session) # :doc:
        session[:_csrf_token] ||= SecureRandom.urlsafe_base64(AUTHENTICITY_TOKEN_LENGTH, padding: false)
        Base64.urlsafe_decode64(session[:_csrf_token])
      end

      def per_form_csrf_token(session, action_path, method) # :doc:
        OpenSSL::HMAC.digest(
          OpenSSL::Digest::SHA256.new,
          real_csrf_token(session),
          [action_path, method.downcase].join("#")
        )
      end

      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

      # The form's authenticity parameter. Override to provide your own.
      def form_authenticity_param # :doc:
        params[request_forgery_protection_token]
      end

      # Checks if the controller allows forgery protection.
      def protect_against_forgery? # :doc:
        allow_forgery_protection
      end

      NULL_ORIGIN_MESSAGE = <<~MSG
        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-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

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

      def normalize_action_path(action_path) # :doc:
        uri = URI.parse(action_path)
        uri.path.chomp("/")
      end
  end
end