configure how unverified request will be handled

can be configured using `:with` option in `protect_from_forgery` method
or `request_forgery_protection_method` config option

possible values:
- :reset_session (default)
- :exception

new applications are generated with:

    protect_from_forgery :with => :exception

actionpack/lib/action_controller/metal/request_forgery_protection.rb

@@ -37,6 +37,10 @@ module ActionController #:nodoc:
       config_accessor :request_forgery_protection_token
       self.request_forgery_protection_token ||= :authenticity_token
 
+      # Controls how unverified request will be handled
+      config_accessor :request_forgery_protection_method
+      self.request_forgery_protection_method ||= :reset_session
+
       # 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?
@@ -64,8 +68,10 @@ module ActionController #:nodoc:
       # Valid Options:
       #
       # * <tt>:only/:except</tt> - Passed to the <tt>before_filter</tt> call. Set which actions are verified.
+      # * <tt>:with</tt> - Set the method to handle unverified request. Valid values: <tt>:exception</tt> and <tt>:reset_session</tt> (default).
       def protect_from_forgery(options = {})
         self.request_forgery_protection_token ||= :authenticity_token
+        self.request_forgery_protection_method = options.delete(:with) if options.key?(:with)
         prepend_before_filter :verify_authenticity_token, options
       end
     end
@@ -80,9 +86,19 @@ module ActionController #:nodoc:
       end
 
       # This is the method that defines the application behavior when a request is found to be unverified.
-      # By default, \Rails resets the session when it finds an unverified request.
+      # By default, \Rails uses <tt>request_forgery_protection_method</tt> when it finds an unverified request:
+      #
+      # * <tt>:reset_session</tt> - Resets the session.
+      # * <tt>:exception</tt>: - Raises ActionController::InvalidAuthenticityToken exception.
       def handle_unverified_request
-        reset_session
+        case request_forgery_protection_method
+        when :exception
+          raise ActionController::InvalidAuthenticityToken
+        when :reset_session
+          reset_session
+        else
+          raise ArgumentError, 'Invalid request forgery protection method, use :exception or :reset_session'
+        end
       end
 
       # Returns true or false if a request is verified. Checks:

actionpack/test/controller/request_forgery_protection_test.rb

@@ -43,7 +43,7 @@ class RequestForgeryProtectionController < ActionController::Base
   protect_from_forgery :only => %w(index meta)
 end
 
-class RequestForgeryProtectionControllerUsingOldBehaviour < ActionController::Base
+class RequestForgeryProtectionControllerUsingException < ActionController::Base
   include RequestForgeryProtectionActions
   protect_from_forgery :only => %w(index meta)
 
@@ -215,7 +215,7 @@ class RequestForgeryProtectionControllerTest < ActionController::TestCase
   end
 end
 
-class RequestForgeryProtectionControllerUsingOldBehaviourTest < ActionController::TestCase
+class RequestForgeryProtectionControllerUsingExceptionTest < ActionController::TestCase
   include RequestForgeryProtectionTests
   def assert_blocked
     assert_raises(ActionController::InvalidAuthenticityToken) do

railties/lib/rails/generators/rails/app/templates/app/controllers/application_controller.rb

@@ -1,3 +1,5 @@
 class ApplicationController < ActionController::Base
-  protect_from_forgery
+  # prevent CSRF attacks by raising an exception,
+  # if your application has an API, you'll probably need to use :reset_session
+  protect_from_forgery :with => :exception
 end

railties/test/application/configuration_test.rb

@@ -275,19 +275,23 @@ module ApplicationTests
 
       require "#{app_path}/config/environment"
 
+      token = "cf50faa3fe97702ca1ae"
+      PostsController.any_instance.stubs(:form_authenticity_token).returns(token)
+      params = {:authenticity_token => token}
+
       get "/posts/1"
       assert_match /patch/, last_response.body
 
-      patch "/posts/1"
+      patch "/posts/1", params
       assert_match /update/, last_response.body
 
-      patch "/posts/1"
+      patch "/posts/1", params
       assert_equal 200, last_response.status
 
-      put "/posts/1"
+      put "/posts/1", params
       assert_match /update/, last_response.body
 
-      put "/posts/1"
+      put "/posts/1", params
       assert_equal 200, last_response.status
     end
 
@@ -528,6 +532,12 @@ module ApplicationTests
       end
       RUBY
 
+      app_file 'app/controllers/application_controller.rb', <<-RUBY
+      class ApplicationController < ActionController::Base
+        protect_from_forgery :with => :reset_session # as we are testing API here
+      end
+      RUBY
+
       app_file 'app/controllers/posts_controller.rb', <<-RUBY
       class PostsController < ApplicationController
         def create