Permit same-origin connections by default

WebSocket always defers the decision to the server, because it didn't
have to deal with legacy compatibility... but the same-origin policy is
still a reasonable default.

Origin checks do not protect against a directly connecting attacker --
they can lie about their host, but can also lie about their origin.
Origin checks protect against a connection from 3rd-party controlled
script in a context where a victim browser's cookies will be passed
along. And if an attacker has breached that protection, they've already
compromised the HTTP session, so treating the WebSocket connection in
the same way seems reasonable.

In case this logic proves incorrect (or anyone just wants to be more
paranoid), we retain a config option to disable it.

actioncable/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Permit same-origin connections by default.
+
+    New option `config.action_cable.allow_same_origin_as_host = false`
+    to disable.
+
+    *Dávid Halász*, *Matthew Draper*
+
 *   Prevent race where the client could receive and act upon a
     subscription confirmation before the channel's `subscribed` method
     completed.

actioncable/README.md

@@ -326,7 +326,10 @@ Rails.application.paths.add "config/cable", with: "somewhere/else/cable.yml"
 
 ### Allowed Request Origins
 
-Action Cable will only accept requests from specified origins, which are passed to the server config as an array. The origins can be instances of strings or regular expressions, against which a check for match will be performed.
+Action Cable will only accept requests from specific origins.
+
+By default, only an origin matching the cable server itself will be permitted.
+Additional origins can be specified using strings or regular expressions, provided in an array.
 
 ```ruby
 Rails.application.config.action_cable.allowed_request_origins = ['http://rubyonrails.com', /http:\/\/ruby.*/]
@@ -334,15 +337,17 @@ Rails.application.config.action_cable.allowed_request_origins = ['http://rubyonr
 
 When running in the development environment, this defaults to "http://localhost:3000".
 
-To disable and allow requests from any origin:
+To disable protection and allow requests from any origin:
 
 ```ruby
 Rails.application.config.action_cable.disable_request_forgery_protection = true
 ```
 
-It is also possible to allow origins that are starting with the actual HTTP HOST header:
+To disable automatic access for same-origin requests, and strictly allow
+only the configured origins:
+
 ```ruby
-Rails.application.config.action_cable.allow_same_origin_as_host = true
+Rails.application.config.action_cable.allow_same_origin_as_host = false
 ```
 
 ### Consumer Configuration

actioncable/lib/action_cable/connection/base.rb

@@ -196,9 +196,9 @@ module ActionCable
           return true if server.config.disable_request_forgery_protection
 
           proto = Rack::Request.new(env).ssl? ? "https" : "http"
-          if Array(server.config.allowed_request_origins).any? { |allowed_origin|  allowed_origin === env["HTTP_ORIGIN"] }
+          if server.config.allow_same_origin_as_host && env["HTTP_ORIGIN"] == "#{proto}://#{env['HTTP_HOST']}"
             true
-          elsif server.config.allow_same_origin_as_host && env["HTTP_ORIGIN"] == "#{proto}://#{env['HTTP_HOST']}"
+          elsif Array(server.config.allowed_request_origins).any? { |allowed_origin|  allowed_origin === env["HTTP_ORIGIN"] }
             true
           else
             logger.error("Request origin not allowed: #{env['HTTP_ORIGIN']}")

actioncable/lib/action_cable/server/configuration.rb

@@ -15,7 +15,7 @@ module ActionCable
         @worker_pool_size = 4
 
         @disable_request_forgery_protection = false
-        @allow_same_origin_as_host = false
+        @allow_same_origin_as_host = true
       end
 
       # Returns constant of subscription adapter specified in config/cable.yml.

actioncable/test/connection/cross_site_forgery_test.rb

@@ -13,12 +13,13 @@ class ActionCable::Connection::CrossSiteForgeryTest < ActionCable::TestCase
   setup do
     @server = TestServer.new
     @server.config.allowed_request_origins = %w( http://rubyonrails.com )
+    @server.config.allow_same_origin_as_host = false
   end
 
   teardown do
     @server.config.disable_request_forgery_protection = false
     @server.config.allowed_request_origins = []
-    @server.config.allow_same_origin_as_host = false
+    @server.config.allow_same_origin_as_host = true
   end
 
   test "disable forgery protection" do