Fixed session ID fixation for ActiveRecord::SessionStore

I have found that Rails will take an invalid session ID specified by the
client and materialize a session based on that session ID. This means
that it is possible, among other things, for a client to use an
arbitrarily weak session ID or for a client to resurrect a previous used
session ID. In other words, we cannot guarantee that all session IDs are
generated by the server and that they are (statistically) unique through
time.

The fix is to always generate a new session ID in #get_session if an
existing session cannot be found under the incoming session ID.

Also added new tests that make sure that an invalid session ID is never
materialized into a new session, regardless of whether it comes in via a
cookie or a URL parameter (when :cookie_only => false).

actionpack/test/activerecord/active_record_store_test.rb

@@ -225,6 +225,36 @@ class ActiveRecordStoreTest < ActionDispatch::IntegrationTest
       assert_equal session_id, cookies['_session_id']
     end
   end
+  
+  def test_incoming_invalid_session_id_via_cookie_should_be_ignored
+    with_test_route_set do
+      open_session do |sess|
+        sess.cookies['_session_id'] = 'INVALID'
+        
+        sess.get '/set_session_value'
+        new_session_id = sess.cookies['_session_id']
+        assert_not_equal 'INVALID', new_session_id
+        
+        sess.get '/get_session_value'
+        new_session_id_2 = sess.cookies['_session_id']
+        assert_equal new_session_id, new_session_id_2
+      end
+    end
+  end
+
+  def test_incoming_invalid_session_id_via_parameter_should_be_ignored
+    with_test_route_set(:cookie_only => false) do
+      open_session do |sess|
+        sess.get '/set_session_value', :_session_id => 'INVALID'
+        new_session_id = sess.cookies['_session_id']
+        assert_not_equal 'INVALID', new_session_id
+        
+        sess.get '/get_session_value'
+        new_session_id_2 = sess.cookies['_session_id']
+        assert_equal new_session_id, new_session_id_2
+      end
+    end
+  end
 
   private
 
@@ -247,6 +277,7 @@ class ActiveRecordStoreTest < ActionDispatch::IntegrationTest
       session_class, ActiveRecord::SessionStore.session_class =
         ActiveRecord::SessionStore.session_class, "ActiveRecord::SessionStore::#{class_name.camelize}".constantize
       yield
+    ensure
       ActiveRecord::SessionStore.session_class = session_class
     end
 end

activerecord/lib/active_record/session_store.rb

@@ -297,8 +297,12 @@ module ActiveRecord
     private
       def get_session(env, sid)
         Base.silence do
-          sid ||= generate_sid
-          session = find_session(sid)
+          unless sid and session = @@session_class.find_by_session_id(sid)
+            # If the sid was nil or if there is no pre-existing session under the sid,
+            # force the generation of a new sid and associate a new session associated with the new sid
+            sid = generate_sid
+            session = @@session_class.new(:session_id => sid, :data => {})
+          end
           env[SESSION_RECORD_KEY] = session
           [sid, session.data]
         end