Purpose Metadata For Signed And Encrypted Cookies

Purpose metadata prevents cookie values from being
copy-pasted and ensures that the cookie is used only
for its originally intended purpose.

The Purpose and Expiry metadata are embedded inside signed/encrypted
cookies and will not be readable on previous versions of Rails.

We can switch off purpose and expiry metadata embedded in
signed and encrypted cookies using

	config.action_dispatch.use_cookies_with_metadata = false

if you want your cookies to be readable on older versions of Rails.

actionpack/lib/action_dispatch/middleware/cookies.rb

@@ -81,6 +81,10 @@ module ActionDispatch
       get_header Cookies::COOKIES_ROTATIONS
     end
 
+    def use_cookies_with_metadata
+      get_header Cookies::USE_COOKIES_WITH_METADATA
+    end
+
     # :startdoc:
   end
 
@@ -182,6 +186,7 @@ module ActionDispatch
     COOKIES_SERIALIZER = "action_dispatch.cookies_serializer".freeze
     COOKIES_DIGEST = "action_dispatch.cookies_digest".freeze
     COOKIES_ROTATIONS = "action_dispatch.cookies_rotations".freeze
+    USE_COOKIES_WITH_METADATA = "action_dispatch.use_cookies_with_metadata".freeze
 
     # Cookies can typically store 4096 bytes.
     MAX_COOKIE_SIZE = 4096
@@ -470,7 +475,7 @@ module ActionDispatch
 
       def [](name)
         if data = @parent_jar[name.to_s]
-          parse name, data
+          parse(name, data, purpose: "cookie.#{name}") || parse(name, data)
         end
       end
 
@@ -481,7 +486,7 @@ module ActionDispatch
           options = { value: options }
         end
 
-        commit(options)
+        commit(name, options)
         @parent_jar[name] = options
       end
 
@@ -497,13 +502,24 @@ module ActionDispatch
           end
         end
 
-        def parse(name, data); data; end
-        def commit(options); end
+        def cookie_metadata(name, options)
+          if request.use_cookies_with_metadata
+            metadata = expiry_options(options)
+            metadata[:purpose] = "cookie.#{name}"
+
+            metadata
+          else
+            {}
+          end
+        end
+
+        def parse(name, data, purpose: nil); data; end
+        def commit(name, options); end
     end
 
     class PermanentCookieJar < AbstractCookieJar # :nodoc:
       private
-        def commit(options)
+        def commit(name, options)
           options[:expires] = 20.years.from_now
         end
     end
@@ -583,14 +599,14 @@ module ActionDispatch
       end
 
       private
-        def parse(name, signed_message)
+        def parse(name, signed_message, purpose: nil)
           deserialize(name) do |rotate|
-            @verifier.verified(signed_message, on_rotation: rotate)
+            @verifier.verified(signed_message, on_rotation: rotate, purpose: purpose)
           end
         end
 
-        def commit(options)
-          options[:value] = @verifier.generate(serialize(options[:value]), expiry_options(options))
+        def commit(name, options)
+          options[:value] = @verifier.generate(serialize(options[:value]), cookie_metadata(name, options))
 
           raise CookieOverflow if options[:value].bytesize > MAX_COOKIE_SIZE
         end
@@ -631,16 +647,16 @@ module ActionDispatch
       end
 
       private
-        def parse(name, encrypted_message)
+        def parse(name, encrypted_message, purpose: nil)
           deserialize(name) do |rotate|
-            @encryptor.decrypt_and_verify(encrypted_message, on_rotation: rotate)
+            @encryptor.decrypt_and_verify(encrypted_message, on_rotation: rotate, purpose: purpose)
           end
         rescue ActiveSupport::MessageEncryptor::InvalidMessage, ActiveSupport::MessageVerifier::InvalidSignature
           parse_legacy_signed_message(name, encrypted_message)
         end
 
-        def commit(options)
-          options[:value] = @encryptor.encrypt_and_sign(serialize(options[:value]), expiry_options(options))
+        def commit(name, options)
+          options[:value] = @encryptor.encrypt_and_sign(serialize(options[:value]), cookie_metadata(name, options))
 
           raise CookieOverflow if options[:value].bytesize > MAX_COOKIE_SIZE
         end

actionpack/lib/action_dispatch/railtie.rb

@@ -21,6 +21,7 @@ module ActionDispatch
     config.action_dispatch.encrypted_signed_cookie_salt = "signed encrypted cookie"
     config.action_dispatch.authenticated_encrypted_cookie_salt = "authenticated encrypted cookie"
     config.action_dispatch.use_authenticated_cookie_encryption = false
+    config.action_dispatch.use_cookies_with_metadata = false
     config.action_dispatch.perform_deep_munge = true
 
     config.action_dispatch.default_headers = {

actionpack/test/dispatch/cookies_test.rb

@@ -289,6 +289,46 @@ class CookiesTest < ActionController::TestCase
       cookies[:user_name] = { value: "assain", expires: 2.hours }
       head :ok
     end
+
+    def encrypted_discount_and_user_id_cookie
+      cookies.encrypted[:user_id] = { value: 50, expires: 1.hour }
+      cookies.encrypted[:discount_percentage] = 10
+
+      head :ok
+    end
+
+    def signed_discount_and_user_id_cookie
+      cookies.signed[:user_id] = { value: 50, expires: 1.hour }
+      cookies.signed[:discount_percentage] = 10
+
+      head :ok
+    end
+
+    def rails_5_2_stable_encrypted_cookie_with_authenticated_encryption_flag_on
+      # cookies.encrypted[:favorite] = { value: "5-2-Stable Chocolate Cookies", expires: 1000.years }
+      cookies[:favorite] = "KvH5lIHvX5vPQkLIK63r/NuIMwzWky8M0Zwk8SZ6DwUv8+srf36geR4nWq5KmhsZIYXA8NRdCZYIfxMKJsOFlz77Gf+Fq8vBBCWJTp95rx39A28TCUTJEyMhCNJO5eie7Skef76Qt5Jo/SCnIADAhzyGQkGBopKRcA==--qXZZFWGbCy6N8AGy--WswoH+xHrNh9MzSXDpB2fA=="
+
+      head :ok
+    end
+
+    def rails_5_2_stable_encrypted_cookie_with_authenticated_encryption_flag_off
+      cookies[:favorite] = "Wmg4amgvcVVvWGcwK3c4WjJEbTdRQUgrWXhBdDliUTR0cVNidXpmVTMrc2RjcitwUzVsWWEwZGtuVGtFUjJwNi0tcVhVMTFMOTQ1d0hIVE1FK0pJc05SQT09--8b2a55c375049a50f7a959b9d42b31ef0b2bb594"
+
+      head :ok
+    end
+
+    def rails_5_2_stable_signed_cookie_with_authenticated_encryption_flag_on
+      # cookies.signed[:favorite] = { value: "5-2-Stable Choco Chip Cookie", expires: 1000.years }
+      cookies[:favorite] = "eyJfcmFpbHMiOnsibWVzc2FnZSI6IkJBaEpJaUUxTFRJdFUzUmhZbXhsSUVOb2IyTnZJRU5vYVhBZ1EyOXZhMmxsQmpvR1JWUT0iLCJleHAiOiIzMDE4LTA3LTExVDE2OjExOjI2Ljc1M1oiLCJwdXIiOm51bGx9fQ==--7df5d885b78b70a501d6e82140ae91b24060ac00"
+
+      head :ok
+    end
+
+    def rails_5_2_stable_signed_cookie_with_authenticated_encryption_flag_off
+      cookies[:favorite] = "BAhJIiE1LTItU3RhYmxlIENob2NvIENoaXAgQ29va2llBjoGRVQ=--50bbdbf8d64f5a3ec3e54878f54d4f55b6cb3aff"
+
+      head :ok
+    end
   end
 
   tests TestController
@@ -1274,6 +1314,8 @@ class CookiesTest < ActionController::TestCase
   end
 
   def test_signed_cookie_with_expires_set_relatively
+    request.env["action_dispatch.use_cookies_with_metadata"] = true
+
     cookies.signed[:user_name] = { value: "assain", expires: 2.hours }
 
     travel 1.hour
@@ -1284,6 +1326,8 @@ class CookiesTest < ActionController::TestCase
   end
 
   def test_encrypted_cookie_with_expires_set_relatively
+    request.env["action_dispatch.use_cookies_with_metadata"] = true
+
     cookies.encrypted[:user_name] = { value: "assain", expires: 2.hours }
 
     travel 1.hour
@@ -1300,6 +1344,128 @@ class CookiesTest < ActionController::TestCase
     end
   end
 
+  def test_purpose_metadata_for_encrypted_cookies
+    get :encrypted_discount_and_user_id_cookie
+
+    cookies[:discount_percentage] = cookies[:user_id]
+    assert_equal 50, cookies.encrypted[:discount_percentage]
+
+    request.env["action_dispatch.use_cookies_with_metadata"] = true
+
+    get :encrypted_discount_and_user_id_cookie
+
+    cookies[:discount_percentage] = cookies[:user_id]
+    assert_nil cookies.encrypted[:discount_percentage]
+  end
+
+  def test_purpose_metadata_for_signed_cookies
+    get :signed_discount_and_user_id_cookie
+
+    cookies[:discount_percentage] = cookies[:user_id]
+    assert_equal 50, cookies.signed[:discount_percentage]
+
+    request.env["action_dispatch.use_cookies_with_metadata"] = true
+
+    get :signed_discount_and_user_id_cookie
+
+    cookies[:discount_percentage] = cookies[:user_id]
+    assert_nil cookies.signed[:discount_percentage]
+  end
+
+  def test_switch_off_metadata_for_encrypted_cookies_if_config_is_false
+    request.env["action_dispatch.use_cookies_with_metadata"] = false
+
+    get :encrypted_discount_and_user_id_cookie
+
+    travel 2.hours
+    assert_equal 50, cookies.encrypted[:user_id]
+
+    cookies[:discount_percentage] = cookies[:user_id]
+    assert_not_equal 10, cookies.encrypted[:discount_percentage]
+    assert_equal 50, cookies.encrypted[:discount_percentage]
+  end
+
+  def test_switch_off_metadata_for_signed_cookies_if_config_is_false
+    request.env["action_dispatch.use_cookies_with_metadata"] = false
+
+    get :signed_discount_and_user_id_cookie
+
+    travel 2.hours
+    assert_equal 50, cookies.signed[:user_id]
+
+    cookies[:discount_percentage] = cookies[:user_id]
+    assert_not_equal 10, cookies.signed[:discount_percentage]
+    assert_equal 50, cookies.signed[:discount_percentage]
+  end
+
+  def test_read_rails_5_2_stable_encrypted_cookies_if_config_is_false
+    request.env["action_dispatch.use_cookies_with_metadata"] = false
+
+    get :rails_5_2_stable_encrypted_cookie_with_authenticated_encryption_flag_on
+
+    assert_equal "5-2-Stable Chocolate Cookies", cookies.encrypted[:favorite]
+
+    freeze_time do
+      travel 1001.years
+      assert_nil cookies.encrypted[:favorite]
+    end
+
+    get :rails_5_2_stable_encrypted_cookie_with_authenticated_encryption_flag_off
+
+    assert_equal "5-2-Stable Chocolate Cookies", cookies.encrypted[:favorite]
+  end
+
+  def test_read_rails_5_2_stable_signed_cookies_if_config_is_false
+    request.env["action_dispatch.use_cookies_with_metadata"] = false
+
+    get :rails_5_2_stable_signed_cookie_with_authenticated_encryption_flag_on
+
+    assert_equal "5-2-Stable Choco Chip Cookie", cookies.signed[:favorite]
+
+    freeze_time do
+      travel 1001.years
+      assert_nil cookies.signed[:favorite]
+    end
+
+    get :rails_5_2_stable_signed_cookie_with_authenticated_encryption_flag_off
+
+    assert_equal "5-2-Stable Choco Chip Cookie", cookies.signed[:favorite]
+  end
+
+  def test_read_rails_5_2_stable_encrypted_cookies_if_use_metadata_config_is_true
+    request.env["action_dispatch.use_cookies_with_metadata"] = true
+
+    get :rails_5_2_stable_encrypted_cookie_with_authenticated_encryption_flag_on
+
+    assert_equal "5-2-Stable Chocolate Cookies", cookies.encrypted[:favorite]
+
+    freeze_time do
+      travel 1001.years
+      assert_nil cookies.encrypted[:favorite]
+    end
+
+    get :rails_5_2_stable_encrypted_cookie_with_authenticated_encryption_flag_off
+
+    assert_equal "5-2-Stable Chocolate Cookies", cookies.encrypted[:favorite]
+  end
+
+  def test_read_rails_5_2_stable_signed_cookies_if_use_metadata_config_is_true
+    request.env["action_dispatch.use_cookies_with_metadata"] = true
+
+    get :rails_5_2_stable_signed_cookie_with_authenticated_encryption_flag_on
+
+    assert_equal "5-2-Stable Choco Chip Cookie", cookies.signed[:favorite]
+
+    freeze_time do
+      travel 1001.years
+      assert_nil cookies.signed[:favorite]
+    end
+
+    get :rails_5_2_stable_signed_cookie_with_authenticated_encryption_flag_off
+
+    assert_equal "5-2-Stable Choco Chip Cookie", cookies.signed[:favorite]
+  end
+
   private
     def assert_cookie_header(expected)
       header = @response.headers["Set-Cookie"]

railties/lib/rails/application.rb

@@ -267,6 +267,7 @@ module Rails
           "action_dispatch.cookies_serializer" => config.action_dispatch.cookies_serializer,
           "action_dispatch.cookies_digest" => config.action_dispatch.cookies_digest,
           "action_dispatch.cookies_rotations" => config.action_dispatch.cookies_rotations,
+          "action_dispatch.use_cookies_with_metadata" => config.action_dispatch.use_cookies_with_metadata,
           "action_dispatch.content_security_policy" => config.content_security_policy,
           "action_dispatch.content_security_policy_report_only" => config.content_security_policy_report_only,
           "action_dispatch.content_security_policy_nonce_generator" => config.content_security_policy_nonce_generator

railties/lib/rails/application/configuration.rb

@@ -120,6 +120,10 @@ module Rails
           if respond_to?(:action_view)
             action_view.default_enforce_utf8 = false
           end
+
+          if respond_to?(:action_dispatch)
+            action_dispatch.use_cookies_with_metadata = true
+          end
         else
           raise "Unknown version #{target_version.to_s.inspect}"
         end

railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt

@@ -8,3 +8,10 @@
 
 # Don't force requests from old versions of IE to be UTF-8 encoded
 # Rails.application.config.action_view.default_enforce_utf8 = false
+
+# Embed purpose and expiry metadata inside signed and encrypted
+# cookies for increased security.
+#
+# This option is not backwards compatible with earlier Rails versions.
+# It's best enabled when your entire app is migrated and stable on 6.0.
+# Rails.application.config.action_dispatch.use_cookies_with_metadata = true

railties/test/application/middleware/cookies_test.rb

@@ -110,14 +110,14 @@ module ApplicationTests
       assert_equal "signed cookie".inspect, last_response.body
 
       get "/foo/read_raw_cookie"
-      assert_equal "signed cookie", verifier_sha512.verify(last_response.body)
+      assert_equal "signed cookie", verifier_sha512.verify(last_response.body, purpose: "cookie.signed_cookie")
 
       get "/foo/write_raw_cookie_sha256"
       get "/foo/read_signed"
       assert_equal "signed cookie".inspect, last_response.body
 
       get "/foo/read_raw_cookie"
-      assert_equal "signed cookie", verifier_sha512.verify(last_response.body)
+      assert_equal "signed cookie", verifier_sha512.verify(last_response.body, purpose: "cookie.signed_cookie")
     end
 
     test "encrypted cookies rotating multiple encryption keys" do
@@ -180,14 +180,14 @@ module ApplicationTests
       assert_equal "encrypted cookie".inspect, last_response.body
 
       get "/foo/read_raw_cookie"
-      assert_equal "encrypted cookie", encryptor.decrypt_and_verify(last_response.body)
+      assert_equal "encrypted cookie", encryptor.decrypt_and_verify(last_response.body, purpose: "cookie.encrypted_cookie")
 
-      get "/foo/write_raw_cookie_sha256"
+      get "/foo/write_raw_cookie_two"
       get "/foo/read_encrypted"
       assert_equal "encrypted cookie".inspect, last_response.body
 
       get "/foo/read_raw_cookie"
-      assert_equal "encrypted cookie", encryptor.decrypt_and_verify(last_response.body)
+      assert_equal "encrypted cookie", encryptor.decrypt_and_verify(last_response.body, purpose: "cookie.encrypted_cookie")
     end
   end
 end

railties/test/application/middleware/session_test.rb

@@ -183,7 +183,7 @@ module ApplicationTests
       encryptor = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len(cipher)], cipher: cipher)
 
       get "/foo/read_raw_cookie"
-      assert_equal 1, encryptor.decrypt_and_verify(last_response.body)["foo"]
+      assert_equal 1, encryptor.decrypt_and_verify(last_response.body, purpose: "cookie._myapp_session")["foo"]
     end
 
     test "session upgrading signature to encryption cookie store works the same way as encrypted cookie store" do
@@ -235,7 +235,7 @@ module ApplicationTests
       encryptor = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len(cipher)], cipher: cipher)
 
       get "/foo/read_raw_cookie"
-      assert_equal 1, encryptor.decrypt_and_verify(last_response.body)["foo"]
+      assert_equal 1, encryptor.decrypt_and_verify(last_response.body, purpose: "cookie._myapp_session")["foo"]
     end
 
     test "session upgrading signature to encryption cookie store upgrades session to encrypted mode" do
@@ -297,7 +297,7 @@ module ApplicationTests
       encryptor = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len(cipher)], cipher: cipher)
 
       get "/foo/read_raw_cookie"
-      assert_equal 2, encryptor.decrypt_and_verify(last_response.body)["foo"]
+      assert_equal 2, encryptor.decrypt_and_verify(last_response.body, purpose: "cookie._myapp_session")["foo"]
     end
 
     test "session upgrading from AES-CBC-HMAC encryption to AES-GCM encryption" do
@@ -364,7 +364,7 @@ module ApplicationTests
         encryptor = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len(cipher)], cipher: cipher)
 
         get "/foo/read_raw_cookie"
-        assert_equal 2, encryptor.decrypt_and_verify(last_response.body)["foo"]
+        assert_equal 2, encryptor.decrypt_and_verify(last_response.body, purpose: "cookie._myapp_session")["foo"]
       ensure
         ENV["RAILS_ENV"] = old_rails_env
       end
@@ -428,7 +428,7 @@ module ApplicationTests
         verifier = ActiveSupport::MessageVerifier.new(app.secrets.secret_token)
 
         get "/foo/read_raw_cookie"
-        assert_equal 2, verifier.verify(last_response.body)["foo"]
+        assert_equal 2, verifier.verify(last_response.body, purpose: "cookie._myapp_session")["foo"]
       ensure
         ENV["RAILS_ENV"] = old_rails_env
       end