diff --git a/activestorage/app/controllers/active_storage/disk_controller.rb b/activestorage/app/controllers/active_storage/disk_controller.rb
index 5c372b3384..97ac584f73 100644
--- a/activestorage/app/controllers/active_storage/disk_controller.rb
+++ b/activestorage/app/controllers/active_storage/disk_controller.rb
@@ -42,11 +42,13 @@ class ActiveStorage::DiskController < ActiveStorage::BaseController
     end
 
     def decode_verified_key
-      ActiveStorage.verifier.verified(params[:encoded_key], purpose: :blob_key)
+      key = ActiveStorage.verifier.verified(params[:encoded_key], purpose: :blob_key)
+      key&.deep_symbolize_keys
     end
 
     def decode_verified_token
-      ActiveStorage.verifier.verified(params[:encoded_token], purpose: :blob_token)
+      token = ActiveStorage.verifier.verified(params[:encoded_token], purpose: :blob_token)
+      token&.deep_symbolize_keys
     end
 
     def acceptable_content?(token)
diff --git a/activesupport/lib/active_support/message_verifier.rb b/activesupport/lib/active_support/message_verifier.rb
index b4268cfc6e..49cb576028 100644
--- a/activesupport/lib/active_support/message_verifier.rb
+++ b/activesupport/lib/active_support/message_verifier.rb
@@ -72,7 +72,7 @@ module ActiveSupport
   #
   # === Alternative serializers
   #
-  # By default MessageVerifier uses Marshal to serialize the message. If you want to use
+  # By default MessageVerifier uses JSON to serialize the message. If you want to use
   # another serialization method, you can set the serializer in the options
   # hash upon initialization:
   #
@@ -115,11 +115,20 @@ module ActiveSupport
     SEPARATOR = "--" # :nodoc:
     SEPARATOR_LENGTH = SEPARATOR.length # :nodoc:
 
+    cattr_accessor :default_message_verifier_serializer, instance_accessor: false, default: :marshal
+
     def initialize(secret, digest: nil, serializer: nil)
       raise ArgumentError, "Secret should not be nil." unless secret
       @secret = secret
       @digest = digest&.to_s || "SHA1"
-      @serializer = serializer || Marshal
+      @serializer = serializer ||
+        if @@default_message_verifier_serializer.equal?(:marshal)
+          Marshal
+        elsif @@default_message_verifier_serializer.equal?(:hybrid)
+          JsonWithMarshalFallback
+        elsif @@default_message_verifier_serializer.equal?(:json)
+          JSON
+        end
     end
 
     # Checks if a signed message could have been generated by signing an object
diff --git a/activesupport/lib/active_support/railtie.rb b/activesupport/lib/active_support/railtie.rb
index 7b5819ffb9..4915bfbe11 100644
--- a/activesupport/lib/active_support/railtie.rb
+++ b/activesupport/lib/active_support/railtie.rb
@@ -167,6 +167,15 @@ module ActiveSupport
       end
     end
 
+    initializer "active_support.set_default_message_verifier_serializer" do |app|
+      config.after_initialize do
+        unless app.config.active_support.default_message_verifier_serializer.nil?
+          ActiveSupport::MessageVerifier.default_message_verifier_serializer =
+            app.config.active_support.default_message_verifier_serializer
+        end
+      end
+    end
+
     initializer "active_support.set_marshal_serialization" do |app|
       config.after_initialize do
         unless app.config.active_support.use_marshal_serialization.nil?
diff --git a/activesupport/test/message_verifier_test.rb b/activesupport/test/message_verifier_test.rb
index adab0369dd..9a0650cdb2 100644
--- a/activesupport/test/message_verifier_test.rb
+++ b/activesupport/test/message_verifier_test.rb
@@ -19,7 +19,7 @@ class MessageVerifierTest < ActiveSupport::TestCase
 
   def setup
     @verifier = ActiveSupport::MessageVerifier.new("Hey, I'm a secret!")
-    @data = { some: "data", now: Time.utc(2010) }
+    @data = { "some" => "data", "now" => Time.utc(2010) }
     @secret = SecureRandom.random_bytes(32)
   end
 
@@ -70,25 +70,6 @@ class MessageVerifierTest < ActiveSupport::TestCase
     ActiveSupport.parse_json_times, Time.zone = previous
   end
 
-  def test_raise_error_when_argument_class_is_not_loaded
-    # To generate the valid message below:
-    #
-    #   AutoloadClass = Struct.new(:foo)
-    #   valid_message = @verifier.generate(foo: AutoloadClass.new('foo'))
-    #
-    valid_message = "BAh7BjoIZm9vbzonTWVzc2FnZVZlcmlmaWVyVGVzdDo6QXV0b2xvYWRDbGFzcwY6CUBmb29JIghmb28GOgZFVA==--f3ef39a5241c365083770566dc7a9eb5d6ace914"
-    exception = assert_raise(ArgumentError, NameError) do
-      @verifier.verified(valid_message)
-    end
-    assert_includes ["uninitialized constant MessageVerifierTest::AutoloadClass",
-                    "undefined class/module MessageVerifierTest::AutoloadClass"], exception.message
-    exception = assert_raise(ArgumentError, NameError) do
-      @verifier.verify(valid_message)
-    end
-    assert_includes ["uninitialized constant MessageVerifierTest::AutoloadClass",
-                    "undefined class/module MessageVerifierTest::AutoloadClass"], exception.message
-  end
-
   def test_raise_error_when_secret_is_nil
     exception = assert_raise(ArgumentError) do
       ActiveSupport::MessageVerifier.new(nil)
@@ -96,12 +77,6 @@ class MessageVerifierTest < ActiveSupport::TestCase
     assert_equal "Secret should not be nil.", exception.message
   end
 
-  def test_backward_compatibility_messages_signed_without_metadata
-    signed_message = "BAh7BzoJc29tZUkiCWRhdGEGOgZFVDoIbm93SXU6CVRpbWUNIIAbgAAAAAAHOgtvZmZzZXRpADoJem9uZUkiCFVUQwY7BkY=--d03c52c91dfe4ccc5159417c660461bcce005e96"
-    assert_equal @data, @verifier.verify(signed_message)
-  end
-
-
   def test_rotating_secret
     old_message = ActiveSupport::MessageVerifier.new("old", digest: "SHA1").generate("old")
 
@@ -124,6 +99,35 @@ class MessageVerifierTest < ActiveSupport::TestCase
     assert_equal "older", verifier.verified(older_message)
   end
 
+  def test_rotations_with_metadata
+    old_message = ActiveSupport::MessageVerifier.new("old").generate("old", purpose: :rotation)
+
+    verifier = ActiveSupport::MessageVerifier.new(@secret)
+    verifier.rotate "old"
+
+    assert_equal "old", verifier.verified(old_message, purpose: :rotation)
+  end
+end
+
+class DefaultMarshalSerializerMessageVerifierTest < MessageVerifierTest
+  def setup
+    @default_verifier = ActiveSupport::MessageVerifier.default_message_verifier_serializer
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = :marshal
+
+    @verifier = ActiveSupport::MessageVerifier.new("Hey, I'm a secret!")
+    @data = { some: "data", now: Time.utc(2010) }
+    @secret = SecureRandom.random_bytes(32)
+  end
+
+  def teardown
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = @default_verifier
+  end
+
+  def test_backward_compatibility_messages_signed_without_metadata
+    signed_message = "BAh7BzoJc29tZUkiCWRhdGEGOgZFVDoIbm93SXU6CVRpbWUNIIAbgAAAAAAHOgtvZmZzZXRpADoJem9uZUkiCFVUQwY7BkY=--d03c52c91dfe4ccc5159417c660461bcce005e96"
+    assert_equal @data, @verifier.verify(signed_message)
+  end
+
   def test_on_rotation_is_called_and_verified_returns_message
     older_message = ActiveSupport::MessageVerifier.new("older", digest: "SHA1").generate({ encoded: "message" })
 
@@ -138,13 +142,127 @@ class MessageVerifierTest < ActiveSupport::TestCase
     assert rotated
   end
 
-  def test_rotations_with_metadata
-    old_message = ActiveSupport::MessageVerifier.new("old").generate("old", purpose: :rotation)
+  def test_raise_error_when_argument_class_is_not_loaded
+    # To generate the valid message below:
+    #
+    #   AutoloadClass = Struct.new(:foo)
+    #   valid_message = @verifier.generate(foo: AutoloadClass.new('foo'))
+    #
+    valid_message = "BAh7BjoIZm9vbzonTWVzc2FnZVZlcmlmaWVyVGVzdDo6QXV0b2xvYWRDbGFzcwY6CUBmb29JIghmb28GOgZFVA==--f3ef39a5241c365083770566dc7a9eb5d6ace914"
+    exception = assert_raise(ArgumentError, NameError) do
+      @verifier.verified(valid_message)
+    end
+    assert_includes ["uninitialized constant MessageVerifierTest::AutoloadClass",
+                    "undefined class/module MessageVerifierTest::AutoloadClass"], exception.message
+    exception = assert_raise(ArgumentError, NameError) do
+      @verifier.verify(valid_message)
+    end
+    assert_includes ["uninitialized constant MessageVerifierTest::AutoloadClass",
+                    "undefined class/module MessageVerifierTest::AutoloadClass"], exception.message
+  end
+end
 
-    verifier = ActiveSupport::MessageVerifier.new(@secret)
-    verifier.rotate "old"
+class MarshalSerializeAndFallbackMessageVerifierTest < DefaultMarshalSerializerMessageVerifierTest
+  def setup
+    @default_verifier = ActiveSupport::MessageVerifier.default_message_verifier_serializer
+    @default_use_marshal = ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization
+    @default_fallback = ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = :hybrid
+    ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization = true
+    ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization = true
 
-    assert_equal "old", verifier.verified(old_message, purpose: :rotation)
+    @verifier = ActiveSupport::MessageVerifier.new("Hey, I'm a secret!")
+    @data = { some: "data", now: Time.utc(2010) }
+    @secret = SecureRandom.random_bytes(32)
+  end
+
+  def teardown
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = @default_verifier
+    ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization = @default_use_marshal
+    ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization = @default_fallback
+  end
+end
+
+class JsonSerializeMarshalFallbackMessageVerifierTest < MessageVerifierTest
+  def setup
+    @default_verifier = ActiveSupport::MessageVerifier.default_message_verifier_serializer
+    @default_use_marshal = ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization
+    @default_fallback = ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = :hybrid
+    ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization = false
+    ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization = true
+
+    @verifier = ActiveSupport::MessageVerifier.new("Hey, I'm a secret!")
+    @data = { "some" => "data", "now" => Time.utc(2010) }
+    @secret = SecureRandom.random_bytes(32)
+  end
+
+  def teardown
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = @default_verifier
+    ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization = @default_use_marshal
+    ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization = @default_fallback
+  end
+
+  def test_on_rotation_is_called_and_verified_returns_message
+    older_message = ActiveSupport::MessageVerifier.new("older", digest: "SHA1").generate({ encoded: "message" })
+
+    verifier = ActiveSupport::MessageVerifier.new(@secret, digest: "SHA512")
+    verifier.rotate "old",   digest: "SHA256"
+    verifier.rotate "older", digest: "SHA1"
+
+    rotated = false
+    message = verifier.verified(older_message, on_rotation: proc { rotated = true })
+
+    assert_equal({ "encoded" => "message" }, message)
+    assert rotated
+  end
+
+  def test_backward_compatibility_messages_signed_marshal_serialized
+    marshal_serialized_signed_message = "BAh7B0kiCXNvbWUGOgZFVEkiCWRhdGEGOwBUSSIIbm93BjsAVEl1OglUaW1lDSCAG8AAAAAABjoJem9uZUkiCFVUQwY7AEY=--ae7480422168507f4a8aec6b1d68bfdfd5c6ef48"
+    assert_equal @data, @verifier.verify(marshal_serialized_signed_message)
+  end
+end
+
+class JsonSerializeAndNoFallbackMessageVerifierTest < JsonSerializeMarshalFallbackMessageVerifierTest
+  def setup
+    @default_verifier = ActiveSupport::MessageVerifier.default_message_verifier_serializer
+    @default_use_marshal = ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization
+    @default_fallback = ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = :hybrid
+    ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization = false
+    ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization = false
+
+    @verifier = ActiveSupport::MessageVerifier.new("Hey, I'm a secret!")
+    @data = { "some" => "data", "now" => Time.utc(2010) }
+    @secret = SecureRandom.random_bytes(32)
+  end
+
+  def teardown
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = @default_verifier
+    ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization = @default_use_marshal
+    ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization = @default_fallback
+  end
+
+  def test_backward_compatibility_messages_signed_marshal_serialized
+    marshal_serialized_signed_message = "BAh7B0kiCXNvbWUGOgZFVEkiCWRhdGEGOwBUSSIIbm93BjsAVEl1OglUaW1lDSCAG8AAAAAABjoJem9uZUkiCFVUQwY7AEY=--ae7480422168507f4a8aec6b1d68bfdfd5c6ef48"
+    assert_raise(JSON::ParserError) do
+      @verifier.verify(marshal_serialized_signed_message)
+    end
+  end
+end
+
+class DefaultJsonSerializerMessageVerifierTest < JsonSerializeAndNoFallbackMessageVerifierTest
+  def setup
+    @default_verifier = ActiveSupport::MessageVerifier.default_message_verifier_serializer
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = :json
+
+    @verifier = ActiveSupport::MessageVerifier.new("Hey, I'm a secret!")
+    @data = { "some" => "data", "now" => Time.utc(2010) }
+    @secret = SecureRandom.random_bytes(32)
+  end
+
+  def teardown
+    ActiveSupport::MessageVerifier.default_message_verifier_serializer = @default_verifier
   end
 end
 
@@ -199,7 +317,22 @@ class MessageVerifierMetadataMarshalTest < MessageVerifierMetadataTest
     end
 end
 
-class MessageVerifierMetadataJSONTest < MessageVerifierMetadataTest
+class MessageVerifierMetadataJsonWithMarshalFallbackTest < MessageVerifierMetadataTest
+  private
+    def verifier_options
+      { serializer: ActiveSupport::JsonWithMarshalFallback }
+    end
+end
+
+class MessageVerifierMetadataJsonTest < MessageVerifierMetadataTest
+  private
+    def verifier_options
+      { serializer: JSON }
+    end
+end
+
+
+class MessageVerifierMetadataCustomJSONTest < MessageVerifierMetadataTest
   private
     def verifier_options
       { serializer: MessageVerifierTest::JSONSerializer.new }
diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index 00e5cf7066..d3db6ba968 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -63,6 +63,7 @@ Below are the default values associated with each target version. In cases of co
 - [`config.action_dispatch.default_headers`](#config-action-dispatch-default-headers): `{ "X-Frame-Options" => "SAMEORIGIN", "X-XSS-Protection" => "0", "X-Content-Type-Options" => "nosniff", "X-Permitted-Cross-Domain-Policies" => "none", "Referrer-Policy" => "strict-origin-when-cross-origin" }`
 - [`config.add_autoload_paths_to_load_path`](#config-add-autoload-paths-to-load-path): `false`
 - [`config.active_support.default_message_encryptor_serializer`](#config-active-support-default-message-encryptor-serializer): `:json`
+- [`config.active_support.default_message_verifier_serializer`](#config-active-support-default-message-verifier-serializer): `:json`
 
 #### Default Values for Target Version 7.0
 
@@ -1909,6 +1910,19 @@ Used to help migrate apps from `Marshal` to `JSON` as the default serializer for
 
 Defaults to `true`.
 
+#### `config.active_support.default_message_verifier_serializer`
+
+Specifies what serializer the `MessageVerifier` class will use by default.
+
+Options are `:json`, `:hybrid`, and `:marshal`. `:hybrid` uses the `JsonWithMarshalFallback` class.
+
+The default value depends on the `config.load_defaults` target version:
+
+| Starting with version | The default value is |
+| --------------------- | -------------------- |
+| (original)            | `:marshal`           |
+| 7.1                   | `:json`              |
+
 ### Configuring Active Job
 
 `config.active_job` provides the following configuration options:
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index 47941d4fa6..c50e6fd510 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -169,6 +169,85 @@ Alternatively, you could load defaults for 7.1
 config.load_defaults 7.1
 ```
 
+### New `ActiveSupport::MessageVerifier` default serializer
+
+As of Rails 7.1, the default serializer in use by the `MessageVerifier` is `JSON`.
+This offers a more secure alternative to the current default serializer.
+
+The `MessageVerifier` offers the ability to migrate the default serializer from `Marshal` to `JSON`.
+
+If you would like to ignore this change in existing applications, set the following: `config.active_support.default_message_verifier_serializer = :marshal`.
+
+In order to roll out the new default when upgrading from `7.0` to `7.1`, there are three configuration variables to keep in mind.
+```
+config.active_support.default_verifier_serializer
+config.active_support.fallback_to_marshal_deserialization
+config.active_support.use_marshal_serialization
+```
+
+`default_message_verifier_serializer` defaults to `:json` as of `7.1` but it offers both a `:hybrid` and `:marshal` option.
+
+In order to migrate an older deployment to `:json`, first ensure that the `default_message_verifier_serializer` is set to `:marshal`.
+```ruby
+# config/application.rb
+config.load_defaults 7.0
+config.active_support.default_message_verifier_serializer = :marshal
+```
+
+Once this is deployed on all Rails processes, set `default_message_verifier_serializer` to `:hybrid` to begin using the
+`ActiveSupport::JsonWithMarshalFallback` class as the serializer. The defaults for this class are to use `Marshal`
+as the serializer and to allow the deserialisation of both `Marshal` and `JSON` serialized payloads.
+
+```ruby
+config.load_defaults 7.0
+config.active_support.default_message_verifier_serializer = :hybrid
+```
+
+Once this is deployed on all Rails processes, set the following configuration options in order to stop the
+`ActiveSupport::JsonWithMarshalFallback` class from using `Marshal` to serialize new payloads.
+
+```ruby
+# config/application.rb
+config.load_defaults 7.0
+config.active_support.default_message_verifier_serializer = :hybrid
+config.active_support.use_marshal_serialization = false
+```
+
+Allow this configuration to run on all processes for a considerable amount of time.
+`ActiveSupport::JsonWithMarshalFallback` logs the following each time the `Marshal` fallback
+is used:
+```
+JsonWithMarshalFallback: Marshal load fallback occurred.
+```
+
+Once those message stop appearing in your logs and you're confident that all `MessageVerifier`
+payloads in transit are `JSON` serialized, the following configuration options will disable the
+Marshal fallback in `ActiveSupport::JsonWithMarshalFallback`.
+
+```ruby
+# config/application.rb
+config.load_defaults 7.0
+config.active_support.default_message_verifier_serializer = :hybrid
+config.active_support.use_marshal_serialization = false
+config.active_support.fallback_to_marshal_deserialization = false
+```
+
+If all goes well, you should now be safe to migrate the Message Verifier from
+`ActiveSupport::JsonWithMarshalFallback` to `ActiveSupport::JSON`.
+To do so, simply swap the `:hybrid` serializer for the `:json` serializer.
+
+```ruby
+# config/application.rb
+config.load_defaults 7.0
+config.active_support.default_message_verifier_serializer = :json
+```
+
+Alternatively, you could load defaults for 7.1
+```ruby
+# config/application.rb
+config.load_defaults 7.1
+```
+
 Upgrading from Rails 6.1 to Rails 7.0
 -------------------------------------
 
diff --git a/railties/lib/rails/application/configuration.rb b/railties/lib/rails/application/configuration.rb
index ae64c59aed..701a3f1e2b 100644
--- a/railties/lib/rails/application/configuration.rb
+++ b/railties/lib/rails/application/configuration.rb
@@ -270,6 +270,7 @@ module Rails
 
           if respond_to?(:active_support)
             active_support.default_message_encryptor_serializer = :json
+            active_support.default_message_verifier_serializer = :json
           end
         else
           raise "Unknown version #{target_version.to_s.inspect}"
diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index e8012cf3d5..457f64229a 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -3189,6 +3189,14 @@ module ApplicationTests
       assert_equal true, ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization
     end
 
+    test "ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization is true by default for upgraded apps" do
+      remove_from_config '.*config\.load_defaults.*\n'
+
+      app "development"
+
+      assert_equal true, ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization
+    end
+
     test "ActiveSupport::JsonWithMarshalFallback.fallback_to_marshal_deserialization can be configured via config.active_support.fallback_to_marshal_deserialization" do
       remove_from_config '.*config\.load_defaults.*\n'
 
@@ -3207,6 +3215,14 @@ module ApplicationTests
       assert_equal true, ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization
     end
 
+    test "ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization is true by default for upgraded apps" do
+      remove_from_config '.*config\.load_defaults.*\n'
+
+      app "development"
+
+      assert_equal true, ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization
+    end
+
     test "ActiveSupport::JsonWithMarshalFallback.use_marshal_serialization can be configured via config.active_support.use_marshal_serialization" do
       remove_from_config '.*config\.load_defaults.*\n'
 
@@ -3246,6 +3262,32 @@ module ApplicationTests
       assert_equal :hybrid, ActiveSupport::MessageEncryptor.default_message_encryptor_serializer
     end
 
+    test "ActiveSupport::MessageVerifier.default_message_verifier_serializer is :json by default for new apps" do
+      app "development"
+
+      assert_equal :json, ActiveSupport::MessageVerifier.default_message_verifier_serializer
+    end
+
+    test "ActiveSupport::MessageVerifier.default_message_verifier_serializer is :marshal by default for upgraded apps" do
+      remove_from_config '.*config\.load_defaults.*\n'
+
+      app "development"
+
+      assert_equal :marshal, ActiveSupport::MessageVerifier.default_message_verifier_serializer
+    end
+
+    test "ActiveSupport::MessageVerifier.default_message_verifier_serializer can be configured via config.active_support.default_message_verifier_serializer" do
+      remove_from_config '.*config\.load_defaults.*\n'
+
+      app_file "config/initializers/default_message_verifier_serializer.rb", <<-RUBY
+        Rails.application.config.active_support.default_message_verifier_serializer = :hybrid
+      RUBY
+
+      app "development"
+
+      assert_equal :hybrid, ActiveSupport::MessageVerifier.default_message_verifier_serializer
+    end
+
     test "unknown_asset_fallback is false by default" do
       app "development"
 
diff --git a/railties/test/application/middleware/cookies_test.rb b/railties/test/application/middleware/cookies_test.rb
index 02b7cd7fc7..4b31b0f91a 100644
--- a/railties/test/application/middleware/cookies_test.rb
+++ b/railties/test/application/middleware/cookies_test.rb
@@ -51,7 +51,7 @@ module ApplicationTests
       assert_equal false, ActionDispatch::Cookies::CookieJar.always_write_cookie
     end
 
-    test "signed cookies with SHA512 digest and rotated out SHA256 and SHA1 digests" do
+    test "signed cookies with SHA512 digest and marshal serializer and rotated out SHA256 and SHA1 digests" do
       app_file "config/routes.rb", <<-RUBY
         Rails.application.routes.draw do
           get  ':controller(/:action)'
@@ -88,8 +88,8 @@ module ApplicationTests
         sha256_secret = Rails.application.key_generator.generate_key("sha256")
 
         ::TestVerifiers = Class.new do
-          class_attribute :sha1, default: ActiveSupport::MessageVerifier.new(sha1_secret, digest: "SHA1")
-          class_attribute :sha256, default: ActiveSupport::MessageVerifier.new(sha256_secret, digest: "SHA256")
+          class_attribute :sha1, default: ActiveSupport::MessageVerifier.new(sha1_secret, digest: "SHA1", serializer: Marshal)
+          class_attribute :sha256, default: ActiveSupport::MessageVerifier.new(sha256_secret, digest: "SHA256", serializer: Marshal)
         end
 
         config.action_dispatch.signed_cookie_digest = "SHA512"
@@ -104,7 +104,77 @@ module ApplicationTests
 
       require "#{app_path}/config/environment"
 
-      verifier_sha512 = ActiveSupport::MessageVerifier.new(app.key_generator.generate_key("sha512 salt"), digest: :SHA512)
+      verifier_sha512 = ActiveSupport::MessageVerifier.new(app.key_generator.generate_key("sha512 salt"), digest: :SHA512, serializer: Marshal)
+
+      get "/foo/write_raw_cookie_sha1"
+      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, 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, purpose: "cookie.signed_cookie")
+    end
+
+    test "signed cookies with SHA512 digest and json serializer and rotated out SHA256 and SHA1 digests" do
+      app_file "config/routes.rb", <<-RUBY
+        Rails.application.routes.draw do
+          get  ':controller(/:action)'
+          post ':controller(/:action)'
+        end
+      RUBY
+
+      controller :foo, <<-RUBY
+        class FooController < ActionController::Base
+          protect_from_forgery with: :null_session
+
+          def write_raw_cookie_sha1
+            cookies[:signed_cookie] = TestVerifiers.sha1.generate("signed cookie")
+            head :ok
+          end
+
+          def write_raw_cookie_sha256
+            cookies[:signed_cookie] = TestVerifiers.sha256.generate("signed cookie")
+            head :ok
+          end
+
+          def read_signed
+            render plain: cookies.signed[:signed_cookie].inspect
+          end
+
+          def read_raw_cookie
+            render plain: cookies[:signed_cookie]
+          end
+        end
+      RUBY
+
+      add_to_config <<-RUBY
+        sha1_secret   = Rails.application.key_generator.generate_key("sha1")
+        sha256_secret = Rails.application.key_generator.generate_key("sha256")
+
+        ::TestVerifiers = Class.new do
+          class_attribute :sha1, default: ActiveSupport::MessageVerifier.new(sha1_secret, digest: "SHA1", serializer: JSON)
+          class_attribute :sha256, default: ActiveSupport::MessageVerifier.new(sha256_secret, digest: "SHA256", serializer: JSON)
+        end
+
+        config.action_dispatch.signed_cookie_digest = "SHA512"
+        config.action_dispatch.signed_cookie_salt = "sha512 salt"
+        config.action_dispatch.cookies_serializer = :json
+
+        config.action_dispatch.cookies_rotations.tap do |cookies|
+          cookies.rotate :signed, sha1_secret,   digest: "SHA1"
+          cookies.rotate :signed, sha256_secret, digest: "SHA256"
+        end
+      RUBY
+
+      require "#{app_path}/config/environment"
+
+      verifier_sha512 = ActiveSupport::MessageVerifier.new(app.key_generator.generate_key("sha512 salt"), digest: :SHA512, serializer: JSON)
 
       get "/foo/write_raw_cookie_sha1"
       get "/foo/read_signed"