diff --git a/config/initializers/cookies_serializer.rb b/config/initializers/cookies_serializer.rb
index b728aeaf9c2..4346142d319 100644
--- a/config/initializers/cookies_serializer.rb
+++ b/config/initializers/cookies_serializer.rb
@@ -32,4 +32,4 @@
 
 # Specify a serializer for the signed and encrypted cookie jars.
 # Valid options are :json, :marshal, and :hybrid.
-Rails.application.config.action_dispatch.cookies_serializer = :marshal
+Rails.application.config.action_dispatch.cookies_serializer = :json
diff --git a/spec/security/cookies_serializer_spec.rb b/spec/security/cookies_serializer_spec.rb
new file mode 100644
index 00000000000..3d8fbf1a8a4
--- /dev/null
+++ b/spec/security/cookies_serializer_spec.rb
@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+# Regression guard against the Marshal.load-via-encrypted-cookie RCE chain.
+# If anyone flips `cookies_serializer` back to :marshal or :hybrid, the
+# 2FA op2fa_remember_token cookie (and any future encrypted cookie) becomes
+# a pre-auth RCE primitive as soon as the signing key is known or leaks.
+
+require "spec_helper"
+
+# Canary class used to detect Marshal.load running against attacker-controlled
+# cookie payloads. Must be a named top-level constant so Marshal can dump and
+# reconstruct it by name. We record the invocation in a class-level flag because
+# the cookie jar swallows exceptions raised during deserialization — an
+# `expect { … }.not_to raise_error` assertion would not catch a regression.
+class CookieSerializerSpecMarshalCanary
+  @triggered = false
+  class << self
+    attr_accessor :triggered
+  end
+
+  def marshal_dump = "canary"
+
+  def marshal_load(_)
+    CookieSerializerSpecMarshalCanary.triggered = true
+  end
+end
+
+RSpec.describe "Cookie serializer" do # rubocop:disable RSpec/DescribeClass
+  it "is configured to :json (NEVER :marshal or :hybrid)" do
+    expect(Rails.application.config.action_dispatch.cookies_serializer).to eq(:json)
+  end
+
+  describe "encrypted cookie jar" do
+    # Forge a cookie whose ciphertext, after authenticated decryption, yields a
+    # Marshal payload. This mimics either (a) a cookie written by an older
+    # version of OpenProject when the serializer was :marshal, or (b) an attacker
+    # who knows SECRET_KEY_BASE (e.g. the historical OVERWRITE_ME for unconfigured
+    # docker containers) and crafts an exploit payload.
+    let(:marshal_payload) { Marshal.dump(CookieSerializerSpecMarshalCanary.new) }
+    let(:forged_cookie) do
+      salt = Rails.application.config.action_dispatch.authenticated_encrypted_cookie_salt
+      cipher = Rails.application.config.action_dispatch.encrypted_cookie_cipher || "aes-256-gcm"
+      key_len = ActiveSupport::MessageEncryptor.key_len(cipher)
+      secret = Rails.application.key_generator.generate_key(salt, key_len)
+
+      encryptor = ActiveSupport::MessageEncryptor.new(
+        secret,
+        cipher: cipher,
+        serializer: ActiveSupport::MessageEncryptor::NullSerializer
+      )
+      encryptor.encrypt_and_sign(marshal_payload, purpose: "cookie.op2fa_remember_token")
+    end
+
+    let(:cookie_jar) do
+      request = ActionDispatch::TestRequest.create
+      request.cookies["op2fa_remember_token"] = forged_cookie
+      ActionDispatch::Cookies::CookieJar.build(request, request.cookies)
+    end
+
+    before { CookieSerializerSpecMarshalCanary.triggered = false }
+
+    it "does not invoke Marshal.load on the cookie payload" do
+      # If the serializer were :marshal/:hybrid, the encrypted cookie jar would
+      # call Marshal.load on the decrypted payload, reconstructing the canary
+      # and setting its `triggered` flag. With :json, the JSON deserializer
+      # rejects the Marshal bytes and the jar returns nil — no Ruby objects
+      # are revived from attacker-controlled data.
+      cookie_jar.encrypted["op2fa_remember_token"]
+
+      expect(CookieSerializerSpecMarshalCanary.triggered).to be(false),
+                                                             "Marshal.load was invoked on attacker-controlled cookie payload — " \
+                                                             "cookies_serializer must be :json, not :marshal or :hybrid."
+    end
+  end
+end