diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..639a25b
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,41 @@
+name: CI
+on:
+  pull_request:
+jobs:
+  rspec:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - { ruby: '3.3', rails: '7.2' }
+          - { ruby: '3.3', rails: '8.0' }
+          - { ruby: '3.3', rails: '8.1' }
+          - { ruby: '3.4', rails: '7.2' }
+          - { ruby: '3.4', rails: '8.0' }
+          - { ruby: '3.4', rails: '8.1' }
+    runs-on: ubuntu-latest
+    env:
+      BUNDLE_GEMFILE: ${{ github.workspace }}/gemfiles/rails_${{ matrix.rails }}.gemfile
+    services:
+      postgres:
+        image: postgres:14
+        env:
+          POSTGRES_USER: postgres
+          POSTGRES_PASSWORD: postgres
+          POSTGRES_DB: bookingsync_engine_test
+        options: --health-cmd pg_isready --health-interval 10s --health-timeout 5s --health-retries 5
+        ports:
+          - 5432:5432
+    steps:
+    - uses: actions/checkout@v4
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+    - run: bundle exec rake spec
+      env:
+        DATABASE_URL: postgres://postgres:postgres@localhost:5432/bookingsync_engine_test
+        BOOKINGSYNC_URL: https://some_url.com
+        BOOKINGSYNC_APP_ID: some_client_id
+        BOOKINGSYNC_APP_SECRET: some_client_secret
+        BOOKINGSYNC_VERIFY_SSL: false
diff --git a/.ruby-version b/.ruby-version
index e70b452..5f6fc5e 100644
--- a/.ruby-version
+++ b/.ruby-version
@@ -1 +1 @@
-2.6.0
+3.3.10
diff --git a/Appraisals b/Appraisals
index 8e8cdc6..12c6625 100644
--- a/Appraisals
+++ b/Appraisals
@@ -1,15 +1,11 @@
-appraise "rails-5.0" do
-  gem "rails", "~> 5.0.7.1"
+appraise "rails-7.2" do
+  gem "rails", "~> 7.2.0"
 end
 
-appraise "rails-5.1" do
-  gem "rails", "~> 5.1.6.1"
+appraise "rails-8.0" do
+  gem "rails", "~> 8.0.0"
 end
 
-appraise "rails-5.2" do
-  gem "rails", "~> 5.2.0"
-end
-
-appraise "rails-6.0" do
-  gem "rails", "~> 6.0"
+appraise "rails-8.1" do
+  gem "rails", "~> 8.1.0"
 end
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7a5eb73..8d40650 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,7 @@
-# Master
+# Master (Unreleased)
+
+* Fix oauth2 2.0 `auth_scheme` breaking change (now defaults to `:basic_auth`)
+* Add `BookingSync::Engine.auth_scheme` configuration option
 
 # 6.0.1 (2021-07-16)
 
@@ -31,7 +34,7 @@
 * Fix `AuthHelpers` include when `helper_method` is missing
 
 # 4.0.0 (2019-04-24)
-
+ª
 * Add support for multi application setup
 * BREAKING CHANGE: Account models method `from_omniauth` now takes the host as second argument
 * BREAKING CHANGE: Rename BookingSync::Engine::Model in BookingSync::Engine::Models::Account
diff --git a/gemfiles/rails_5.1.gemfile b/gemfiles/rails_5.1.gemfile
deleted file mode 100644
index d4fc944..0000000
--- a/gemfiles/rails_5.1.gemfile
+++ /dev/null
@@ -1,7 +0,0 @@
-# This file was generated by Appraisal
-
-source "https://rubygems.org"
-
-gem "rails", "~> 5.1.6.1"
-
-gemspec path: "../"
diff --git a/gemfiles/rails_5.2.gemfile b/gemfiles/rails_7.2.gemfile
similarity index 79%
rename from gemfiles/rails_5.2.gemfile
rename to gemfiles/rails_7.2.gemfile
index 5a706dc..92cc2b2 100644
--- a/gemfiles/rails_5.2.gemfile
+++ b/gemfiles/rails_7.2.gemfile
@@ -2,6 +2,6 @@
 
 source "https://rubygems.org"
 
-gem "rails", "~> 5.2.0"
+gem "rails", "~> 7.2.0"
 
 gemspec path: "../"
diff --git a/gemfiles/rails_6.0.gemfile b/gemfiles/rails_8.0.gemfile
similarity index 79%
rename from gemfiles/rails_6.0.gemfile
rename to gemfiles/rails_8.0.gemfile
index 10c6284..3b3765b 100644
--- a/gemfiles/rails_6.0.gemfile
+++ b/gemfiles/rails_8.0.gemfile
@@ -2,6 +2,6 @@
 
 source "https://rubygems.org"
 
-gem "rails", "~> 6.0"
+gem "rails", "~> 8.0.0"
 
 gemspec path: "../"
diff --git a/gemfiles/rails_5.0.gemfile b/gemfiles/rails_8.1.gemfile
similarity index 77%
rename from gemfiles/rails_5.0.gemfile
rename to gemfiles/rails_8.1.gemfile
index 8f0b552..cedb65f 100644
--- a/gemfiles/rails_5.0.gemfile
+++ b/gemfiles/rails_8.1.gemfile
@@ -2,6 +2,6 @@
 
 source "https://rubygems.org"
 
-gem "rails", "~> 5.0.7.1"
+gem "rails", "~> 8.1.0"
 
 gemspec path: "../"
diff --git a/lib/bookingsync-engine.rb b/lib/bookingsync-engine.rb
index 66abee7..b14b76a 100644
--- a/lib/bookingsync-engine.rb
+++ b/lib/bookingsync-engine.rb
@@ -11,6 +11,9 @@ module BookingSyncEngine
   cattr_accessor :oauth_client_connection_options
   self.oauth_client_connection_options = { request: { timeout: 2 } }
 
+  cattr_accessor :oauth_client_auth_scheme
+  self.oauth_client_auth_scheme = :request_body
+
   cattr_accessor :token_refresh_timeout_retry_count
   self.token_refresh_timeout_retry_count = 2
 
diff --git a/lib/bookingsync/engine.rb b/lib/bookingsync/engine.rb
index 5a1400f..4591c60 100644
--- a/lib/bookingsync/engine.rb
+++ b/lib/bookingsync/engine.rb
@@ -83,7 +83,8 @@ def self.oauth_client(client_id: ENV["BOOKINGSYNC_APP_ID"], client_secret: ENV["
 
       client_options = {
         site: ENV["BOOKINGSYNC_URL"] || 'https://www.bookingsync.com',
-        connection_opts: connection_options
+        connection_opts: connection_options,
+        auth_scheme: ::BookingSyncEngine.oauth_client_auth_scheme
       }
       client_options[:ssl] = { verify: ENV['BOOKINGSYNC_VERIFY_SSL'] != 'false' }
       OAuth2::Client.new(client_id, client_secret, client_options)
diff --git a/spec/controllers/authenticated_controller_spec.rb b/spec/controllers/authenticated_controller_spec.rb
index 7b4f780..a55f2b4 100644
--- a/spec/controllers/authenticated_controller_spec.rb
+++ b/spec/controllers/authenticated_controller_spec.rb
@@ -12,8 +12,8 @@
       it "renders autosubmitted form" do
         get :index
         expect(response.status).to eq(200)
-        expect(response.body).to include("action='/auth/bookingsync' method='post'")
-        expect(response.body).to include("<input type='hidden' name='account_id' value=''>")
+        expect(response.body).to include("action=\"/auth/bookingsync\" method=\"post\"")
+        expect(response.body).to include("<input type=\"hidden\" name=\"account_id\" value=\"\">")
         expect(response.header["Content-Type"]).to include("text/html")
       end
     end
@@ -24,8 +24,8 @@
       it "renders autosubmitted form" do
         get :index
         expect(response.status).to eq(200)
-        expect(response.body).to include("action='/auth/bookingsync' method='post'")
-        expect(response.body).to include("<input type='hidden' name='account_id' value=''>")
+        expect(response.body).to include("action=\"/auth/bookingsync\" method=\"post\"")
+        expect(response.body).to include("<input type=\"hidden\" name=\"account_id\" value=\"\">")
       end
     end
   end
@@ -37,8 +37,8 @@
       it "renders autosubmitted form" do
         get :index, xhr: true
         expect(response.status).to eq(401)
-        expect(response.body).to include("action='/auth/bookingsync' method='post'")
-        expect(response.body).to include("<input type='hidden' name='account_id' value=''>")
+        expect(response.body).to include("action=\"/auth/bookingsync\" method=\"post\"")
+        expect(response.body).to include("<input type=\"hidden\" name=\"account_id\" value=\"\">")
       end
     end
 
@@ -48,8 +48,8 @@
       it "renders autosubmitted form" do
         get :index, xhr: true
         expect(response.status).to eq(401)
-        expect(response.body).to include("action='/auth/bookingsync' method='post'")
-        expect(response.body).to include("<input type='hidden' name='account_id' value=''>")
+        expect(response.body).to include("action=\"/auth/bookingsync\" method=\"post\"")
+        expect(response.body).to include("<input type=\"hidden\" name=\"account_id\" value=\"\">")
       end
     end
   end
diff --git a/spec/dummy/config/application.rb b/spec/dummy/config/application.rb
index b2507c3..98fabcc 100644
--- a/spec/dummy/config/application.rb
+++ b/spec/dummy/config/application.rb
@@ -1,6 +1,16 @@
 require File.expand_path('../boot', __FILE__)
 
-require 'rails/all'
+require "rails"
+require "active_model/railtie"
+require "active_record/railtie"
+require "action_controller/railtie"
+require "action_mailer/railtie"
+require "action_view/railtie"
+# require "active_storage/engine"
+# require "action_mailbox/engine"
+# require "action_text/engine"
+# require "action_cable/engine"
+require "rails/test_unit/railtie"
 
 Bundler.require(*Rails.groups)
 require "bookingsync"
diff --git a/spec/dummy/config/boot.rb b/spec/dummy/config/boot.rb
index ef36047..6266cfc 100644
--- a/spec/dummy/config/boot.rb
+++ b/spec/dummy/config/boot.rb
@@ -1,5 +1,5 @@
 # Set up gems listed in the Gemfile.
 ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../../../Gemfile', __FILE__)
 
-require 'bundler/setup' if File.exists?(ENV['BUNDLE_GEMFILE'])
+require 'bundler/setup' if File.exist?(ENV['BUNDLE_GEMFILE'])
 $LOAD_PATH.unshift File.expand_path('../../../../lib', __FILE__)
diff --git a/spec/dummy/config/database.yml b/spec/dummy/config/database.yml
index 9878676..723a1af 100644
--- a/spec/dummy/config/database.yml
+++ b/spec/dummy/config/database.yml
@@ -9,3 +9,4 @@ test:
   encoding: unicode
   database: bookingsync_engine_test
   pool: 5
+  url: <%= ENV['DATABASE_URL'] %>
diff --git a/spec/dummy/config/environments/development.rb b/spec/dummy/config/environments/development.rb
index 9d26e12..00ca9a3 100644
--- a/spec/dummy/config/environments/development.rb
+++ b/spec/dummy/config/environments/development.rb
@@ -1,16 +1,18 @@
 Dummy::Application.configure do
   # Settings specified here will take precedence over those in config/application.rb.
 
-  # In the development environment your application's code is reloaded on
-  # every request. This slows down response time but is perfect for development
+  # In the development environment your application's code is reloaded any time
+  # it changes. This slows down response time but is perfect for development
   # since you don't have to restart the web server when you make code changes.
-  config.cache_classes = false
+  config.enable_reloading = true
 
   # Do not eager load code on boot.
   config.eager_load = false
 
-  # Show full error reports and disable caching.
-  config.consider_all_requests_local       = true
+  # Show full error reports.
+  config.consider_all_requests_local = true
+
+  # Disable caching.
   config.action_controller.perform_caching = false
 
   # Don't care if the mailer can't send.
@@ -19,11 +21,6 @@
   # Print deprecation notices to the Rails logger.
   config.active_support.deprecation = :log
 
-  # Raise an error on page load if there are pending migrations
+  # Raise an error on page load if there are pending migrations.
   config.active_record.migration_error = :page_load
-
-  # Debug mode disables concatenation and preprocessing of assets.
-  # This option may cause significant delays in view rendering with a large
-  # number of complex assets.
-  config.assets.debug = true
-end
+end
\ No newline at end of file
diff --git a/spec/dummy/config/environments/test.rb b/spec/dummy/config/environments/test.rb
index 49b5517..8600ab8 100644
--- a/spec/dummy/config/environments/test.rb
+++ b/spec/dummy/config/environments/test.rb
@@ -5,23 +5,22 @@
   # test suite. You never need to work with it otherwise. Remember that
   # your test database is "scratch space" for the test suite and is wiped
   # and recreated between test runs. Don't rely on the data there!
-  config.cache_classes = true
+  config.enable_reloading = false
 
-  # Do not eager load code on boot. This avoids loading your whole application
-  # just for the purpose of running a single test. If you are using a tool that
-  # preloads Rails for running tests, you may have to set it to true.
-  config.eager_load = false
+  # Eager loading loads your whole application. When running a single test locally,
+  # this probably isn't necessary. It's a good idea to do in a CI environment.
+  config.eager_load = ENV["CI"].present?
 
-  # Configure static asset server for tests with Cache-Control for performance.
-  config.serve_static_files  = true
-  config.static_cache_control = "public, max-age=3600"
+  # Configure public file server for tests with Cache-Control for performance.
+  config.public_file_server.enabled = true
+  config.public_file_server.headers = { "Cache-Control" => "public, max-age=3600" }
 
   # Show full error reports and disable caching.
-  config.consider_all_requests_local       = true
+  config.consider_all_requests_local = true
   config.action_controller.perform_caching = false
 
   # Raise exceptions instead of rendering exception templates.
-  config.action_dispatch.show_exceptions = false
+  config.action_dispatch.show_exceptions = :none
 
   # Disable request forgery protection in test environment.
   config.action_controller.allow_forgery_protection = false
@@ -33,4 +32,4 @@
 
   # Print deprecation notices to the stderr.
   config.active_support.deprecation = :stderr
-end
+end
\ No newline at end of file