diff --git a/.rubocop/todo.rspec.yml b/.rubocop/todo.rspec.yml
index 1729baba2..c9c01c984 100644
--- a/.rubocop/todo.rspec.yml
+++ b/.rubocop/todo.rspec.yml
@@ -38,9 +38,6 @@ RSpec/ContextWording:
     - 'spec/jobs/imap_authentication_migration_cleanup_job_spec.rb'
     - 'spec/jobs/ticket_article_communicate_email_job_spec.rb'
     - 'spec/lib/application_handle_info_spec.rb'
-    - 'spec/lib/auth/developer_spec.rb'
-    - 'spec/lib/auth/ldap_spec.rb'
-    - 'spec/lib/auth_spec.rb'
     - 'spec/lib/core_ext/string_spec.rb'
     - 'spec/lib/external_credential/google_spec.rb'
     - 'spec/lib/external_credential/microsoft365_spec.rb'
@@ -178,8 +175,6 @@ RSpec/ExampleLength:
     - 'spec/jobs/migrate_ldap_samaccountname_to_uid_job_spec.rb'
     - 'spec/jobs/ticket_user_ticket_counter_job_spec.rb'
     - 'spec/jobs/user_device_log_job_spec.rb'
-    - 'spec/lib/auth/internal_spec.rb'
-    - 'spec/lib/auth/ldap_spec.rb'
     - 'spec/lib/auto_wizard_spec.rb'
     - 'spec/lib/core_ext/string_spec.rb'
     - 'spec/lib/external_credential/google_spec.rb'
@@ -395,8 +390,6 @@ RSpec/MessageSpies:
     - 'spec/jobs/app_version_restart_job_spec.rb'
     - 'spec/jobs/search_index_job_spec.rb'
     - 'spec/jobs/sla_ticket_rebuild_escalation_job_spec.rb'
-    - 'spec/lib/auth/developer_spec.rb'
-    - 'spec/lib/auth/ldap_spec.rb'
     - 'spec/lib/import/base_factory_examples.rb'
     - 'spec/lib/import/helper_spec.rb'
     - 'spec/lib/import/otrs/article/attachment_factory_spec.rb'
@@ -472,10 +465,6 @@ RSpec/MultipleExpectations:
     - 'spec/jobs/migrate_ldap_samaccountname_to_uid_job_spec.rb'
     - 'spec/jobs/search_index_job_spec.rb'
     - 'spec/jobs/ticket_user_ticket_counter_job_spec.rb'
-    - 'spec/lib/auth/developer_spec.rb'
-    - 'spec/lib/auth/internal_spec.rb'
-    - 'spec/lib/auth/ldap_spec.rb'
-    - 'spec/lib/auth_spec.rb'
     - 'spec/lib/cache_spec.rb'
     - 'spec/lib/core_ext/string_spec.rb'
     - 'spec/lib/email_address_validation_spec.rb'
@@ -667,7 +656,6 @@ RSpec/VerifiedDoubles:
   Exclude:
     - 'spec/db/migrate/issue_2460_fix_corrupted_twitter_ids_spec.rb'
     - 'spec/jobs/communicate_twitter_job_spec.rb'
-    - 'spec/lib/auth/ldap_spec.rb'
     - 'spec/lib/external_sync_spec.rb'
     - 'spec/lib/import/zendesk/object_attribute/base_examples.rb'
     - 'spec/lib/import/zendesk/object_attribute/checkbox_spec.rb'
diff --git a/app/assets/javascripts/app/models/user.coffee b/app/assets/javascripts/app/models/user.coffee
index 6c1a4beb1..d92327242 100644
--- a/app/assets/javascripts/app/models/user.coffee
+++ b/app/assets/javascripts/app/models/user.coffee
@@ -122,7 +122,7 @@ class App.User extends App.Model
     false
 
   maxLoginFailedReached: ->
-    return @login_failed > (App.Config.get('password_max_login_failed') || 10)
+    return @login_failed > (parseInt(App.Config.get('password_max_login_failed')))
 
   imageUrl: ->
     return if !@image
diff --git a/app/controllers/application_controller/authenticates.rb b/app/controllers/application_controller/authenticates.rb
index 277414b6e..503649b2f 100644
--- a/app/controllers/application_controller/authenticates.rb
+++ b/app/controllers/application_controller/authenticates.rb
@@ -66,8 +66,8 @@ module ApplicationController::Authenticates
         raise Exceptions::Forbidden, 'API password access disabled!'
       end
 
-      user = User.authenticate(username, password)
-      return authentication_check_prerequesits(user, 'basic_auth', auth_param) if user
+      auth = Auth.new(username, password)
+      return authentication_check_prerequesits(auth.user, 'basic_auth', auth_param) if auth.valid?
 
       authentication_errors.push('Invalid BasicAuth credentials')
     end
@@ -146,14 +146,6 @@ module ApplicationController::Authenticates
     raise Exceptions::NotAuthorized, authentication_errors.join(', ')
   end
 
-  def authenticate_with_password
-    user = User.authenticate(params[:username], params[:password])
-    raise_unified_login_error if !user
-
-    session.delete(:switched_from_user_id)
-    authentication_check_prerequesits(user, 'session', {})
-  end
-
   def authentication_check_prerequesits(user, auth_type, auth_param)
     raise Exceptions::Forbidden, 'Maintenance mode enabled!' if in_maintenance_mode?(user)
 
diff --git a/app/controllers/sessions_controller.rb b/app/controllers/sessions_controller.rb
index f429940ab..50b82f3ba 100644
--- a/app/controllers/sessions_controller.rb
+++ b/app/controllers/sessions_controller.rb
@@ -214,6 +214,14 @@ class SessionsController < ApplicationController
 
   private
 
+  def authenticate_with_password
+    auth = Auth.new(params[:username], params[:password])
+    raise_unified_login_error if !auth.valid?
+
+    session.delete(:switched_from_user_id)
+    authentication_check_prerequesits(auth.user, 'session', {})
+  end
+
   def initiate_session_for(user)
     request.env['rack.session.options'][:expire_after] = 1.year if params[:remember_me]
     session[:persistent] = true
diff --git a/app/controllers/users_controller.rb b/app/controllers/users_controller.rb
index daaa4ce90..87324b0ae 100644
--- a/app/controllers/users_controller.rb
+++ b/app/controllers/users_controller.rb
@@ -578,8 +578,9 @@ curl http://localhost/api/v1/users/password_change -v -u #{login}:#{password} -H
       render json: { message: 'failed', notice: ['Current password needed!'] }, status: :ok
       return
     end
-    user = User.authenticate(current_user.login, params[:password_old])
-    if !user
+
+    current_password_verified = PasswordHash.verified?(current_user.password, params[:password_old])
+    if !current_password_verified
       render json: { message: 'failed', notice: ['Current password is wrong!'] }, status: :ok
       return
     end
@@ -596,20 +597,19 @@ curl http://localhost/api/v1/users/password_change -v -u #{login}:#{password} -H
       return
     end
 
-    user.update!(password: params[:password_new])
+    current_user.update!(password: params[:password_new])
 
-    if user.email.present?
+    if current_user.email.present?
       NotificationFactory::Mailer.notification(
         template: 'password_change',
-        user:     user,
+        user:     current_user,
         objects:  {
-          user:         user,
-          current_user: current_user,
+          user: current_user,
         }
       )
     end
 
-    render json: { message: 'ok', user_login: user.login }, status: :ok
+    render json: { message: 'ok', user_login: current_user.login }, status: :ok
   end
 
 =begin
diff --git a/app/models/user.rb b/app/models/user.rb
index fb1c9cc70..32007b4ad 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -288,54 +288,6 @@ returns
 
 =begin
 
-authenticate user
-
-  result = User.authenticate(username, password)
-
-returns
-
-  result = user_model # user model if authentication was successfully
-
-=end
-
-  def self.authenticate(username, password)
-
-    # do not authenticate with nothing
-    return if username.blank? || password.blank?
-
-    user = User.identify(username)
-    return if !user
-
-    return if !Auth.can_login?(user)
-
-    return user if Auth.valid?(user, password)
-
-    sleep 1
-    user.login_failed += 1
-    user.save!
-    nil
-  end
-
-=begin
-
-checks if a user has reached the maximum of failed login tries
-
-  user = User.find(123)
-  result = user.max_login_failed?
-
-returns
-
-  result = true | false
-
-=end
-
-  def max_login_failed?
-    max_login_failed = Setting.get('password_max_login_failed').to_i || 10
-    login_failed > max_login_failed
-  end
-
-=begin
-
 tries to find the matching instance by the given identifier. Currently email and login is supported.
 
   user = User.indentify('User123')
@@ -352,6 +304,8 @@ returns
 =end
 
   def self.identify(identifier)
+    return if identifier.blank?
+
     # try to find user based on login
     user = User.find_by(login: identifier.downcase)
     return user if user
diff --git a/db/migrate/20210728103633_move_auth_backends_to_database.rb b/db/migrate/20210728103633_move_auth_backends_to_database.rb
new file mode 100644
index 000000000..64a8709d3
--- /dev/null
+++ b/db/migrate/20210728103633_move_auth_backends_to_database.rb
@@ -0,0 +1,54 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+class MoveAuthBackendsToDatabase < ActiveRecord::Migration[6.0]
+  def change
+
+    # return if it's a new setup
+    return if !Setting.exists?(name: 'system_init_done')
+
+    Setting.create_if_not_exists(
+      title:       'Authentication via %s',
+      name:        'auth_internal',
+      area:        'Security::Authentication',
+      description: 'Enables user authentication via %s.',
+      preferences: {
+        title_i18n:       ['internal database'],
+        description_i18n: ['internal database'],
+        permission:       ['admin.security'],
+      },
+      state:       {
+        priority: 1,
+        adapter:  'Auth::Internal',
+      },
+      frontend:    false
+    )
+    Setting.create_if_not_exists(
+      title:       'Authentication via %s',
+      name:        'auth_developer',
+      area:        'Security::Authentication',
+      description: 'Enables user authentication via %s.',
+      preferences: {
+        title_i18n:       ['developer password'],
+        description_i18n: ['developer password'],
+        permission:       ['admin.security'],
+      },
+      state:       {
+        priority: 2,
+        adapter:  'Auth::Developer',
+      },
+      frontend:    false
+    )
+
+    begin
+      auth_ldap = Setting.find_by(name: 'auth_ldap')
+
+      auth_ldap.state_initial[:value][:priority] = 3
+      auth_ldap.state_current[:value][:priority] = 3
+
+      auth_ldap.save!
+    rescue => e
+      Rails.logger.error "Error while updating 'auth_ldap' Setting priority"
+      Rails.logger.error e
+    end
+  end
+end
diff --git a/db/migrate/20210729183242_set_user_source_ldap_from_external_sync.rb b/db/migrate/20210729183242_set_user_source_ldap_from_external_sync.rb
new file mode 100644
index 000000000..c9f9efff8
--- /dev/null
+++ b/db/migrate/20210729183242_set_user_source_ldap_from_external_sync.rb
@@ -0,0 +1,16 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+class SetUserSourceLdapFromExternalSync < ActiveRecord::Migration[6.0]
+  def change
+    return if !Setting.exists?(name: 'system_init_done')
+
+    ldap_user_ids = ExternalSync.where(
+      source: 'Ldap::User',
+      object: 'User'
+    ).pluck(:o_id)
+
+    User.where(id: ldap_user_ids).find_each do |user|
+      user.update!(source: 'Ldap')
+    end
+  end
+end
diff --git a/db/seeds/settings.rb b/db/seeds/settings.rb
index 6674947d9..92b84915a 100644
--- a/db/seeds/settings.rb
+++ b/db/seeds/settings.rb
@@ -1122,6 +1122,38 @@ Setting.create_if_not_exists(
   },
   frontend:    false
 )
+Setting.create_if_not_exists(
+  title:       'Authentication via %s',
+  name:        'auth_internal',
+  area:        'Security::Authentication',
+  description: 'Enables user authentication via %s.',
+  preferences: {
+    title_i18n:       ['internal database'],
+    description_i18n: ['internal database'],
+    permission:       ['admin.security'],
+  },
+  state:       {
+    priority: 1,
+    adapter:  'Auth::Backend::Internal',
+  },
+  frontend:    false
+)
+Setting.create_if_not_exists(
+  title:       'Authentication via %s',
+  name:        'auth_developer',
+  area:        'Security::Authentication',
+  description: 'Enables user authentication via %s.',
+  preferences: {
+    title_i18n:       ['developer password'],
+    description_i18n: ['developer password'],
+    permission:       ['admin.security'],
+  },
+  state:       {
+    priority: 2,
+    adapter:  'Auth::Backend::Developer',
+  },
+  frontend:    false
+)
 Setting.create_if_not_exists(
   title:       'Authentication via %s',
   name:        'auth_ldap',
@@ -1133,7 +1165,8 @@ Setting.create_if_not_exists(
     permission:       ['admin.security'],
   },
   state:       {
-    adapter:       'Auth::Ldap',
+    priority:      3,
+    adapter:       'Auth::Backend::Ldap',
     host:          'localhost',
     port:          389,
     bind_dn:       'cn=Manager,dc=example,dc=org',
diff --git a/lib/auth.rb b/lib/auth.rb
index 3adb45fc5..7038e290f 100644
--- a/lib/auth.rb
+++ b/lib/auth.rb
@@ -1,115 +1,58 @@
 # Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
 
 class Auth
-  include ApplicationLib
 
-=begin
+  attr_reader :user, :password, :auth_user
 
-checks if a given user can login. Checks for
- - valid user
- - active state
- - max failed logins
+  delegate :user, to: :auth_user
 
-  result = Auth.can_login?(user)
+  attr_accessor :increase_login_failed_attempts
 
-returns
+  # Initializes a Auth object for the given user.
+  #
+  # @param username [String] the user name for the user object which needs an authentication.
+  #
+  # @example
+  #  auth = Auth.new('master@example.com', 'some+password')
+  def initialize(username, password)
+    @lookup_backend_instance = {}
 
-  result = true | false
+    @auth_user = username.present? ? Auth::User.new(username) : nil
+    @password = password
 
-=end
+    @increase_login_failed_attempts = false
+  end
 
-  def self.can_login?(user)
-    return false if !user.is_a?(User)
-    return false if !user.active?
+  # Validates the given credentials for the user to the configured auth backends which should
+  # be performed.
+  #
+  # @return [Boolean] true if the user was authenticated, otherwise false.
+  def valid?
+    if !auth_user || !auth_user.can_login?
+      avoid_brute_force_attack
 
-    return true if !user.max_login_failed?
+      return false
+    end
 
-    Rails.logger.info "Max login failed reached for user #{user.login}."
+    if backends.valid?
+      auth_user.update_last_login
+      return true
+    end
 
+    avoid_brute_force_attack
+
+    auth_user.increase_login_failed if increase_login_failed_attempts
     false
   end
 
-=begin
+  private
 
-checks if a given user and password match against multiple auth backends
- - valid user
- - active state
- - max failed logins
-
-  result = Auth.valid?(user, password)
-
-returns
-
-  result = true | false
-
-=end
-
-  def self.valid?(user, password)
-    # try to login against configure auth backends
-    backends.any? do |config|
-      next if !backend_validates?(
-        config:   config,
-        user:     user,
-        password: password,
-      )
-
-      Rails.logger.info "Authentication against #{config[:adapter]} for user #{user.login} ok."
-
-      # remember last login date
-      user.update_last_login
-
-      true
-    end
+  # Sleep for a second to avoid brute force attacks.
+  def avoid_brute_force_attack
+    sleep 1
   end
 
-=begin
-
-returns a list of all Auth backend configurations
-
-  result = Auth.backends
-
-returns
-
-  result = [
-    {
-      adapter: 'Auth::Internal',
-    },
-    {
-      adapter: 'Auth::Developer',
-    },
-    ...
-  ]
-
-=end
-
-  def self.backends
-
-    # use std. auth backends
-    config = [
-      {
-        adapter: 'Auth::Internal',
-      },
-      {
-        adapter: 'Auth::Developer',
-      },
-    ]
-
-    # added configured backends
-    Setting.where(area: 'Security::Authentication').each do |setting|
-      next if setting.state_current[:value].blank?
-
-      config.push setting.state_current[:value]
-    end
-
-    config
+  def backends
+    Auth::Backend.new(self)
   end
-
-  def self.backend_validates?(config:, user:, password:)
-    return false if !config[:adapter]
-
-    instance = config[:adapter].constantize.new(config)
-
-    instance.valid?(user, password)
-  end
-  private_class_method :backend_validates?
 end
diff --git a/lib/auth/backend.rb b/lib/auth/backend.rb
new file mode 100644
index 000000000..181237d23
--- /dev/null
+++ b/lib/auth/backend.rb
@@ -0,0 +1,41 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+class Auth
+  class Backend
+
+    attr_reader :auth
+
+    def initialize(auth)
+      @auth = auth
+    end
+
+    def valid?
+      instances.any? do |instance|
+        next if !instance.valid?
+
+        Rails.logger.info "Authentication against #{instance.class.name} for user #{auth.user.login} ok."
+
+        true
+      end
+    end
+
+    private
+
+    def instances
+      configs.filter_map do |config|
+        config[:adapter].constantize.new(config, auth)
+      rescue => e
+        Rails.logger.error "Failed to load Auth::Backend from Setting '#{config}'"
+        Rails.logger.error e
+        nil
+      end
+    end
+
+    def configs
+      Setting.where(area: 'Security::Authentication')
+      .map { |setting| setting.state_current[:value] } # extract current Setting value as config
+      .reject(&:blank?)
+      .sort { |a, b| a.fetch(:priority, 999) <=> b.fetch(:priority, 999) } # sort by priority and fallback to append if not set
+    end
+  end
+end
diff --git a/lib/auth/backend/base.rb b/lib/auth/backend/base.rb
new file mode 100644
index 000000000..532bda2cf
--- /dev/null
+++ b/lib/auth/backend/base.rb
@@ -0,0 +1,40 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+class Auth
+  class Backend
+    class Base
+
+      delegate :user, :password, to: :auth
+
+      attr_reader :config, :auth
+
+      # Base initialization for Auth backend object.
+      #
+      # @param config [Hash] backend configuration hash.
+      # @param auth [Auth] the Auth object for the authentication.
+      #
+      # @example
+      #  auth = Auth::Backend::Internal.new('master@example.com', auth)
+      def initialize(config, auth)
+        @config = config
+        @auth   = auth
+      end
+
+      def valid?
+        return false if !perform?
+
+        authenticated?
+      end
+
+      private
+
+      def perform?
+        raise NotImplementedError
+      end
+
+      def authenticated?
+        raise NotImplementedError
+      end
+    end
+  end
+end
diff --git a/lib/auth/backend/developer.rb b/lib/auth/backend/developer.rb
new file mode 100644
index 000000000..4c63b5a83
--- /dev/null
+++ b/lib/auth/backend/developer.rb
@@ -0,0 +1,52 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+class Auth
+  class Backend
+    class Developer < Auth::Backend::Base
+
+      private
+
+      # Special development/test password validation.
+      # * For the developer mode the password 'test' is allowed for every User.
+      # * For the test environment the password can be blank if also the user password is currently blank.
+      #
+      # @returns [Boolean] true if the validation works, otherwise false.
+      def authenticated?
+        if valid_for_developer_mode? || valid_for_test_environment?
+          Rails.logger.info "System in test/developer mode, authentication for user #{user.login} ok."
+          return true
+        end
+
+        false
+      end
+
+      # Overwrites the default behaviour to check for a allowed environment.
+      #
+      # @returns [Boolean] true if the environment is development or test.
+      def perform?
+        allowed_environment?
+      end
+
+      # Check for allowed environments.
+      #
+      # @returns [Boolean] true if one allowed environment is active.
+      def allowed_environment?
+        Setting.get('developer_mode') == true || Rails.env.test?
+      end
+
+      # Validate password for test environment.
+      #
+      # @returns [Boolean] true if password and user password is blank, otherwise false.
+      def valid_for_test_environment?
+        Rails.env.test? && password.blank? && user.password.blank?
+      end
+
+      # Validate password for test environment.
+      #
+      # @returns [Boolean] true if the given password is 'test', otherwise false.
+      def valid_for_developer_mode?
+        Setting.get('developer_mode') == true && password == 'test'
+      end
+    end
+  end
+end
diff --git a/lib/auth/backend/internal.rb b/lib/auth/backend/internal.rb
new file mode 100644
index 000000000..459c45c13
--- /dev/null
+++ b/lib/auth/backend/internal.rb
@@ -0,0 +1,42 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+class Auth
+  class Backend
+    class Internal < Auth::Backend::Base
+
+      private
+
+      # Validation against the internal database.
+      #
+      # @returns [Boolean] true if the validation works, otherwise false.
+      def authenticated?
+        return true if hash_matches?
+
+        auth.increase_login_failed_attempts = true
+
+        false
+      end
+
+      # Overwrites the default behaviour to only perform this authentication if an internal password exists.
+      #
+      # @returns [Boolean] true if a internal password for the user is present.
+      def perform?
+        return false if password.blank?
+        return false if !user.verified && user.source == 'signup'
+
+        user.password.present?
+      end
+
+      def hash_matches?
+        # Because of legacy reason a special check exists and afterwards the
+        # password will be saved in the current format.
+        if PasswordHash.legacy?(user.password, password)
+          user.update!(password: password)
+          return true
+        end
+
+        PasswordHash.verified?(user.password, password)
+      end
+    end
+  end
+end
diff --git a/lib/auth/backend/ldap.rb b/lib/auth/backend/ldap.rb
new file mode 100644
index 000000000..bf090aaa4
--- /dev/null
+++ b/lib/auth/backend/ldap.rb
@@ -0,0 +1,74 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+class Auth
+  class Backend
+    class Ldap < Auth::Backend::Base
+
+      private
+
+      # Validation against the configured ldap integration.
+      #
+      # @returns [Boolean] true if the validation works, otherwise false.
+      def authenticated?
+        ldap_user = ::Ldap::User.new
+
+        # get from config or fallback to login
+        # for a list of user attributes which should
+        # be used for logging in
+        login_attributes = config[:login_attributes] || %w[login]
+
+        authed = login_attributes.any? do |attribute|
+          ldap_user.valid?(user[attribute], password)
+        end
+
+        log_auth_result(authed)
+        authed
+      rescue => e
+        message = "Can't connect to ldap backend #{e}"
+        Rails.logger.info message
+        Rails.logger.info e
+        log(
+          status:   'failed',
+          response: message,
+        )
+        false
+      end
+
+      # Checks the default behaviour and as a addition if the ldap integration is currently active.
+      #
+      # @returns [Boolean] true if the ldap integration is active and the default behaviour matches.
+      def perform?
+        user.source == 'Ldap' && Setting.get('ldap_integration')
+      end
+
+      # Logs the auth result
+      #
+      # @param authed [Boolean] true if the user is authed, otherwise false.
+      def log_auth_result(authed)
+        result = authed ? 'success' : 'failed'
+        log(
+          status: result,
+        )
+      end
+
+      # Created the http log for the current authentication.
+      #
+      # @param status [String] the status of the ldap authentication.
+      # @param response [String] the response message.
+      def log(status:, response: nil)
+        HttpLog.create(
+          direction:     'out',
+          facility:      'ldap',
+          url:           "bind -> #{user.login}",
+          status:        status,
+          ip:            nil,
+          request:       { content: user.login },
+          response:      { content: response || status },
+          method:        'tcp',
+          created_by_id: 1,
+          updated_by_id: 1,
+        )
+      end
+    end
+  end
+end
diff --git a/lib/auth/base.rb b/lib/auth/base.rb
deleted file mode 100644
index 8cda7595f..000000000
--- a/lib/auth/base.rb
+++ /dev/null
@@ -1,14 +0,0 @@
-# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
-
-class Auth
-  class Base
-
-    def initialize(config)
-      @config = config
-    end
-
-    def valid?(_user, _password)
-      raise "Missing implementation of method 'valid?' for class '#{self.class.name}'"
-    end
-  end
-end
diff --git a/lib/auth/developer.rb b/lib/auth/developer.rb
deleted file mode 100644
index fc07234d1..000000000
--- a/lib/auth/developer.rb
+++ /dev/null
@@ -1,15 +0,0 @@
-# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
-
-class Auth
-  class Developer < Auth::Base
-
-    def valid?(user, password)
-      return false if user.blank?
-      return false if Setting.get('developer_mode') != true
-      return false if password != 'test'
-
-      Rails.logger.info "System in developer mode, authentication for user #{user.login} ok."
-      true
-    end
-  end
-end
diff --git a/lib/auth/internal.rb b/lib/auth/internal.rb
deleted file mode 100644
index cb9adb766..000000000
--- a/lib/auth/internal.rb
+++ /dev/null
@@ -1,29 +0,0 @@
-# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
-
-class Auth
-  class Internal < Auth::Base
-
-    def valid?(user, password)
-
-      return false if user.blank?
-
-      if PasswordHash.legacy?(user.password, password)
-        update_password(user, password)
-        return true
-      end
-
-      password_verified = PasswordHash.verified?(user.password, password)
-
-      raise Exceptions::Forbidden, 'Please verify your account before you can login!' if !user.verified && user.source == 'signup' && password_verified
-
-      password_verified
-    end
-
-    private
-
-    def update_password(user, password)
-      user.password = PasswordHash.crypt(password)
-      user.save
-    end
-  end
-end
diff --git a/lib/auth/ldap.rb b/lib/auth/ldap.rb
deleted file mode 100644
index 330447ed5..000000000
--- a/lib/auth/ldap.rb
+++ /dev/null
@@ -1,59 +0,0 @@
-# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
-
-class Auth
-  class Ldap < Auth::Base
-
-    def valid?(user, password)
-      return false if !Setting.get('ldap_integration')
-
-      ldap_user = ::Ldap::User.new
-
-      # get from config or fallback to login
-      # for a list of user attributes which should
-      # be used for logging in
-      login_attributes = @config[:login_attributes] || %w[login]
-
-      authed = login_attributes.any? do |attribute|
-        ldap_user.valid?(user[attribute], password)
-      end
-
-      log_auth_result(user, authed)
-      authed
-    rescue => e
-      message = "Can't connect to ldap backend #{e}"
-      Rails.logger.info message
-      Rails.logger.info e
-      log(
-        user:     user,
-        status:   'failed',
-        response: message,
-      )
-      false
-    end
-
-    private
-
-    def log_auth_result(user, authed)
-      result = authed ? 'success' : 'failed'
-      log(
-        user:   user,
-        status: result,
-      )
-    end
-
-    def log(user:, status:, response: nil)
-      HttpLog.create(
-        direction:     'out',
-        facility:      'ldap',
-        url:           "bind -> #{user.login}",
-        status:        status,
-        ip:            nil,
-        request:       { content: user.login },
-        response:      { content: response || status },
-        method:        'tcp',
-        created_by_id: 1,
-        updated_by_id: 1,
-      )
-    end
-  end
-end
diff --git a/lib/auth/user.rb b/lib/auth/user.rb
new file mode 100644
index 000000000..f5a39d50c
--- /dev/null
+++ b/lib/auth/user.rb
@@ -0,0 +1,52 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+class Auth
+  class User < SimpleDelegator
+
+    attr_reader :user
+
+    def initialize(username)
+      @user = ::User.identify(username)
+      super(@user)
+    end
+
+    # Checks if a given user can login. Check for the following criteria:
+    # * valid user
+    # * user is active
+    # * user has not reached the maximum of failed login tries
+    #
+    # @return [Boolean] true if the user can login, false otherwise.
+    def can_login?
+      return false if !exists?
+      return false if !active?
+
+      !max_login_failed?
+    end
+
+    # Increase the current failed login count for the user.
+    def increase_login_failed
+      self.login_failed += 1
+      save!
+    end
+
+    private
+
+    # Checks if a user has reached the maximum of failed login tries.
+    #
+    # @return [Boolean] true if the user has reached the maximum of failed login tries, otherwise false.
+    def max_login_failed?
+      max_login_failed = Setting.get('password_max_login_failed').to_i
+      return false if login_failed <= max_login_failed
+
+      Rails.logger.info "Max login failed reached for user #{login}."
+      true
+    end
+
+    # Checks if a valid user exists.
+    #
+    # @return [Boolean] true if a valid user exists, otherwise false.
+    def exists?
+      present? && __getobj__.is_a?(::User)
+    end
+  end
+end
diff --git a/lib/sequencer/unit/import/ldap/user/attributes/static.rb b/lib/sequencer/unit/import/ldap/user/attributes/static.rb
index cd8aa2828..2649ca4b2 100644
--- a/lib/sequencer/unit/import/ldap/user/attributes/static.rb
+++ b/lib/sequencer/unit/import/ldap/user/attributes/static.rb
@@ -19,6 +19,9 @@ class Sequencer
                     # because otherwise disabled instances won't get
                     # re-activated if they should get synced again
                     active: true,
+
+                    # Set the source to 'Ldap' for the authentication handling.
+                    source: 'Ldap',
                   }
                 end
               end
diff --git a/spec/db/migrate/set_user_source_ldap_from_external_sync_spec.rb b/spec/db/migrate/set_user_source_ldap_from_external_sync_spec.rb
new file mode 100644
index 000000000..047661eb9
--- /dev/null
+++ b/spec/db/migrate/set_user_source_ldap_from_external_sync_spec.rb
@@ -0,0 +1,30 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+require 'rails_helper'
+
+RSpec.describe SetUserSourceLdapFromExternalSync, type: :db_migration, db_strategy: :reset do
+  let(:users) { create_list(:user, 2) }
+  let(:other_user) { create(:user) }
+
+  before do
+    2.times do |count|
+      index = count - 1
+
+      create(:external_sync,
+             source:    'Ldap::User',
+             source_id: "uid=#{users[index].login},ou=People,dc=example,dc=org",
+             object:    'User',
+             o_id:      users[index].id)
+    end
+  end
+
+  context 'when having users from the ldap integration' do
+    it 'source key for users are filled' do
+      expect { migrate }.to change { users[0].reload.source }.to('Ldap').and change { users[1].reload.source }.to('Ldap')
+    end
+
+    it 'other user should not be touched' do
+      expect { migrate }.not_to change { other_user.reload.source }
+    end
+  end
+end
diff --git a/spec/factories/setting.rb b/spec/factories/setting.rb
index d08c96859..0df9ca5ae 100644
--- a/spec/factories/setting.rb
+++ b/spec/factories/setting.rb
@@ -3,7 +3,7 @@
 FactoryBot.define do
   factory :setting do
     title       { 'ABC API Token' }
-    name        { 'abc_api_token' }
+    name        { Faker::Name.unique.name }
     area        { 'Integration::ABC' }
     description { 'API Token for ABC to access ABC.' }
     frontend    { false }
diff --git a/spec/lib/auth/backend_examples.rb b/spec/lib/auth/backend/backend_examples.rb
similarity index 100%
rename from spec/lib/auth/backend_examples.rb
rename to spec/lib/auth/backend/backend_examples.rb
diff --git a/spec/lib/auth/base_spec.rb b/spec/lib/auth/backend/base_spec.rb
similarity index 51%
rename from spec/lib/auth/base_spec.rb
rename to spec/lib/auth/backend/base_spec.rb
index 9a8506f04..3299753f2 100644
--- a/spec/lib/auth/base_spec.rb
+++ b/spec/lib/auth/backend/base_spec.rb
@@ -1,21 +1,15 @@
 # Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
 
 require 'rails_helper'
-require 'lib/auth/backend_examples'
+require 'lib/auth/backend/backend_examples'
 
-RSpec.describe Auth::Base do
+RSpec.describe Auth::Backend::Base do
 
   let(:user) { create(:user) }
-  let(:instance) { described_class.new({ adapter: described_class.name }) }
+  let(:auth) { Auth.new(user.login, 'not_used') }
+  let(:instance) { described_class.new({ adapter: described_class.name }, auth) }
 
   describe '#valid?' do
     it_behaves_like 'Auth backend'
-
-    it "requires an implementation of the 'valid?' method" do
-
-      expect do
-        instance.valid?(user, 'password')
-      end.to raise_error(RuntimeError)
-    end
   end
 end
diff --git a/spec/lib/auth/backend/developer_spec.rb b/spec/lib/auth/backend/developer_spec.rb
new file mode 100644
index 000000000..b024b7f82
--- /dev/null
+++ b/spec/lib/auth/backend/developer_spec.rb
@@ -0,0 +1,94 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+require 'rails_helper'
+require 'lib/auth/backend/backend_examples'
+
+RSpec.describe Auth::Backend::Developer do
+
+  let(:user) { create(:user) }
+  let(:password) { 'not_used' }
+  let(:auth) { Auth.new(user.login, password) }
+  let(:instance) { described_class.new({ adapter: described_class.name }, auth) }
+
+  describe '#valid?' do
+    it_behaves_like 'Auth backend'
+
+    context 'when Setting developer_mode is true' do
+
+      before do
+        Setting.set('developer_mode', true)
+      end
+
+      context 'when password is "test"' do
+
+        let(:password) { 'test' }
+
+        it 'authenticates' do
+          expect(instance.valid?).to be true
+        end
+      end
+
+      context 'when password matches actual User password' do
+
+        let(:user) { create(:user, password: 'secure') }
+        let(:password) { user.password_plain }
+
+        it "doesn't authenticate" do
+          expect(instance.valid?).to be false
+        end
+      end
+    end
+
+    context 'when Rails.env is "test"' do
+
+      before do
+        allow(Rails).to receive(:env) { 'test'.inquiry } # rubocop:disable Rails/Inquiry
+      end
+
+      context 'when password is blank' do
+
+        let(:password) { '' }
+
+        it 'authenticates' do
+          expect(instance.valid?).to be true
+        end
+      end
+
+      context 'when password matches actual User password' do
+
+        let(:user) { create(:user, password: 'secure') }
+        let(:password) { user.password_plain }
+
+        it "doesn't authenticate" do
+          expect(instance.valid?).to be false
+        end
+      end
+    end
+
+    context 'when Rails.env is "production"' do
+
+      before do
+        allow(Rails).to receive(:env) { 'production'.inquiry } # rubocop:disable Rails/Inquiry
+      end
+
+      context 'when password is blank' do
+
+        let(:password) { '' }
+
+        it "doesn't authenticate" do
+          expect(instance.valid?).to be false
+        end
+      end
+
+      context 'when password matches actual User password' do
+
+        let(:user) { create(:user, password: 'secure') }
+        let(:password) { user.password_plain }
+
+        it "doesn't authenticate" do
+          expect(instance.valid?).to be false
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/auth/backend/internal_spec.rb b/spec/lib/auth/backend/internal_spec.rb
new file mode 100644
index 000000000..7704b329e
--- /dev/null
+++ b/spec/lib/auth/backend/internal_spec.rb
@@ -0,0 +1,89 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+require 'rails_helper'
+require 'lib/auth/backend/backend_examples'
+
+RSpec.describe Auth::Backend::Internal do
+
+  let(:user) { create(:user) }
+  let(:password) { 'secure' }
+  let(:auth) { Auth.new(user.login, password) }
+  let(:instance) { described_class.new({ adapter: described_class.name }, auth) }
+
+  describe '#valid?' do
+    it_behaves_like 'Auth backend'
+
+    context 'when password is given' do
+      let(:user) { create(:user, password: password) }
+
+      it 'authenticates' do
+        expect(instance.valid?).to be true
+      end
+    end
+
+    context 'when given password matches stored hash' do
+
+      let(:password) { user.password }
+      let(:user) { create(:user, password: 'secure') }
+
+      it "doesn't authenticate" do
+        expect(instance.valid?).to be false
+      end
+    end
+
+    context 'when given password is blank' do
+      let(:password) { '' }
+      let(:user) { create(:user, password: 'secure') }
+
+      it "doesn't authenticate" do
+        expect(instance.valid?).to be false
+      end
+    end
+
+    context 'with legacy SHA2 passwords' do
+      let(:user) { create(:user, password: PasswordHash.sha2(password)) }
+
+      it 'is password hash crypted' do
+        expect(PasswordHash.crypted?(user.password)).to be true
+      end
+
+      it 'is password hash legacy' do
+        expect(PasswordHash.legacy?(user.password, password)).to be true
+      end
+
+      it 'valid authentication' do
+        expect(instance.valid?).to be true
+      end
+
+      it 'is password hash not legacy after validation' do
+        instance.valid?
+        expect(PasswordHash.legacy?(user.reload.password, password)).to be false
+      end
+
+      it 'is password hash crypted after validation' do
+        instance.valid?
+        expect(PasswordHash.crypted?(user.password)).to be true
+      end
+    end
+
+    context 'when affecting Auth#increase_login_failed_attempts' do
+
+      context 'when authentication fails' do
+        let(:password) { 'wrong' }
+        let(:user) { create(:user, password: 'secure') }
+
+        it 'sets Auth#increase_login_failed_attempts flag to true' do
+          expect { instance.valid? }.to change(auth, :increase_login_failed_attempts).from(false).to(true)
+        end
+      end
+
+      context 'when authentication succeeds' do
+        let(:user) { create(:user, password: password) }
+
+        it "doesn't change Auth#increase_login_failed_attempts flag" do
+          expect { instance.valid? }.not_to change(auth, :increase_login_failed_attempts)
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/auth/backend/ldap_spec.rb b/spec/lib/auth/backend/ldap_spec.rb
new file mode 100644
index 000000000..b3727b569
--- /dev/null
+++ b/spec/lib/auth/backend/ldap_spec.rb
@@ -0,0 +1,91 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+require 'rails_helper'
+require 'lib/auth/backend/backend_examples'
+
+RSpec.describe ::Auth::Backend::Ldap do
+
+  let(:user) { create(:user, source: 'Ldap') }
+  let(:password) { 'secure' }
+  let(:auth) { Auth.new(user.login, password) }
+  let(:config) do
+    {
+      adapter: described_class.name
+    }
+  end
+  let(:instance) { described_class.new(config, auth) }
+  let(:ldap_integration) { true }
+
+  before do
+    Setting.set('ldap_integration', ldap_integration)
+  end
+
+  describe '#valid?' do
+    let(:ldap_user) { instance_double(Ldap::User) }
+
+    before do
+      allow(Ldap::User).to receive(:new).with(any_args).and_return(ldap_user)
+      allow(ldap_user).to receive(:valid?).with(any_args).and_return(true)
+    end
+
+    it_behaves_like 'Auth backend'
+
+    it 'authenticates users' do
+      expect(instance.valid?).to be true
+    end
+
+    context 'when custom login attribute is configured' do
+
+      let(:config) do
+        super().merge(
+          login_attributes: %w[firstname]
+        )
+      end
+
+      it 'authenticates' do
+        allow(ldap_user).to receive(:valid?).with(user.firstname, password).and_return(true)
+
+        expect(instance.valid?).to be true
+      end
+    end
+
+    context 'when Setting ldap_integration is false' do
+
+      let(:ldap_integration) { false }
+
+      it "doesn't authenticate" do
+        expect(instance.valid?).to be false
+      end
+    end
+
+    context 'when LDAP authentication fails' do
+
+      it "doesn't authenticate" do
+        allow(ldap_user).to receive(:valid?).with(any_args).and_return(false)
+
+        expect(instance.valid?).to be false
+      end
+    end
+
+    context 'when User#source does not match Ldap' do
+
+      context 'when blank' do
+
+        let(:user) { create(:user) }
+
+        it "doesn't authenticate" do
+          expect(instance.valid?).to be false
+        end
+      end
+
+      context 'when some other value' do
+
+        let(:user) { create(:user, source: 'some other value') }
+
+        it "doesn't authenticate" do
+          expect(instance.valid?).to be false
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/auth/backend_spec.rb b/spec/lib/auth/backend_spec.rb
new file mode 100644
index 000000000..29cb36e9b
--- /dev/null
+++ b/spec/lib/auth/backend_spec.rb
@@ -0,0 +1,108 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+require 'rails_helper'
+
+RSpec.describe Auth::Backend do
+
+  let(:user) { create(:user) }
+  let(:password) { 'secure' }
+  let(:auth) { Auth.new(user.login, password) }
+  let(:instance) { described_class.new(auth) }
+
+  describe '#valid?' do
+
+    context 'when invalid Setting is present in DB' do
+
+      context 'when value is blank' do
+
+        before do
+          create(:setting,
+                 area:  'Security::Authentication',
+                 state: {},)
+        end
+
+        it "doesn't raise an exception" do
+          expect { instance.valid? }.not_to raise_exception
+        end
+      end
+
+      context "when adapter can't be constantized" do
+        before do
+          create(:setting,
+                 area:  'Security::Authentication',
+                 state: {
+                   adapter: 'This::Will::Never::Work'
+                 },)
+        end
+
+        it "doesn't raise an exception" do
+          expect { instance.valid? }.not_to raise_exception
+        end
+      end
+    end
+
+    context 'when backend prioritization is relevant' do
+
+      let(:previous_class_namespace) { 'Auth::Backend::TopPrio' }
+      let(:later_class_namespace) { 'Auth::Backend::LeastPrio' }
+
+      let(:previous_backend_class) { Class.new(Auth::Backend::Base) }
+      let(:later_backend_class) { Class.new(Auth::Backend::Base) }
+
+      let(:previous_backend_instance) { instance_double(previous_class_namespace) }
+      let(:later_backend_instance) { instance_double(later_class_namespace) }
+
+      before do
+        stub_const previous_class_namespace, previous_backend_class
+        stub_const later_class_namespace, later_backend_class
+
+        Setting.where(area: 'Security::Authentication').destroy_all
+
+        create(:setting,
+               area:  'Security::Authentication',
+               state: {
+                 adapter:  previous_class_namespace,
+                 priority: 1
+               },)
+        create(:setting,
+               area:  'Security::Authentication',
+               state: {
+                 adapter:  later_class_namespace,
+                 priority: 2
+               },)
+
+        allow(previous_class_namespace.constantize).to receive(:new).and_return(previous_backend_instance)
+        allow(later_class_namespace.constantize).to receive(:new).and_return(later_backend_instance)
+
+        allow(previous_backend_instance).to receive(:valid?)
+        allow(later_backend_instance).to receive(:valid?)
+      end
+
+      context 'when previous backend was valid' do
+
+        before do
+          allow(previous_backend_instance).to receive(:valid?).and_return(true)
+        end
+
+        it "doesn't call valid on later backend" do
+          instance.valid?
+
+          expect(later_backend_instance).not_to have_received(:valid?)
+        end
+      end
+
+      context 'when previous backend was not valid' do
+
+        before do
+          allow(previous_backend_instance).to receive(:valid?).and_return(false)
+        end
+
+        it 'calls valid on later backend' do
+          instance.valid?
+
+          expect(later_backend_instance).to have_received(:valid?)
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/auth/developer_spec.rb b/spec/lib/auth/developer_spec.rb
deleted file mode 100644
index 80f7227cb..000000000
--- a/spec/lib/auth/developer_spec.rb
+++ /dev/null
@@ -1,49 +0,0 @@
-# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
-
-require 'rails_helper'
-require 'lib/auth/backend_examples'
-
-RSpec.describe Auth::Developer do
-
-  let(:user) { create(:user) }
-  let(:instance) { described_class.new({ adapter: described_class.name }) }
-
-  describe '#valid?' do
-    it_behaves_like 'Auth backend'
-
-    it "authenticates users with password 'test'" do
-
-      allow(Setting).to receive(:get)
-      allow(Setting).to receive(:get).with('developer_mode').and_return(true)
-
-      result = instance.valid?(user, 'test')
-
-      expect(result).to be true
-    end
-
-    context 'invalid' do
-
-      let(:password) { 'zammad' }
-
-      it "doesn't authenticate if developer mode is off" do
-
-        allow(Setting).to receive(:get)
-        allow(Setting).to receive(:get).with('developer_mode').and_return(false)
-
-        result = instance.valid?(user, password)
-
-        expect(result).to be false
-      end
-
-      it "doesn't authenticate with correct password" do
-
-        allow(Setting).to receive(:get)
-        allow(Setting).to receive(:get).with('developer_mode').and_return(true)
-
-        result = instance.valid?(user, password)
-
-        expect(result).to be false
-      end
-    end
-  end
-end
diff --git a/spec/lib/auth/internal_spec.rb b/spec/lib/auth/internal_spec.rb
deleted file mode 100644
index b899a53e9..000000000
--- a/spec/lib/auth/internal_spec.rb
+++ /dev/null
@@ -1,40 +0,0 @@
-# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
-
-require 'rails_helper'
-require 'lib/auth/backend_examples'
-
-RSpec.describe Auth::Internal do
-
-  let(:password) { 'zammad' }
-  let(:user) { create(:user, password: password) }
-  let(:instance) { described_class.new({ adapter: described_class.name }) }
-
-  describe '#valid?' do
-    it_behaves_like 'Auth backend'
-
-    it 'authenticates via password' do
-      result = instance.valid?(user, password)
-      expect(result).to be true
-    end
-
-    it "doesn't authenticate via plain password" do
-      result = instance.valid?(user, user.password)
-      expect(result).to be_falsy
-    end
-
-    it 'converts legacy sha2 passwords' do
-
-      sha2 = PasswordHash.sha2(password)
-      user = create(:user, password: sha2)
-
-      expect(PasswordHash.crypted?(user.password)).to be true
-      expect(PasswordHash.legacy?(user.password, password)).to be true
-
-      result = instance.valid?(user, password)
-      expect(result).to be true
-
-      expect(PasswordHash.legacy?(user.password, password)).to be false
-      expect(PasswordHash.crypted?(user.password)).to be true
-    end
-  end
-end
diff --git a/spec/lib/auth/ldap_spec.rb b/spec/lib/auth/ldap_spec.rb
deleted file mode 100644
index ebbd50809..000000000
--- a/spec/lib/auth/ldap_spec.rb
+++ /dev/null
@@ -1,71 +0,0 @@
-# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
-
-require 'rails_helper'
-require 'lib/auth/backend_examples'
-require 'auth/ldap'
-
-RSpec.describe ::Auth::Ldap do
-
-  let(:user) { create(:user) }
-  let(:password) { 'somepassword' }
-  let(:instance) { described_class.new({ adapter: described_class.name }) }
-
-  describe '#valid?' do
-    it_behaves_like 'Auth backend'
-
-    it 'authenticates users' do
-
-      allow(Setting).to receive(:get)
-      allow(Setting).to receive(:get).with('ldap_integration').and_return(true)
-
-      ldap_user = double(valid?: true)
-      allow(::Ldap::User).to receive(:new).and_return(ldap_user)
-
-      result = instance.valid?(user, password)
-      expect(result).to be true
-    end
-
-    it 'authenticates via configurable user attributes' do
-
-      allow(Setting).to receive(:get)
-      allow(Setting).to receive(:get).with('ldap_integration').and_return(true)
-
-      instance = described_class.new(
-        adapter:          described_class.name,
-        login_attributes: %w[firstname],
-      )
-
-      ldap_user = double
-      allow(ldap_user).to receive(:valid?).with(user.firstname, password).and_return(true)
-
-      allow(::Ldap::User).to receive(:new).and_return(ldap_user)
-
-      result = instance.valid?(user, password)
-      expect(result).to be true
-    end
-
-    context 'invalid' do
-
-      it "doesn't authenticate if 'ldap_integration' Setting is disabled" do
-
-        allow(Setting).to receive(:get)
-        allow(Setting).to receive(:get).with('ldap_integration').and_return(false)
-
-        result = instance.valid?(user, password)
-        expect(result).to be false
-      end
-
-      it "doesn't authenticate if ldap says 'nope'" do
-
-        allow(Setting).to receive(:get)
-        allow(Setting).to receive(:get).with('ldap_integration').and_return(true)
-
-        ldap_user = double(valid?: false)
-        allow(::Ldap::User).to receive(:new).and_return(ldap_user)
-
-        result = instance.valid?(user, password)
-        expect(result).to be false
-      end
-    end
-  end
-end
diff --git a/spec/lib/auth/user_spec.rb b/spec/lib/auth/user_spec.rb
new file mode 100644
index 000000000..39d552ad7
--- /dev/null
+++ b/spec/lib/auth/user_spec.rb
@@ -0,0 +1,97 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+require 'rails_helper'
+
+RSpec.describe Auth::User do
+
+  let(:user) { create(:user) }
+  let(:instance) { described_class.new(user.login) }
+
+  describe '.can_login?' do
+    it 'responds to can_login?' do
+      expect(instance).to respond_to(:can_login?)
+    end
+
+    shared_examples 'check loginable' do
+      it 'checks if users can login' do
+        expect(instance.can_login?).to be true
+      end
+    end
+
+    shared_examples 'check not loginable' do
+      it 'check that user can not login' do
+        expect(instance.can_login?).to be false
+      end
+    end
+
+    context 'with valid user login' do
+      include_examples 'check loginable'
+    end
+
+    context 'with to many failed logins' do
+      let(:user) { create(:user, login_failed: 999) }
+
+      include_examples 'check not loginable'
+    end
+
+    context 'with not active user' do
+      let(:user) { create(:user, active: false) }
+
+      include_examples 'check not loginable'
+    end
+
+    context 'with invalid instance username parameter' do
+      let(:instance) { described_class.new('not_existing') }
+
+      include_examples 'check not loginable'
+    end
+
+    context 'with empty instance username parameter' do
+      let(:instance) { described_class.new('') }
+
+      include_examples 'check not loginable'
+    end
+
+    context 'with given default password_max_login_failed' do
+      context 'with 5 attempts' do
+        let(:user) { create(:user, login_failed: 5) }
+
+        include_examples 'check loginable'
+      end
+
+      context 'with 6 attempts' do
+        let(:user) { create(:user, login_failed: 6) }
+
+        include_examples 'check not loginable'
+      end
+    end
+
+    context 'when "password_max_login_failed" Setting is changed' do
+
+      context 'when changed to lower value' do
+        before do
+          Setting.set('password_max_login_failed', 5)
+          user.update(login_failed: 6)
+        end
+
+        include_examples 'check not loginable'
+      end
+
+      context 'when changed to nil' do
+        before do
+          Setting.set('password_max_login_failed', nil)
+        end
+
+        include_examples 'check loginable'
+
+        context 'when User login failed once' do
+          before do
+            user.update(login_failed: 1)
+          end
+
+          include_examples 'check not loginable'
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/auth_spec.rb b/spec/lib/auth_spec.rb
index 489a46b1d..7bedbcefb 100644
--- a/spec/lib/auth_spec.rb
+++ b/spec/lib/auth_spec.rb
@@ -3,75 +3,242 @@
 require 'rails_helper'
 
 RSpec.describe Auth do
-
-  describe '.can_login?' do
-    it 'responds to can_login?' do
-      expect(described_class).to respond_to(:can_login?)
-    end
-
-    it 'checks if users can login' do
-      user   = create(:user)
-      result = described_class.can_login?(user)
-      expect(result).to be true
-    end
-
-    context 'not loginable' do
-
-      it 'fails if user has too many failed logins' do
-        user   = create(:user, login_failed: 999)
-        result = described_class.can_login?(user)
-        expect(result).to be false
-      end
-
-      it "fails if user isn't active" do
-        user   = create(:user, active: false)
-        result = described_class.can_login?(user)
-        expect(result).to be false
-      end
-
-      it 'fails if parameter is no User instance' do
-        result = described_class.can_login?('user')
-        expect(result).to be false
-      end
-    end
-
-    context 'given default password_max_login_failed' do
-      it 'passes with 5 attempts' do
-        user   = create(:user, login_failed: 5)
-        result = described_class.can_login?(user)
-        expect(result).to be true
-      end
-
-      it 'fails with 6 attempts' do
-        user   = create(:user, login_failed: 6)
-        result = described_class.can_login?(user)
-        expect(result).to be false
-      end
-    end
-  end
+  let(:password) { 'zammad' }
+  let(:user) { create(:user, password: password) }
+  let(:instance) { described_class.new(user.login, password) }
 
   describe '.valid?' do
     it 'responds to valid?' do
-      expect(described_class).to respond_to(:valid?)
+      expect(instance).to respond_to(:valid?)
     end
 
-    it 'authenticates users' do
-      password = 'zammad'
-      user     = create(:user, password: password)
-      result   = described_class.valid?(user, password)
-      expect(result).to be true
-    end
-  end
+    context 'with an internal user' do
+      context 'with valid credentials' do
+        it 'check for valid credentials' do
+          expect(instance.valid?).to be true
+        end
 
-  describe '.backends' do
-    it 'responds to backends' do
-      expect(described_class).to respond_to(:backends)
+        it 'check for not increased failed login count' do
+          expect { instance.valid? }.not_to change { user.reload.login_failed }
+        end
+
+        context 'when not case-sensitive' do
+          let(:instance) { described_class.new(user.login.upcase, password) }
+
+          it 'returns true' do
+            expect(instance.valid?).to be true
+          end
+        end
+
+        context 'when email is used' do
+          let(:instance) { described_class.new(user.email, password) }
+
+          it 'check for valid credentials' do
+            expect(instance.valid?).to be true
+          end
+        end
+
+        context 'when previous login was' do
+          context 'when never logged in' do
+            it 'updates #last_login and #updated_at' do
+              expect { instance.valid? }.to change { user.reload.last_login }.and change { user.reload.updated_at }
+            end
+          end
+
+          context 'when less than 10 minutes ago' do
+            before do
+              instance.valid?
+              travel 9.minutes
+            end
+
+            it 'does not update #last_login and #updated_at' do
+              expect { instance.valid? }.to not_change { user.reload.last_login }.and not_change { user.reload.updated_at }
+            end
+          end
+
+          context 'when more than 10 minutes ago' do
+            before do
+              instance.valid?
+              travel 11.minutes
+            end
+
+            it 'updates #last_login and #updated_at' do
+              expect { instance.valid? }.to change { user.reload.last_login }.and change { user.reload.updated_at }
+            end
+          end
+        end
+      end
+
+      context 'with valid user and invalid password' do
+        let(:instance) { described_class.new(user.login, 'wrong') }
+
+        it 'check for invalid credentials' do
+          expect(instance.valid?).to be false
+        end
+
+        it 'check for increased failed login count' do
+          expect { instance.valid? }.to change { user.reload.login_failed }.from(0).to(1)
+        end
+
+        it 'failed login avoids brute force attack' do
+          allow(instance).to receive(:sleep)
+          instance.valid?
+          expect(instance).to have_received(:sleep).with(1)
+        end
+      end
+
+      context 'with inactive user login' do
+        let(:user) { create(:user, active: false) }
+
+        it 'returns false' do
+          expect(instance.valid?).to be false
+        end
+      end
+
+      context 'with non-existent user login' do
+        let(:instance) { described_class.new('not_existing', password) }
+
+        it 'returns false' do
+          expect(instance.valid?).to be false
+        end
+      end
+
+      context 'with empty user login' do
+        let(:instance) { described_class.new('', password) }
+
+        it 'returns false' do
+          expect(instance.valid?).to be false
+        end
+      end
+
+      context 'when password is empty' do
+        before do
+          # Remove adapter from auth developer setting, to avoid execution for this test case, because of special empty
+          # password handling in adapter.
+          Setting.set('auth_developer', {})
+        end
+
+        context 'with empty password string' do
+          let(:password) { '' }
+
+          it 'returns false' do
+            expect(instance.valid?).to be false
+          end
+        end
+
+        shared_examples 'check empty password' do
+          context 'when password is an empty string' do
+            let(:password) { '' }
+
+            it 'returns false' do
+              expect(instance.valid?).to be false
+            end
+          end
+
+          context 'when password is nil' do
+            let(:password) { nil }
+
+            it 'returns false' do
+              expect(instance.valid?).to be false
+            end
+          end
+        end
+
+        context 'with empty password string when the stored password is an empty string' do
+          before { user.update_column(:password, '') }
+
+          include_examples 'check empty password'
+        end
+
+        context 'with empty password string when the stored hash represents an empty string' do
+          before { user.update(password: PasswordHash.crypt('')) }
+
+          include_examples 'check empty password'
+        end
+      end
     end
 
-    it 'returns a list of Hashes' do
-      result = described_class.backends
-      expect(result).to be_an(Array)
-      expect(result.first).to be_a(Hash)
+    context 'with a ldap user' do
+      let(:password_ldap) { 'zammad_ldap' }
+      let(:ldap_user) { instance_double(Ldap::User) }
+
+      before do
+        Setting.set('ldap_integration', true)
+
+        allow(Ldap::User).to receive(:new).with(any_args).and_return(ldap_user)
+      end
+
+      context 'with a ldap user without internal password' do
+        let(:user) { create(:user, source: 'Ldap') }
+        let(:password) { password_ldap }
+
+        context 'with valid credentials' do
+          before do
+            allow(ldap_user).to receive(:valid?).with(any_args).and_return(true)
+          end
+
+          it 'returns true' do
+            expect(instance.valid?).to be true
+          end
+        end
+
+        context 'with invalid credentials' do
+          let(:password) { 'wrong' }
+
+          before do
+            allow(ldap_user).to receive(:valid?).with(any_args).and_return(false)
+          end
+
+          it 'returns false' do
+            expect(instance.valid?).to be false
+          end
+
+          it 'check for not increased failed login count' do
+            expect { instance.valid? }.not_to change { user.reload.login_failed }
+          end
+        end
+      end
+
+      context 'with a ldap user which also has a internal password' do
+        let(:user) { create(:user, source: 'Ldap', password: password) }
+        let(:password) { password_ldap }
+
+        context 'with valid ldap credentials' do
+          before do
+            allow(ldap_user).to receive(:valid?).with(any_args).and_return(true)
+          end
+
+          it 'returns true' do
+            expect(instance.valid?).to be true
+          end
+        end
+
+        context 'with invalid ldap credentials' do
+          let(:instance) { described_class.new(user.login, 'wrong') }
+
+          before do
+            allow(ldap_user).to receive(:valid?).with(any_args).and_return(false)
+          end
+
+          it 'returns false' do
+            expect(instance.valid?).to be false
+          end
+
+          it 'check for not increased failed login count' do
+            expect { instance.valid? }.to change { user.reload.login_failed }.from(0).to(1)
+          end
+        end
+
+        context 'with valid internal credentials' do
+          before do
+            allow(ldap_user).to receive(:valid?).with(any_args).and_return(false)
+          end
+
+          it 'returns true' do
+            expect(instance.valid?).to be true
+          end
+        end
+      end
     end
   end
 end
diff --git a/spec/lib/auto_wizard_spec.rb b/spec/lib/auto_wizard_spec.rb
index 131140eb6..02b0c13d0 100644
--- a/spec/lib/auto_wizard_spec.rb
+++ b/spec/lib/auto_wizard_spec.rb
@@ -63,7 +63,7 @@ RSpec.describe AutoWizard do
           .and change { User.last.firstname }.to('Test Master')
           .and change { User.last.lastname }.to('Agent')
           .and change { User.last.email }.to('master_unit_test01@example.com')
-          .and change { User.authenticate(User.last.email, 'test') }.from(nil)
+          .and change { Auth.new(User.last.email, 'test').valid? }.from(false)
       end
     end
 
@@ -90,7 +90,7 @@ RSpec.describe AutoWizard do
           .and change { User.last.firstname }.to('Test Master')
           .and change { User.last.lastname }.to('Agent')
           .and change { User.last.email }.to('master_unit_test01@example.com')
-          .and change { User.authenticate(User.last.email, 'test') }.from(nil)
+          .and change { Auth.new(User.last.email, 'test').valid? }.from(false)
       end
     end
 
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index 7cd984aeb..77666cd5f 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -35,154 +35,6 @@ RSpec.describe User, type: :model do
   it_behaves_like 'UserPerformsGeoLookup'
 
   describe 'Class methods:' do
-    describe '.authenticate' do
-      subject(:user) { create(:user, password: password) }
-
-      let(:password) { Faker::Internet.password }
-
-      context 'with valid credentials' do
-        context 'using #login' do
-          it 'returns the matching user' do
-            expect(described_class.authenticate(user.login, password))
-              .to eq(user)
-          end
-
-          it 'is not case-sensitive' do
-            expect(described_class.authenticate(user.login.upcase, password))
-              .to eq(user)
-          end
-        end
-
-        context 'using #email' do
-          it 'returns the matching user' do
-            expect(described_class.authenticate(user.email, password))
-              .to eq(user)
-          end
-
-          it 'is not case-sensitive' do
-            expect(described_class.authenticate(user.email.upcase, password))
-              .to eq(user)
-          end
-        end
-
-        context 'but exceeding failed login limit' do
-          before { user.update(login_failed: 999) }
-
-          it 'returns nil' do
-            expect(described_class.authenticate(user.login, password))
-              .to be(nil)
-          end
-        end
-
-        context 'when previous login was' do
-          context 'never' do
-            it 'updates #last_login and #updated_at' do
-              expect { described_class.authenticate(user.login, password) }
-                .to change { user.reload.last_login }
-                .and change { user.reload.updated_at }
-            end
-          end
-
-          context 'less than 10 minutes ago' do
-            before do
-              described_class.authenticate(user.login, password)
-              travel 9.minutes
-            end
-
-            it 'does not update #last_login and #updated_at' do
-              expect { described_class.authenticate(user.login, password) }
-                .to not_change { user.reload.last_login }
-                .and not_change { user.reload.updated_at }
-            end
-          end
-
-          context 'more than 10 minutes ago' do
-            before do
-              described_class.authenticate(user.login, password)
-              travel 11.minutes
-            end
-
-            it 'updates #last_login and #updated_at' do
-              expect { described_class.authenticate(user.login, password) }
-                .to change { user.reload.last_login }
-                .and change { user.reload.updated_at }
-            end
-          end
-        end
-      end
-
-      context 'with valid user and invalid password' do
-        it 'increments failed login count' do
-          expect(described_class).to receive(:sleep).with(1)
-          expect { described_class.authenticate(user.login, password.next) }
-            .to change { user.reload.login_failed }.by(1)
-        end
-
-        it 'returns nil' do
-          expect(described_class).to receive(:sleep).with(1)
-          expect(described_class.authenticate(user.login, password.next)).to be(nil)
-        end
-      end
-
-      context 'with inactive user’s login' do
-        before { user.update(active: false) }
-
-        it 'returns nil' do
-          expect(described_class.authenticate(user.login, password)).to be(nil)
-        end
-      end
-
-      context 'with non-existent user login' do
-        it 'returns nil' do
-          expect(described_class.authenticate('john.doe', password)).to be(nil)
-        end
-      end
-
-      context 'with empty login string' do
-        it 'returns nil' do
-          expect(described_class.authenticate('', password)).to be(nil)
-        end
-      end
-
-      context 'with empty password string' do
-        it 'returns nil' do
-          expect(described_class.authenticate(user.login, '')).to be(nil)
-        end
-      end
-
-      context 'with empty password string when the stored password is an empty string' do
-        before { user.update_column(:password, '') }
-
-        context 'when password is an empty string' do
-          it 'returns nil' do
-            expect(described_class.authenticate(user.login, '')).to be(nil)
-          end
-        end
-
-        context 'when password is nil' do
-          it 'returns nil' do
-            expect(described_class.authenticate(user.login, nil)).to be(nil)
-          end
-        end
-      end
-
-      context 'with empty password string when the stored hash represents an empty string' do
-        before { user.update(password: PasswordHash.crypt('')) }
-
-        context 'when password is an empty string' do
-          it 'returns nil' do
-            expect(described_class.authenticate(user.login, '')).to be(nil)
-          end
-        end
-
-        context 'when password is nil' do
-          it 'returns nil' do
-            expect(described_class.authenticate(user.login, nil)).to be(nil)
-          end
-        end
-      end
-    end
-
     describe '.identify' do
       it 'returns users by given login' do
         expect(described_class.identify(user.login)).to eq(user)
@@ -191,37 +43,14 @@ RSpec.describe User, type: :model do
       it 'returns users by given email' do
         expect(described_class.identify(user.email)).to eq(user)
       end
+
+      it 'returns nil for empty username' do
+        expect(described_class.identify('')).to eq(nil)
+      end
     end
   end
 
   describe 'Instance methods:' do
-    describe '#max_login_failed?' do
-      it { is_expected.to respond_to(:max_login_failed?) }
-
-      context 'with "password_max_login_failed" setting' do
-        before do
-          Setting.set('password_max_login_failed', 5)
-          user.update(login_failed: 5)
-        end
-
-        it 'returns true once user’s #login_failed count exceeds the setting' do
-          expect { user.update(login_failed: 6) }
-            .to change(user, :max_login_failed?).to(true)
-        end
-      end
-
-      context 'without password_max_login_failed setting' do
-        before do
-          Setting.set('password_max_login_failed', nil)
-          user.update(login_failed: 0)
-        end
-
-        it 'defaults to 0' do
-          expect { user.update(login_failed: 1) }
-            .to change(user, :max_login_failed?).to(true)
-        end
-      end
-    end
 
     describe '#out_of_office?' do
       context 'without any out_of_office_* attributes set' do
diff --git a/spec/support/request.rb b/spec/support/request.rb
index 9783a9fcc..7b04c4459 100644
--- a/spec/support/request.rb
+++ b/spec/support/request.rb
@@ -70,12 +70,6 @@ module ZammadSpecSupportRequest
     password = options[:password] || user.password.to_s
     login    = options[:login] || user.login
 
-    # mock authentication otherwise login won't
-    # if user has no password (which is expensive to create)
-    if password.blank?
-      allow(User).to receive(:authenticate).with(login, '') { user.update_last_login }.and_return(user)
-    end
-
     case via
     when :api_client
       # ensure that always the correct header value is set