diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 947a8e865..c5b173773 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -5,7 +5,7 @@ class ApplicationController < ActionController::Base
   include ApplicationController::HandlesTransitions
   include ApplicationController::Authenticates
   include ApplicationController::SetsHeaders
-  include ApplicationController::ChecksMaintainance
+  include ApplicationController::ChecksMaintenance
   include ApplicationController::RendersModels
   include ApplicationController::HasUser
   include ApplicationController::HasResponseExtentions
diff --git a/app/controllers/application_controller/authenticates.rb b/app/controllers/application_controller/authenticates.rb
index 6df649d19..ef3ab3eff 100644
--- a/app/controllers/application_controller/authenticates.rb
+++ b/app/controllers/application_controller/authenticates.rb
@@ -50,15 +50,6 @@ module ApplicationController::Authenticates
       return authentication_check_prerequesits(user, 'session', auth_param) if user
     end
 
-    # check sso based authentication
-    sso_user = User.sso(params)
-    if sso_user
-      if authentication_check_prerequesits(sso_user, 'session', auth_param)
-        session[:persistent] = true
-        return sso_user
-      end
-    end
-
     # check http basic based authentication
     authenticate_with_http_basic do |username, password|
       request.session_options[:skip] = true # do not send a session cookie
@@ -135,21 +126,37 @@ module ApplicationController::Authenticates
     false
   end
 
+  def authenticate_with_password
+    user = User.authenticate(params[:username], params[:password])
+    raise Exceptions::NotAuthorized, 'Wrong Username or Password combination.' if !user
+
+    session.delete(:switched_from_user_id)
+    authentication_check_prerequesits(user, 'session', {})
+  end
+
+  def authenticate_with_sso
+    user = begin
+              login = request.env['REMOTE_USER'] ||
+                      request.env['HTTP_REMOTE_USER'] ||
+                      request.headers['X-Forwarded-User']
+
+              User.lookup(login: login&.downcase)
+            end
+
+    raise Exceptions::NotAuthorized, 'no valid session' if !user
+
+    session.delete(:switched_from_user_id)
+    authentication_check_prerequesits(user, 'session', {})
+  end
+
   def authentication_check_prerequesits(user, auth_type, auth_param)
-    if check_maintenance_only(user)
-      raise Exceptions::NotAuthorized, 'Maintenance mode enabled!'
-    end
-
+    raise Exceptions::NotAuthorized, 'Maintenance mode enabled!' if in_maintenance_mode?(user)
     raise Exceptions::NotAuthorized, 'User is inactive!' if !user.active
-
-    # check scopes / permission check
-    if auth_param[:permission] && !user.permissions?(auth_param[:permission])
-      raise Exceptions::NotAuthorized, 'Not authorized (user)!'
-    end
+    raise Exceptions::NotAuthorized, 'Not authorized (user)!' if auth_param[:permission] && !user.permissions?(auth_param[:permission])
 
     current_user_set(user, auth_type)
     user_device_log(user, auth_type)
     logger.debug { "#{auth_type} for '#{user.login}'" }
-    true
+    user
   end
 end
diff --git a/app/controllers/application_controller/checks_maintainance.rb b/app/controllers/application_controller/checks_maintenance.rb
similarity index 52%
rename from app/controllers/application_controller/checks_maintainance.rb
rename to app/controllers/application_controller/checks_maintenance.rb
index 7d1ef0ae5..3cdaeaab4 100644
--- a/app/controllers/application_controller/checks_maintainance.rb
+++ b/app/controllers/application_controller/checks_maintenance.rb
@@ -1,16 +1,9 @@
-module ApplicationController::ChecksMaintainance
+module ApplicationController::ChecksMaintenance
   extend ActiveSupport::Concern
 
   private
 
-  def check_maintenance(user)
-    return false if !check_maintenance_only(user)
-
-    raise Exceptions::NotAuthorized, 'Maintenance mode enabled!'
-  end
-
-  # check maintenance mode
-  def check_maintenance_only(user)
+  def in_maintenance_mode?(user)
     return false if Setting.get('maintenance_mode') != true
     return false if user.permissions?('admin.maintenance')
 
diff --git a/app/controllers/sessions_controller.rb b/app/controllers/sessions_controller.rb
index 8376799bc..d242a4646 100644
--- a/app/controllers/sessions_controller.rb
+++ b/app/controllers/sessions_controller.rb
@@ -2,112 +2,32 @@
 
 class SessionsController < ApplicationController
   prepend_before_action :authentication_check, only: %i[switch_to_user list delete]
-  skip_before_action :verify_csrf_token, only: %i[show destroy create_omniauth failure_omniauth create_sso]
+  skip_before_action :verify_csrf_token, only: %i[show destroy create_omniauth failure_omniauth]
 
   # "Create" a login, aka "log the user in"
   def create
-
-    # in case, remove switched_from_user_id
-    session[:switched_from_user_id] = nil
-
-    # authenticate user
-    user = User.authenticate(params[:username], params[:password])
-
-    # check maintenance mode
-    check_maintenance(user)
-
-    # auth failed
-    raise Exceptions::NotAuthorized, 'Wrong Username or Password combination.' if !user
-
-    # remember me - set session cookie to expire later
-    expire_after = nil
-    if params[:remember_me]
-      expire_after = 1.year
-    end
-    request.env['rack.session.options'][:expire_after] = expire_after
-
-    # set session user
-    current_user_set(user)
-
-    # log device
-    return if !user_device_log(user, 'session')
-
-    # log new session
-    user.activity_stream_log('session started', user.id, true)
-
-    # add session user assets
-    assets = {}
-    assets = user.assets(assets)
-
-    # auto population of default collections
-    collections, assets = SessionHelper.default_collections(user, assets)
-
-    # get models
-    models = SessionHelper.models(user)
-
-    # sessions created via this
-    # controller are persistent
-    session[:persistent] = true
+    user = authenticate_with_password
+    initiate_session_for(user)
 
     # return new session data
-    render  status: :created,
-            json:   {
-              session:     user,
-              config:      config_frontend,
-              models:      models,
-              collections: collections,
-              assets:      assets,
-            }
+    render status: :created,
+           json:   SessionHelper.json_hash(user).merge(config: config_frontend)
   end
 
   def show
-
-    user_id = nil
-
-    # no valid sessions
-    if session[:user_id]
-      user_id = session[:user_id]
-    end
-
-    if !user_id || !User.exists?(user_id)
-      # get models
-      models = SessionHelper.models()
-
-      render json: {
-        error:       'no valid session',
-        config:      config_frontend,
-        models:      models,
-        collections: {
-          Locale.to_app_model => Locale.where(active: true)
-        },
-      }
-      return
-    end
-
-    # Save the user ID in the session so it can be used in
-    # subsequent requests
-    user = User.find(user_id)
-
-    # log device
-    return if !user_device_log(user, 'session')
-
-    # add session user assets
-    assets = {}
-    assets = user.assets(assets)
-
-    # auto population of default collections
-    collections, assets = SessionHelper.default_collections(user, assets)
-
-    # get models
-    models = SessionHelper.models(user)
+    user = authentication_check_only || authenticate_with_sso
+    initiate_session_for(user)
 
     # return current session
+    render json: SessionHelper.json_hash(user).merge(config: config_frontend)
+  rescue Exceptions::NotAuthorized => e
+    raise if e.message != 'no valid session'
+
     render json: {
-      session:     user,
+      error:       e.message,
       config:      config_frontend,
-      models:      models,
-      collections: collections,
-      assets:      assets,
+      models:      SessionHelper.models,
+      collections: { Locale.to_app_model => Locale.where(active: true) }
     }
   end
 
@@ -146,8 +66,7 @@ class SessionsController < ApplicationController
       authorization = Authorization.create_from_hash(auth, current_user)
     end
 
-    # check maintenance mode
-    if check_maintenance_only(authorization.user)
+    if in_maintenance_mode?(authorization.user)
       redirect_to '/#'
       return
     end
@@ -169,36 +88,6 @@ class SessionsController < ApplicationController
     raise Exceptions::UnprocessableEntity, "Message from #{params[:strategy]}: #{params[:message]}"
   end
 
-  def create_sso
-
-    # in case, remove switched_from_user_id
-    session[:switched_from_user_id] = nil
-
-    user = User.sso(params)
-
-    # Log the authorizing user in.
-    if user
-
-      # check maintenance mode
-      if check_maintenance_only(user)
-        redirect_to '/#'
-        return
-      end
-
-      # set current session user
-      current_user_set(user)
-
-      # log new session
-      user.activity_stream_log('session started', user.id, true)
-
-      # remember last login date
-      user.update_last_login
-    end
-
-    # redirect to app
-    redirect_to '/#'
-  end
-
   # "switch" to user
   def switch_to_user
     permission_check(['admin.session', 'admin.user'])
@@ -308,6 +197,12 @@ class SessionsController < ApplicationController
 
   private
 
+  def initiate_session_for(user)
+    request.env['rack.session.options'][:expire_after] = 1.year if params[:remember_me]
+    session[:persistent] = true
+    user.activity_stream_log('session started', user.id, true)
+  end
+
   def config_frontend
 
     # config
@@ -333,4 +228,5 @@ class SessionsController < ApplicationController
 
     config
   end
+
 end
diff --git a/app/models/user.rb b/app/models/user.rb
index 36481bcd1..d8a550efa 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -333,27 +333,6 @@ returns
 
 =begin
 
-authenticate user again sso
-
-  result = User.sso(sso_params)
-
-returns
-
-  result = user_model # user model if authentication was successfully
-
-=end
-
-  def self.sso(params)
-
-    # try to login against configure auth backends
-    user_auth = Sso.check(params)
-    return if !user_auth
-
-    user_auth
-  end
-
-=begin
-
 create user from from omni auth hash
 
   result = User.create_from_hash!(hash)
diff --git a/config/routes/auth.rb b/config/routes/auth.rb
index 92aa9fdd5..701bc6c75 100644
--- a/config/routes/auth.rb
+++ b/config/routes/auth.rb
@@ -5,9 +5,6 @@ Zammad::Application.routes.draw do
   match '/auth/:provider/callback',         to: 'sessions#create_omniauth',      via: %i[post get puts delete]
   match '/auth/failure',                    to: 'sessions#failure_omniauth',     via: %i[post get]
 
-  # sso
-  match '/auth/sso',                        to: 'sessions#create_sso',           via: %i[post get]
-
   # sessions
   match api_path + '/signin',               to: 'sessions#create',               via: :post
   match api_path + '/signshow',             to: 'sessions#show',                 via: %i[get post]
diff --git a/lib/session_helper.rb b/lib/session_helper.rb
index d459e3426..ce1e41774 100644
--- a/lib/session_helper.rb
+++ b/lib/session_helper.rb
@@ -1,8 +1,20 @@
 module SessionHelper
-  def self.default_collections(user, assets = {})
+  def self.json_hash(user)
+    collections, assets = default_collections(user)
+
+    {
+      session:     user,
+      models:      models(user),
+      collections: collections,
+      assets:      assets,
+    }
+  end
+
+  def self.default_collections(user)
 
     # auto population collections, store all here
     default_collection = {}
+    assets = user.assets({})
 
     # load collections to deliver from external files
     dir = File.expand_path('..', __dir__)
diff --git a/lib/sso.rb b/lib/sso.rb
deleted file mode 100644
index 6acae6579..000000000
--- a/lib/sso.rb
+++ /dev/null
@@ -1,53 +0,0 @@
-# Copyright (C) 2012-2016 Zammad Foundation, http://zammad-foundation.org/
-
-class Sso
-  include ApplicationLib
-
-=begin
-
-authenticate user via username and password
-
-  result = Sso.check( params )
-
-returns
-
-  result = user_model # if authentication was successfully
-
-=end
-
-  def self.check(params)
-
-    # use std. auth backends
-    config = [
-      {
-        adapter: 'Sso::Env',
-      },
-    ]
-
-    # added configured backends
-    Setting.where( area: 'Security::SSO' ).each do |setting|
-      if setting.state_current[:value]
-        config.push setting.state_current[:value]
-      end
-    end
-
-    # try to login against configure auth backends
-    user_auth = nil
-    config.each do |config_item|
-      next if !config_item[:adapter]
-
-      user_auth = config_item[:adapter].constantize.check( params, config_item )
-
-      # auth not ok
-      next if !user_auth
-
-      Rails.logger.info "Authentication against #{config_item[:adapter]} for user #{user_auth.login} ok."
-
-      # remember last login date
-      user_auth.update_last_login
-
-      return user_auth
-    end
-    nil
-  end
-end
diff --git a/lib/sso/env.rb b/lib/sso/env.rb
deleted file mode 100644
index 974891041..000000000
--- a/lib/sso/env.rb
+++ /dev/null
@@ -1,19 +0,0 @@
-# Copyright (C) 2012-2016 Zammad Foundation, http://zammad-foundation.org/
-
-module Sso::Env
-  def self.check( _params, _config_item )
-
-    # try to find user based on login
-    if ENV['REMOTE_USER']
-      user = User.where( login: ENV['REMOTE_USER'], active: true ).first
-      return user if user
-    end
-
-    if ENV['HTTP_REMOTE_USER']
-      user = User.where( login: ENV['HTTP_REMOTE_USER'], active: true ).first
-      return user if user
-    end
-
-    false
-  end
-end
diff --git a/spec/requests/session_spec.rb b/spec/requests/session_spec.rb
new file mode 100644
index 000000000..31edec401
--- /dev/null
+++ b/spec/requests/session_spec.rb
@@ -0,0 +1,178 @@
+require 'rails_helper'
+
+RSpec.describe 'Sessions endpoints', type: :request do
+  # The frontend sends a device fingerprint in the request parameters during authentication
+  # (as part of App.Auth.loginCheck() and App.WebSocket.auth()).
+  #
+  # Without this parameter, the controller will raise a 422 Unprocessable Entity error
+  # (in ApplicationController::HandlesDevices#user_device_log).
+  let(:fingerprint) { { fingerprint: 'foo' } }
+
+  describe 'GET /api/v1/signshow (single sign-on)' do
+    context 'with invalid user login' do
+      let(:login) { User.pluck(:login).max.next }
+
+      context 'in "REMOTE_USER" request env var' do
+        let(:env) { { 'REMOTE_USER' => login } }
+
+        it 'returns invalid session response' do
+          get '/api/v1/signshow', as: :json, env: env, params: fingerprint
+
+          expect(response).to have_http_status(:ok)
+          expect(json_response)
+            .to include('error' => 'no valid session')
+            .and not_include('session')
+        end
+      end
+
+      context 'in "HTTP_REMOTE_USER" request env var' do
+        let(:env) { { 'HTTP_REMOTE_USER' => login } }
+
+        it 'returns invalid session response' do
+          get '/api/v1/signshow', as: :json, env: env, params: fingerprint
+
+          expect(response).to have_http_status(:ok)
+          expect(json_response)
+            .to include('error' => 'no valid session')
+            .and not_include('session')
+        end
+      end
+
+      context 'in "X-Forwarded-User" request header' do
+        let(:headers) { { 'X-Forwarded-User' => login } }
+
+        it 'returns invalid session response' do
+          get '/api/v1/signshow', as: :json, headers: headers, params: fingerprint
+
+          expect(response).to have_http_status(:ok)
+          expect(json_response)
+            .to include('error' => 'no valid session')
+            .and not_include('session')
+        end
+      end
+    end
+
+    context 'with valid user login' do
+      let(:user) { User.last }
+      let(:login) { user.login }
+
+      context 'in Maintenance Mode' do
+        before { Setting.set('maintenance_mode', true) }
+
+        context 'in "REMOTE_USER" request env var' do
+          let(:env) { { 'REMOTE_USER' => login } }
+
+          it 'returns 401 unauthorized' do
+            get '/api/v1/signshow', as: :json, env: env, params: fingerprint
+
+            expect(response).to have_http_status(:unauthorized)
+            expect(json_response).to include('error' => 'Maintenance mode enabled!')
+          end
+        end
+
+        context 'in "HTTP_REMOTE_USER" request env var' do
+          let(:env) { { 'HTTP_REMOTE_USER' => login } }
+
+          it 'returns 401 unauthorized' do
+            get '/api/v1/signshow', as: :json, env: env, params: fingerprint
+
+            expect(response).to have_http_status(:unauthorized)
+            expect(json_response).to include('error' => 'Maintenance mode enabled!')
+          end
+        end
+
+        context 'in "X-Forwarded-User" request header' do
+          let(:headers) { { 'X-Forwarded-User' => login } }
+
+          it 'returns 401 unauthorized' do
+            get '/api/v1/signshow', as: :json, headers: headers, params: fingerprint
+
+            expect(response).to have_http_status(:unauthorized)
+            expect(json_response).to include('error' => 'Maintenance mode enabled!')
+          end
+        end
+      end
+
+      context 'in "REMOTE_USER" request env var' do
+        let(:env) { { 'REMOTE_USER' => login } }
+
+        it 'returns a new user-session response' do
+          get '/api/v1/signshow', as: :json, env: env, params: fingerprint
+
+          expect(json_response)
+            .to include('session' => hash_including('login' => login))
+            .and not_include('error')
+        end
+
+        it 'sets the :user_id session parameter' do
+          expect { get '/api/v1/signshow', as: :json, env: env, params: fingerprint }
+            .to change { request&.session&.fetch(:user_id) }.to(user.id)
+        end
+
+        it 'sets the :persistent session parameter' do
+          expect { get '/api/v1/signshow', as: :json, env: env, params: fingerprint }
+            .to change { request&.session&.fetch(:persistent) }.to(true)
+        end
+
+        it 'adds an activity stream entry for the user’s session' do
+          expect { get '/api/v1/signshow', as: :json, env: env, params: fingerprint }
+            .to change(ActivityStream, :count).by(1)
+        end
+      end
+
+      context 'in "HTTP_REMOTE_USER" request env var' do
+        let(:env) { { 'HTTP_REMOTE_USER' => login } }
+
+        it 'returns a new user-session response' do
+          get '/api/v1/signshow', as: :json, env: env, params: fingerprint
+
+          expect(json_response)
+            .to include('session' => hash_including('login' => login))
+            .and not_include('error')
+        end
+
+        it 'sets the :user_id session parameter' do
+          expect { get '/api/v1/signshow', as: :json, env: env, params: fingerprint }
+            .to change { request&.session&.fetch(:user_id) }.to(user.id)
+        end
+
+        it 'sets the :persistent session parameter' do
+          expect { get '/api/v1/signshow', as: :json, env: env, params: fingerprint }
+            .to change { request&.session&.fetch(:persistent) }.to(true)
+        end
+
+        it 'adds an activity stream entry for the user’s session' do
+          expect { get '/api/v1/signshow', as: :json, env: env, params: fingerprint }
+            .to change(ActivityStream, :count).by(1)
+        end
+      end
+
+      context 'in "X-Forwarded-User" request header' do
+        let(:headers) { { 'X-Forwarded-User' => login } }
+
+        it 'returns a new user-session response' do
+          get '/api/v1/signshow', as: :json, headers: headers, params: fingerprint
+
+          expect(json_response)
+            .to include('session' => hash_including('login' => login))
+            .and not_include('error')
+        end
+
+        it 'sets the :user_id session parameter on the client' do
+          expect { get '/api/v1/signshow', as: :json, headers: headers, params: fingerprint }
+            .to change { request&.session&.fetch(:user_id) }.to(user.id)
+        end
+
+        it 'sets the :persistent session parameter' do
+          expect { get '/api/v1/signshow', as: :json, headers: headers, params: fingerprint }
+            .to change { request&.session&.fetch(:persistent) }.to(true)
+        end
+
+        it 'adds an activity stream entry for the user’s session' do
+          expect { get '/api/v1/signshow', as: :json, headers: headers, params: fingerprint }
+            .to change(ActivityStream, :count).by(1)
+        end
+      end
+    end
+  end
+end