diff --git a/app/controllers/sessions_controller.rb b/app/controllers/sessions_controller.rb
index 65677a3dd3c..c29f4609e93 100644
--- a/app/controllers/sessions_controller.rb
+++ b/app/controllers/sessions_controller.rb
@@ -5,7 +5,8 @@ class SessionsController < Devise::SessionsController
   skip_before_action :check_2fa_requirement, only: [:destroy]
 
   prepend_before_action :check_initial_setup, only: [:new]
-  prepend_before_action :authenticate_with_two_factor, only: [:create]
+  prepend_before_action :authenticate_with_two_factor,
+    if: :two_factor_enabled?, only: [:create]
   prepend_before_action :store_redirect_path, only: [:new]
 
   before_action :auto_sign_in_with_provider, only: [:new]
@@ -56,10 +57,10 @@ class SessionsController < Devise::SessionsController
   end
 
   def find_user
-    if user_params[:login]
-      User.by_login(user_params[:login])
-    elsif user_params[:otp_attempt] && session[:otp_user_id]
+    if session[:otp_user_id]
       User.find(session[:otp_user_id])
+    elsif user_params[:login]
+      User.by_login(user_params[:login])
     end
   end
 
@@ -83,11 +84,13 @@ class SessionsController < Devise::SessionsController
     end
   end
 
+  def two_factor_enabled?
+    find_user.try(:two_factor_enabled?)
+  end
+
   def authenticate_with_two_factor
     user = self.resource = find_user
 
-    return unless user && user.two_factor_enabled?
-
     if user_params[:otp_attempt].present? && session[:otp_user_id]
       if valid_otp_attempt?(user)
         # Remove any lingering user data from login
diff --git a/spec/controllers/sessions_controller_spec.rb b/spec/controllers/sessions_controller_spec.rb
new file mode 100644
index 00000000000..83cc8ec6d26
--- /dev/null
+++ b/spec/controllers/sessions_controller_spec.rb
@@ -0,0 +1,101 @@
+require 'spec_helper'
+
+describe SessionsController do
+  describe '#create' do
+    before do
+      @request.env['devise.mapping'] = Devise.mappings[:user]
+    end
+
+    context 'when using standard authentications' do
+      context 'invalid password' do
+        it 'does not authenticate user' do
+          post(:create, user: { login: 'invalid', password: 'invalid' })
+
+          expect(response)
+            .to set_flash.now[:alert].to /Invalid login or password/
+        end
+      end
+
+      context 'when using valid password' do
+        let(:user) { create(:user) }
+
+        it 'authenticates user correctly' do
+          post(:create, user: { login: user.username, password: user.password })
+
+          expect(response).to set_flash.to /Signed in successfully/
+          expect(subject.current_user). to eq user
+        end
+      end
+    end
+
+    context 'when using two-factor authentication' do
+      let(:user) { create(:user, :two_factor) }
+
+      def authenticate_2fa(user_params)
+        post(:create, { user: user_params }, { otp_user_id: user.id })
+      end
+
+      ##
+      # See #14900 issue
+      #
+      context 'when authenticating with login and OTP of another user' do
+        context 'when another user has 2FA enabled' do
+          let(:another_user) { create(:user, :two_factor) }
+
+          context 'when OTP is valid for another user' do
+            it 'does not authenticate' do
+              authenticate_2fa(login: another_user.username,
+                               otp_attempt: another_user.current_otp)
+
+              expect(subject.current_user).to_not eq another_user
+            end
+          end
+
+          context 'when OTP is invalid for another user' do
+            it 'does not authenticate' do
+              authenticate_2fa(login: another_user.username,
+                               otp_attempt: 'invalid')
+
+              expect(subject.current_user).to_not eq another_user
+            end
+          end
+
+          context 'when authenticating with OTP' do
+            context 'when OTP is valid' do
+              it 'authenticates correctly' do
+                authenticate_2fa(otp_attempt: user.current_otp)
+
+                expect(subject.current_user).to eq user
+              end
+            end
+
+            context 'when OTP is invalid' do
+              before { authenticate_2fa(otp_attempt: 'invalid') }
+
+              it 'does not authenticate' do
+                expect(subject.current_user).to_not eq user
+              end
+
+              it 'warns about invalid OTP code' do
+                expect(response).to set_flash.now[:alert]
+                  .to /Invalid two-factor code/
+              end
+            end
+          end
+
+          context 'when another user does not have 2FA enabled' do
+            let(:another_user) { create(:user) }
+
+            it 'does not leak that 2FA is disabled for another user' do
+              authenticate_2fa(login: another_user.username,
+                               otp_attempt: 'invalid')
+
+              expect(response).to set_flash.now[:alert]
+                .to /Invalid two-factor code/
+            end
+          end
+        end
+      end
+    end
+  end
+end