Merge branch 'fix/2fa-authentication-spoofing' into 'master'

Fix 2FA authentication spoofing

## Summary

This is security fix for vulnerability described at 
https://gitlab.com/gitlab-org/gitlab-ce/issues/14900.

Attacker was able to bypass password authentication of users that have 2FA enabled, and consequently sign is as a different user, without knowing his password, if he managed to guess 2FA One Time Password for that user.

It was also possible to enumerate users and check if they have 2FA enabled, because GitLab responded with different error for each case.

## Fix

This MR attempts to change default user search scope if `otp_user_id` session variable has been set. If it is present, it means that user has 2FA enabled, and has already been verified with login and password. In this case we should look for user with `otp_user_id` first, before picking it up by `login`.

Both, 2FA authentication spoofing and 2FA discovery have been covered by specs.

## Further work

Current 2FA code is a bit tricky, so it probably needs some refactoring.



See merge request !1947

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

spec/controllers/sessions_controller_spec.rb

+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