Validate that SAML requests are originated from gitlab

If the request wasn't initiated by gitlab we shouldn't add the new
identity to the user, and instead show that we weren't able to link
the identity to the user.

This should fix: https://gitlab.com/gitlab-org/gitlab-ce/issues/56509

app/controllers/omniauth_callbacks_controller.rb

@@ -40,6 +40,8 @@ class OmniauthCallbacksController < Devise::OmniauthCallbacksController
 
   def saml
     omniauth_flow(Gitlab::Auth::Saml)
+  rescue Gitlab::Auth::Saml::IdentityLinker::UnverifiedRequest
+    redirect_unverified_saml_initiation
   end
 
   def omniauth_error
@@ -92,8 +94,7 @@ class OmniauthCallbacksController < Devise::OmniauthCallbacksController
       return render_403 unless link_provider_allowed?(oauth['provider'])
 
       log_audit_event(current_user, with: oauth['provider'])
-
-      identity_linker ||= auth_module::IdentityLinker.new(current_user, oauth)
+      identity_linker ||= auth_module::IdentityLinker.new(current_user, oauth, session)
 
       link_identity(identity_linker)
 
@@ -194,6 +195,10 @@ class OmniauthCallbacksController < Devise::OmniauthCallbacksController
     redirect_to new_user_session_path
   end
 
+  def redirect_unverified_saml_initiation
+    redirect_to profile_account_path, notice: _('Request to link SAML account must be authorized')
+  end
+
   def handle_disabled_provider
     label = Gitlab::Auth::OAuth::Provider.label_for(oauth['provider'])
     flash[:alert] = _("Signing in using %{label} has been disabled") % { label: label }

changelogs/unreleased/security-sarcila-verify-saml-request-origin.yml

+---
+title: Prevent GitLab accounts takeover if SAML is configured
+merge_request:
+author:
+type: security

lib/gitlab/auth/omniauth_identity_linker_base.rb

@@ -3,12 +3,13 @@
 module Gitlab
   module Auth
     class OmniauthIdentityLinkerBase
-      attr_reader :current_user, :oauth
+      attr_reader :current_user, :oauth, :session
 
-      def initialize(current_user, oauth)
+      def initialize(current_user, oauth, session = {})
         @current_user = current_user
         @oauth = oauth
         @changed = false
+        @session = session
       end
 
       def link

lib/gitlab/auth/saml/identity_linker.rb

@@ -4,6 +4,30 @@ module Gitlab
   module Auth
     module Saml
       class IdentityLinker < OmniauthIdentityLinkerBase
+        extend ::Gitlab::Utils::Override
+
+        UnverifiedRequest = Class.new(StandardError)
+
+        override :link
+        def link
+          raise_unless_request_is_gitlab_initiated! if unlinked?
+
+          super
+        end
+
+        protected
+
+        def raise_unless_request_is_gitlab_initiated!
+          raise UnverifiedRequest unless valid_gitlab_initiated_request?
+        end
+
+        def valid_gitlab_initiated_request?
+          OriginValidator.new(session).gitlab_initiated?(saml_response)
+        end
+
+        def saml_response
+          oauth.fetch(:extra, {}).fetch(:response_object, {})
+        end
       end
     end
   end

lib/gitlab/auth/saml/origin_validator.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Auth
+    module Saml
+      class OriginValidator
+        AUTH_REQUEST_SESSION_KEY = "last_authn_request_id".freeze
+
+        def initialize(session)
+          @session = session || {}
+        end
+
+        def store_origin(authn_request)
+          session[AUTH_REQUEST_SESSION_KEY] = authn_request.uuid
+        end
+
+        def gitlab_initiated?(saml_response)
+          return false if identity_provider_initiated?(saml_response)
+
+          matches?(saml_response)
+        end
+
+        private
+
+        attr_reader :session
+
+        def matches?(saml_response)
+          saml_response.in_response_to == expected_request_id
+        end
+
+        def identity_provider_initiated?(saml_response)
+          saml_response.in_response_to.blank?
+        end
+
+        def expected_request_id
+          session[AUTH_REQUEST_SESSION_KEY]
+        end
+      end
+    end
+  end
+end

lib/omni_auth/strategies/saml.rb

+# frozen_string_literal: true
+
+module OmniAuth
+  module Strategies
+    class SAML
+      extend ::Gitlab::Utils::Override
+
+      # NOTE: This method duplicates code from omniauth-saml
+      #       so that we can access authn_request to store it
+      #       See: https://github.com/omniauth/omniauth-saml/issues/172
+      override :request_phase
+      def request_phase
+        authn_request = OneLogin::RubySaml::Authrequest.new
+
+        store_authn_request_id(authn_request)
+
+        with_settings do |settings|
+          redirect(authn_request.create(settings, additional_params_for_authn_request))
+        end
+      end
+
+      private
+
+      def store_authn_request_id(authn_request)
+        Gitlab::Auth::Saml::OriginValidator.new(session).store_origin(authn_request)
+      end
+    end
+  end
+end

locale/gitlab.pot

@@ -13242,6 +13242,9 @@ msgstr ""
 msgid "Request Access"
 msgstr ""
 
+msgid "Request to link SAML account must be authorized"
+msgstr ""
+
 msgid "Requested %{time_ago}"
 msgstr ""
 

spec/controllers/omniauth_callbacks_controller_spec.rb

@@ -247,16 +247,26 @@ describe OmniauthCallbacksController, type: :controller do
   end
 
   describe '#saml' do
+    let(:last_request_id) { 'ONELOGIN_4fee3b046395c4e751011e97f8900b5273d56685' }
     let(:user) { create(:omniauth_user, :two_factor, extern_uid: 'my-uid', provider: 'saml') }
     let(:mock_saml_response) { File.read('spec/fixtures/authentication/saml_response.xml') }
     let(:saml_config) { mock_saml_config_with_upstream_two_factor_authn_contexts }
 
+    def stub_last_request_id(id)
+      session['last_authn_request_id'] = id
+    end
+
     before do
+      stub_last_request_id(last_request_id)
       stub_omniauth_saml_config({ enabled: true, auto_link_saml_user: true, allow_single_sign_on: ['saml'],
                                   providers: [saml_config] })
       mock_auth_hash_with_saml_xml('saml', +'my-uid', user.email, mock_saml_response)
-      request.env["devise.mapping"] = Devise.mappings[:user]
+      request.env['devise.mapping'] = Devise.mappings[:user]
       request.env['omniauth.auth'] = Rails.application.env_config['omniauth.auth']
+    end
+
+    context 'with GitLab initiated request' do
+      before do
         post :saml, params: { SAMLResponse: mock_saml_response }
       end
 
@@ -278,4 +288,30 @@ describe OmniauthCallbacksController, type: :controller do
         end
       end
     end
+
+    context 'with IdP initiated request' do
+      let(:user) { create(:user) }
+      let(:last_request_id) { '99999' }
+
+      before do
+        sign_in user
+      end
+
+      it 'lets the user know their account isn\'t linked yet' do
+        post :saml, params: { SAMLResponse: mock_saml_response }
+
+        expect(flash[:notice]).to eq 'Request to link SAML account must be authorized'
+      end
+
+      it 'redirects to profile account page' do
+        post :saml, params: { SAMLResponse: mock_saml_response }
+
+        expect(response).to redirect_to(profile_account_path)
+      end
+
+      it 'doesn\'t link a new identity to the user' do
+        expect { post :saml, params: { SAMLResponse: mock_saml_response } }.not_to change { user.identities.count }
+      end
+    end
+  end
 end

spec/lib/gitlab/auth/saml/identity_linker_spec.rb

@@ -6,10 +6,17 @@ describe Gitlab::Auth::Saml::IdentityLinker do
   let(:user) { create(:user) }
   let(:provider) { 'saml' }
   let(:uid) { user.email }
-  let(:oauth) { { 'provider' => provider, 'uid' => uid } }
+  let(:in_response_to) { '12345' }
+  let(:saml_response) { instance_double(OneLogin::RubySaml::Response, in_response_to: in_response_to) }
+  let(:session) { { 'last_authn_request_id' => in_response_to } }
 
-  subject { described_class.new(user, oauth) }
+  let(:oauth) do
+    OmniAuth::AuthHash.new(provider: provider, uid: uid, extra: { response_object: saml_response })
+  end
+
+  subject { described_class.new(user, oauth, session) }
 
+  context 'with valid GitLab initiated request' do
     context 'linked identity exists' do
       let!(:identity) { user.identities.create!(provider: provider, extern_uid: uid) }
 
@@ -47,4 +54,13 @@ describe Gitlab::Auth::Saml::IdentityLinker do
         expect(subject).to be_changed
       end
     end
+  end
+
+  context 'with identity provider initiated request' do
+    let(:session) { { 'last_authn_request_id' => nil } }
+
+    it 'attempting to link accounts raises an exception' do
+      expect { subject.link }.to raise_error(Gitlab::Auth::Saml::IdentityLinker::UnverifiedRequest)
+    end
+  end
 end

spec/lib/gitlab/auth/saml/origin_validator_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Auth::Saml::OriginValidator do
+  let(:session) { instance_double(ActionDispatch::Request::Session) }
+
+  subject { described_class.new(session) }
+
+  describe '#store_origin' do
+    it 'stores the SAML request ID' do
+      request_id = double
+      authn_request = instance_double(OneLogin::RubySaml::Authrequest, uuid: request_id)
+
+      expect(session).to receive(:[]=).with('last_authn_request_id', request_id)
+
+      subject.store_origin(authn_request)
+    end
+  end
+
+  describe '#gitlab_initiated?' do
+    it 'returns false if InResponseTo is not present' do
+      saml_response = instance_double(OneLogin::RubySaml::Response, in_response_to: nil)
+
+      expect(subject.gitlab_initiated?(saml_response)).to eq(false)
+    end
+
+    it 'returns false if InResponseTo does not match stored value' do
+      saml_response = instance_double(OneLogin::RubySaml::Response, in_response_to: "abc")
+      allow(session).to receive(:[]).with('last_authn_request_id').and_return('123')
+
+      expect(subject.gitlab_initiated?(saml_response)).to eq(false)
+    end
+
+    it 'returns true if InResponseTo matches stored value' do
+      saml_response = instance_double(OneLogin::RubySaml::Response, in_response_to: "123")
+      allow(session).to receive(:[]).with('last_authn_request_id').and_return('123')
+
+      expect(subject.gitlab_initiated?(saml_response)).to eq(true)
+    end
+  end
+end

spec/lib/omni_auth/strategies/saml_spec.rb

+require 'spec_helper'
+
+describe OmniAuth::Strategies::SAML, type: :strategy do
+  let(:idp_sso_target_url) { 'https://login.example.com/idp' }
+  let(:strategy) { [OmniAuth::Strategies::SAML, { idp_sso_target_url: idp_sso_target_url }] }
+
+  describe 'POST /users/auth/saml' do
+    it 'redirects to the provider login page' do
+      post '/users/auth/saml'
+
+      expect(last_response).to redirect_to(/\A#{Regexp.quote(idp_sso_target_url)}/)
+    end
+
+    it 'stores request ID during request phase' do
+      request_id = double
+      allow_any_instance_of(OneLogin::RubySaml::Authrequest).to receive(:uuid).and_return(request_id)
+
+      post '/users/auth/saml'
+      expect(session['last_authn_request_id']).to eq(request_id)
+    end
+  end
+end

spec/support/omniauth_strategy.rb

+module StrategyHelpers
+  include Rack::Test::Methods
+  include ActionDispatch::Assertions::ResponseAssertions
+  include Shoulda::Matchers::ActionController
+  include OmniAuth::Test::StrategyTestCase
+
+  def post(*args)
+    super.tap do
+      @response = ActionDispatch::TestResponse.from_response(last_response)
+    end
+  end
+
+  def auth_hash
+    last_request.env['omniauth.auth']
+  end
+
+  def self.without_test_mode
+    original_mode = OmniAuth.config.test_mode
+    original_on_failure = OmniAuth.config.on_failure
+
+    OmniAuth.config.test_mode = false
+    OmniAuth.config.on_failure = OmniAuth::FailureEndpoint
+
+    yield
+  ensure
+    OmniAuth.config.test_mode = original_mode
+    OmniAuth.config.on_failure = original_on_failure
+  end
+end
+
+RSpec.configure do |config|
+  config.include StrategyHelpers, type: :strategy
+
+  config.around(:all, type: :strategy) do |example|
+    StrategyHelpers.without_test_mode do
+      example.run
+    end
+  end
+end