Validate session key when authorizing with GCP to create a cluster

It was previously possible to link a GCP account to another
user's GitLab account by having them visit the callback URL,
as there was no check that they were the initiator of the
request.

We now reject the callback unless the state parameter
matches the one added to the initiating user's session.

3 files changed, 67 insertions, 30 deletions

diff --git a/app/controllers/google_api/authorizations_controller.rb b/app/controllers/google_api/authorizations_controller.rb
index dd9f5af61b3..ed0995e7ffd 100644
--- a/app/controllers/google_api/authorizations_controller.rb
+++ b/app/controllers/google_api/authorizations_controller.rb
@@ -2,6 +2,10 @@
 
 module GoogleApi
   class AuthorizationsController < ApplicationController
+    include Gitlab::Utils::StrongMemoize
+
+    before_action :validate_session_key!
+
     def callback
       token, expires_at = GoogleApi::CloudPlatform::Client
         .new(nil, callback_google_api_auth_url)
@@ -11,21 +15,27 @@ module GoogleApi
       session[GoogleApi::CloudPlatform::Client.session_key_for_expires_at] =
         expires_at.to_s
 
-      state_redirect_uri = redirect_uri_from_session_key(params[:state])
-
-      if state_redirect_uri
-        redirect_to state_redirect_uri
-      else
-        redirect_to root_path
-      end
+      redirect_to redirect_uri_from_session
     end
 
     private
 
-    def redirect_uri_from_session_key(state)
-      key = GoogleApi::CloudPlatform::Client
-        .session_key_for_redirect_uri(params[:state])
-      session[key] if key
+    def validate_session_key!
+      access_denied! unless redirect_uri_from_session.present?
+    end
+
+    def redirect_uri_from_session
+      strong_memoize(:redirect_uri_from_session) do
+        if params[:state].present?
+          session[session_key_for_redirect_uri(params[:state])]
+        else
+          nil
+        end
+      end
+    end
+
+    def session_key_for_redirect_uri(state)
+      GoogleApi::CloudPlatform::Client.session_key_for_redirect_uri(state)
     end
   end
 end
diff --git a/changelogs/unreleased/security-kubernetes-google-login-csrf.yml b/changelogs/unreleased/security-kubernetes-google-login-csrf.yml
new file mode 100644
index 00000000000..2f87100a8dd
--- /dev/null
+++ b/changelogs/unreleased/security-kubernetes-google-login-csrf.yml
@@ -0,0 +1,5 @@
+---
+title: Validate session key when authorizing with GCP to create a cluster
+merge_request:
+author:
+type: security
diff --git a/spec/controllers/google_api/authorizations_controller_spec.rb b/spec/controllers/google_api/authorizations_controller_spec.rb
index 1e8e82da4f3..d9ba85cf56a 100644
--- a/spec/controllers/google_api/authorizations_controller_spec.rb
+++ b/spec/controllers/google_api/authorizations_controller_spec.rb
@@ -6,7 +6,7 @@ describe GoogleApi::AuthorizationsController do
     let(:token) { 'token' }
     let(:expires_at) { 1.hour.since.strftime('%s') }
 
-    subject { get :callback, params: { code: 'xxx', state: @state } }
+    subject { get :callback, params: { code: 'xxx', state: state } }
 
     before do
       sign_in(user)
@@ -15,35 +15,57 @@ describe GoogleApi::AuthorizationsController do
         .to receive(:get_token).and_return([token, expires_at])
     end
 
-    it 'sets token and expires_at in session' do
-      subject
+    shared_examples_for 'access denied' do
+      it 'returns a 404' do
+        subject
 
-      expect(session[GoogleApi::CloudPlatform::Client.session_key_for_token])
-        .to eq(token)
-      expect(session[GoogleApi::CloudPlatform::Client.session_key_for_expires_at])
-        .to eq(expires_at)
+        expect(session[GoogleApi::CloudPlatform::Client.session_key_for_token]).to be_nil
+        expect(response).to have_http_status(:not_found)
+      end
     end
 
-    context 'when redirect uri key is stored in state' do
-      set(:project) { create(:project) }
-      let(:redirect_uri) { project_clusters_url(project).to_s }
+    context 'session key is present' do
+      let(:session_key) { 'session-key' }
+      let(:redirect_uri) { 'example.com' }
 
       before do
-        @state = GoogleApi::CloudPlatform::Client
-          .new_session_key_for_redirect_uri do |key|
-          session[key] = redirect_uri
+        session[GoogleApi::CloudPlatform::Client.session_key_for_redirect_uri(session_key)] = redirect_uri
+      end
+
+      context 'session key matches state param' do
+        let(:state) { session_key }
+
+        it 'sets token and expires_at in session' do
+          subject
+
+          expect(session[GoogleApi::CloudPlatform::Client.session_key_for_token])
+            .to eq(token)
+          expect(session[GoogleApi::CloudPlatform::Client.session_key_for_expires_at])
+            .to eq(expires_at)
+        end
+
+        it 'redirects to the URL stored in state param' do
+          expect(subject).to redirect_to(redirect_uri)
         end
       end
 
-      it 'redirects to the URL stored in state param' do
-        expect(subject).to redirect_to(redirect_uri)
+      context 'session key does not match state param' do
+        let(:state) { 'bad-key' }
+
+        it_behaves_like 'access denied'
       end
-    end
 
-    context 'when redirection url is not stored in state' do
-      it 'redirects to root_path' do
-        expect(subject).to redirect_to(root_path)
+      context 'state param is blank' do
+        let(:state) { '' }
+
+        it_behaves_like 'access denied'
       end
     end
+
+    context 'state param is present, but session key is blank' do
+      let(:state) { 'session-key' }
+
+      it_behaves_like 'access denied'
+    end
   end
 end