No redirects in prometheus service

Do not allow redirects in the prometheus service to prevent SSRFs.

app/models/project_services/prometheus_service.rb

@@ -71,7 +71,7 @@ class PrometheusService < MonitoringService
   end
 
   def prometheus_client
-    RestClient::Resource.new(api_url) if api_url && manual_configuration? && active?
+    RestClient::Resource.new(api_url, max_redirects: 0) if api_url && manual_configuration? && active?
   end
 
   def prometheus_available?

spec/models/project_services/prometheus_service_spec.rb

@@ -13,6 +13,23 @@ describe PrometheusService, :use_clean_rails_memory_store_caching do
     it { is_expected.to belong_to :project }
   end
 
+  context 'redirects' do
+    it 'does not follow redirects' do
+      redirect_to = 'https://redirected.example.com'
+      redirect_req_stub = stub_prometheus_request(prometheus_query_url('1'), status: 302, headers: {location: redirect_to})
+      redirected_req_stub = stub_prometheus_request(redirect_to, body: {'status': 'success'})
+
+      result = service.test
+
+      # result = { success: false, result: error }
+      expect(result[:success]).to be_falsy
+      expect(result[:result]).to be_instance_of(Gitlab::PrometheusClient::Error)
+
+      expect(redirect_req_stub).to have_been_requested
+      expect(redirected_req_stub).not_to have_been_requested
+    end
+  end
+
   describe 'Validations' do
     context 'when manual_configuration is enabled' do
       before do

spec/support/helpers/prometheus_helpers.rb

@@ -49,11 +49,11 @@ module PrometheusHelpers
     "https://prometheus.example.com/api/v1/series?#{query}"
   end
 
-  def stub_prometheus_request(url, body: {}, status: 200)
+  def stub_prometheus_request(url, body: {}, status: 200, headers: {})
     WebMock.stub_request(:get, url)
       .to_return({
         status: status,
-        headers: { 'Content-Type' => 'application/json' },
+        headers: { 'Content-Type' => 'application/json' }.merge(headers),
         body: body.to_json
       })
   end