Monkey kubeclient to not follow any redirects.

This should prevent any malicious server from responding with a location
that will redirect us and expose internal services, as kubeclient's
rest-client will no longer follow redirects.

changelogs/unreleased/security-kubeclient-ssrf.yml

+---
+title: Monkey kubeclient to not follow any redirects.
+merge_request:
+author:
+type: security

config/initializers/kubeclient.rb

@@ -13,4 +13,25 @@ class Kubeclient::Client
     ns_prefix = build_namespace_prefix(namespace)
     rest_client["#{ns_prefix}#{entity_name_plural}/#{name}:#{port}/proxy"].url
   end
+
+  # Monkey patch to set `max_redirects: 0`, so that kubeclient
+  # does not follow redirects and expose internal services.
+  # See https://gitlab.com/gitlab-org/gitlab-ce/issues/53158
+  def create_rest_client(path = nil)
+    path ||= @api_endpoint.path
+    options = {
+      ssl_ca_file: @ssl_options[:ca_file],
+      ssl_cert_store: @ssl_options[:cert_store],
+      verify_ssl: @ssl_options[:verify_ssl],
+      ssl_client_cert: @ssl_options[:client_cert],
+      ssl_client_key: @ssl_options[:client_key],
+      proxy: @http_proxy_uri,
+      user: @auth_options[:username],
+      password: @auth_options[:password],
+      open_timeout: @timeouts[:open],
+      read_timeout: @timeouts[:read],
+      max_redirects: 0
+    }
+    RestClient::Resource.new(@api_endpoint.merge(path).to_s, options)
+  end
 end