diff --git a/lib/bundler/vendor/compact_index_client/lib/compact_index_client/updater.rb b/lib/bundler/vendor/compact_index_client/lib/compact_index_client/updater.rb
index 6c5a4da57ac..40c61644e36 100644
--- a/lib/bundler/vendor/compact_index_client/lib/compact_index_client/updater.rb
+++ b/lib/bundler/vendor/compact_index_client/lib/compact_index_client/updater.rb
@@ -26,7 +26,7 @@ def initialize(fetcher)
     def update(local_path, remote_path, retrying = nil)
       headers = {}
 
-      Dir.mktmpdir(local_path.basename.to_s, local_path.dirname) do |local_temp_dir|
+      Dir.mktmpdir("bundler-compact-index-") do |local_temp_dir|
         local_temp_path = Pathname.new(local_temp_dir).join(local_path.basename)
 
         # first try to fetch any new bytes on the existing file
diff --git a/spec/install/gems/compact_index_spec.rb b/spec/install/gems/compact_index_spec.rb
index 0edd1d20e7a..b34d9e872d5 100644
--- a/spec/install/gems/compact_index_spec.rb
+++ b/spec/install/gems/compact_index_spec.rb
@@ -695,4 +695,12 @@ def start
     expect(File.read(versions)).to start_with("created_at")
     expect(the_bundle).to include_gems "rack 1.0.0"
   end
+
+  it "works when cache dir is world-writable" do
+    install_gemfile! <<-G, :artifice => "compact_index"
+      File.umask(0000)
+      source "#{source_uri}"
+      gem "rack"
+    G
+  end
 end