Use a temporary file for storing unencrypted files while editing

When we're editing the contents of encrypted files, we should use the
`Tempfile` class because it creates temporary files with restrictive
permissions.  This prevents other users on the same system from reading
the contents of those files while the user is editing them.

[CVE-2023-38037]

Author: tenderlove

activesupport/lib/active_support/encrypted_file.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 require "pathname"
-require "tmpdir"
+require "tempfile"
 require "active_support/message_encryptor"
 
 module ActiveSupport
@@ -69,17 +69,16 @@ def change(&block)
 
     private
       def writing(contents)
-        tmp_file = "#{Process.pid}.#{content_path.basename.to_s.chomp('.enc')}"
-        tmp_path = Pathname.new File.join(Dir.tmpdir, tmp_file)
-        tmp_path.binwrite contents
+        Tempfile.create(["", "-" + content_path.basename.to_s.chomp(".enc")]) do |tmp_file|
+          tmp_path = Pathname.new(tmp_file)
+          tmp_path.binwrite contents
 
-        yield tmp_path
+          yield tmp_path
 
-        updated_contents = tmp_path.binread
+          updated_contents = tmp_path.binread
 
-        write(updated_contents) if updated_contents != contents
-      ensure
-        FileUtils.rm(tmp_path) if tmp_path&.exist?
+          write(updated_contents) if updated_contents != contents
+        end
       end
 
 

activesupport/test/encrypted_file_test.rb

@@ -49,6 +49,14 @@ class EncryptedFileTest < ActiveSupport::TestCase
     assert_equal "#{@content} and went by the lake", @encrypted_file.read
   end
 
+  test "change sets restricted permissions" do
+    @encrypted_file.write(@content)
+    @encrypted_file.change do |file|
+      assert_predicate file, :owned?
+      assert_equal "100600", file.stat.mode.to_s(8), "Incorrect mode for #{file}"
+    end
+  end
+
   test "raise MissingKeyError when key is missing" do
     assert_raise ActiveSupport::EncryptedFile::MissingKeyError do
       encrypted_file(@content_path, key_path: "", env_key: "").read

railties/lib/rails/secrets.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "yaml"
+require "tempfile"
 require "active_support/message_encryptor"
 
 module Rails
@@ -87,17 +88,18 @@ def preprocess(path)
         end
 
         def writing(contents)
-          tmp_file = "#{File.basename(path)}.#{Process.pid}"
-          tmp_path = File.join(Dir.tmpdir, tmp_file)
-          IO.binwrite(tmp_path, contents)
+          file_name = "#{File.basename(path)}.#{Process.pid}"
 
-          yield tmp_path
+          Tempfile.create(["", "-" + file_name]) do |tmp_file|
+            tmp_path = Pathname.new(tmp_file)
+            tmp_path.binwrite contents
 
-          updated_contents = IO.binread(tmp_path)
+            yield tmp_path
 
-          write(updated_contents) if updated_contents != contents
-        ensure
-          FileUtils.rm(tmp_path) if File.exist?(tmp_path)
+            updated_contents = tmp_path.binread
+
+            write(updated_contents) if updated_contents != contents
+          end
         end
 
         def encryptor