diff --git a/lib/rdoc/rdoc.rb b/lib/rdoc/rdoc.rb
index df04646f3c..bb58513d2c 100644
--- a/lib/rdoc/rdoc.rb
+++ b/lib/rdoc/rdoc.rb
@@ -443,7 +443,7 @@ def remove_unparseable files
     files.reject do |file, *|
       file =~ /\.(?:class|eps|erb|scpt\.txt|svg|ttf|yml)$/i or
         (file =~ /tags$/i and
-         open(file, 'rb') { |io|
+         File.open(file, 'rb') { |io|
            io.read(100) =~ /\A(\f\n[^,]+,\d+$|!_TAG_)/
          })
     end
diff --git a/test/rdoc/test_rdoc_rdoc.rb b/test/rdoc/test_rdoc_rdoc.rb
index 3910dd4656..a83d5a1b88 100644
--- a/test/rdoc/test_rdoc_rdoc.rb
+++ b/test/rdoc/test_rdoc_rdoc.rb
@@ -456,6 +456,18 @@ def test_remove_unparseable_tags_vim
     end
   end
 
+  def test_remove_unparseable_CVE_2021_31799
+    temp_dir do
+      file_list = ['| touch evil.txt && echo tags']
+      file_list.each do |f|
+        FileUtils.touch f
+      end
+
+      assert_equal file_list, @rdoc.remove_unparseable(file_list)
+      assert_equal file_list, Dir.children('.')
+    end
+  end
+
   def test_setup_output_dir
     Dir.mktmpdir {|d|
       path = File.join d, 'testdir'