Sinatra vulnerable to Reflected File Download attack

@@ -396,13 +396,20 @@ def content_type(type = nil, params = {})
       response['Content-Type'] = mime_type
     end
 
+    # https://html.spec.whatwg.org/#multipart-form-data
+    MULTIPART_FORM_DATA_REPLACEMENT_TABLE = {
+      '"'  => '%22',
+      "\r" => '%0D',
+      "\n" => '%0A'
+    }.freeze
+
     # Set the Content-Disposition to "attachment" with the specified filename,
     # instructing the user agents to prompt to save.
     def attachment(filename = nil, disposition = :attachment)
       response['Content-Disposition'] = disposition.to_s.dup
       return unless filename
 
-      params = format('; filename="%s"', File.basename(filename))
+      params = format('; filename="%s"', File.basename(filename).gsub(/["\r\n]/, MULTIPART_FORM_DATA_REPLACEMENT_TABLE))
       response['Content-Disposition'] << params
       ext = File.extname(filename)
       content_type(ext) unless response['Content-Type'] || ext.empty?

@@ -781,6 +781,18 @@ def attachment_app(filename=nil)
       assert_equal '<sinatra></sinatra>', body
     end
 
+    it 'escapes filename in the Content-Disposition header according to the multipart form data spec in WHATWG living standard' do
+      mock_app do
+        get('/attachment') do
+          attachment "test.xml\";\r\next=.txt"
+          response.write("<sinatra></sinatra>")
+        end
+      end
+
+      get '/attachment'
+      assert_equal 'attachment; filename="test.xml%22;%0D%0Aext=.txt"', response['Content-Disposition']
+      assert_equal '<sinatra></sinatra>', body
+    end
   end
 
   describe 'send_file' do