In this section, we'll explain what XML external entity injection is, describe some common examples, explain how to find and exploit various kinds of XXE injection, and summarize how to prevent XXE injection attacks.
What is XML external entity injection?
XML external entity injection (also known as XXE) is a web security vulnerability that allows an attacker to interfere with an application's processing of XML data. It often allows an attacker to view files on the application server filesystem, and to interact with any backend or external systems that the application itself can access.
In some situations, an attacker can escalate an XXE attack to compromise the underlying server or other backend infrastructure, by leveraging the XXE vulnerability to perform server-side request forgery (SSRF) attacks.
How do XXE vulnerabilities arise?
Some applications use the XML format to transmit data between the browser and the server. Applications that do this virtually always use a standard library or platform API to process the XML data on the server. XXE vulnerabilities arise because the XML specification contains various potentially dangerous features, and standard parsers support these features even if they are not normally used by the application.
XML external entities are a type of custom XML entity whose defined values are loaded from outside of the DTD in which they are declared. External entities are particularly interesting from a security perspective because they allow an entity to be defined based on the contents of a file path or URL.
An XML document with correct syntax is called "Well Formed".
An XML document validated against a DTD is both "Well Formed" and "Valid".
What is a DTD?
DTD stands for Document Type Definition. A DTD defines the structure and the legal elements and attributes of an XML document.
The purpose of a DTD is to define the structure and the legal elements and attributes of an XML document:
<!DOCTYPE note [ <!ELEMENT note (to,from,heading,body)> <!ELEMENT to (#PCDATA)> <!ELEMENT from (#PCDATA)> <!ELEMENT heading (#PCDATA)> <!ELEMENT body (#PCDATA)> ]>
The DTD above is interpreted like this:
- !DOCTYPE note - Defines that the root element of the document is 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"
What are the types of XXE attacks?
- Exploiting XXE to retrieve files, where an external entity is defined containing the contents of a file, and returned in the application's response.
- Exploiting XXE to perform SSRF attacks, where an external entity is defined based on a URL to a back-end system.
- Exploiting blind XXE exfiltrate data out-of-band, where sensitive data is transmitted from the application server to a system that the attacker controls.
- Exploiting blind XXE to retrieve data via error messages, where the attacker can trigger a parsing error message containing sensitive data.
How to find and test for XXE vulnerabilities?
The vast majority of XXE vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner.
Manually testing for XXE vulnerabilities generally involves:
- Testing for file retrieval by defining an external entity based on a well-known operating system file and using that entity in data that is returned in the application's response.
- Testing for blind XXE vulnerabilities by defining an external entity based on a URL to a system that you control, and monitoring for interactions with that system. Burp Collaborator client is perfect for this purpose.
- Testing for vulnerable inclusion of user-supplied non-XML data within a server-side XML document by using an XInclude attack to try to retrieve a well-known operating system file.
How to prevent XXE vulnerabilities?
Virtually all XXE vulnerabilities arise because the application's XML parsing library supports potentially dangerous XML features that the application does not need or intend to use. The easiest and most effective way to prevent XXE attacks is to disable those features.
Generally, it is sufficient to disable resolution of external entities and disable support for XInclude. This can usually be done via configuration options or by programmatically overriding default behavior. Consult the documentation for your XML parsing library or API for details about how to disable unnecessary capabilities.