XML External Entities (XXE)

Lecture

XML External Entities (XXE) Explained - PwnFunction

XML Format

What is XML?

XML stands for "eXtensible Markup Language". XML is a language designed for storing and transporting data.
Like HTML, XML uses a tree-like structure of tags and data. Unlike HTML, XML does not use predefined tags, and so tags can be given names that describe the data. Earlier in the web's history, XML was in vogue as a data transport format (the "X" in "AJAX" stands for "XML"). But its popularity has now declined in favor of the JSON format.
XML is used in:
  • APIs
  • UI layouts & styles
  • Configs
  • RSS
  • ...
Example:
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<?xml version="1.0"?>
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<Person>
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<Name>John</Name>
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<Age>20</Age>
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</Person>
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XML Entities

XML entities are a way of representing an item of data within an XML document, instead of using the data itself. Entities are the "variables" for XML.
Various entities are built in to the specification of the XML language. For example, the entities &lt; and &gt; represent the characters < and >. These are metacharacters used to denote XML tags, and so must generally be represented using their entities when they appear within data.

Document Type Definition (DTD)

The XML Document Type Definition (DTD) contains declarations that can define the structure of an XML document, the types of data values it can contain, and other items. The DTD is declared within the optional DOCTYPE element at the start of the XML document. The DTD can be fully self-contained within the document itself (known as an "internal DTD") or can be loaded from elsewhere (known as an "external DTD") or can be hybrid of the two.

Custom Entities

XML allows custom entities to be defined within the DTD. For example:
<!DOCTYPE foo [ <!ENTITY myentity "my entity value" > ]>
This definition means that any usage of the entity reference &myentity; within the XML document will be replaced with the defined value: "my entity value".
Example:
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<?xml version="1.0"?>
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<!DOCTYPE Person [
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<!ENTITY name "John">
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]>
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<Person>
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<Name>&name;</Name>
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<Age>20</Age>
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</Person>
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External Entities

XML external entities are a type of custom entity whose definition is located outside of the DTD where they are declared.
The declaration of an external entity uses the SYSTEM keyword and must specify a URL from which the value of the entity should be loaded. For example:
<!DOCTYPE foo [ <!ENTITY ext SYSTEM "http://normal-website.com" > ]>
The URL can use the file:// protocol, and so external entities can be loaded from file. For example:
<!DOCTYPE foo [ <!ENTITY ext SYSTEM "file:///path/to/file" > ]>
XML external entities provide the primary means by which XML External Entity (XXE) attacks arise.

XML External Entity (XXE)

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 back-end or external systems that the application itself can access.
XXE
In some situations, an attacker can escalate an XXE attack to compromise the underlying server or other back-end infrastructure, by leveraging the XXE vulnerability to perform Server-Side Request Forgery (SSRF) attacks.
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.

Arbitrary Read

To perform an XXE injection attack that retrieves an arbitrary file from the server's filesystem, you need to modify the submitted XML in two ways:
  • Introduce (or edit) a DOCTYPE element that defines an external entity containing the path to the file.
  • Edit a data value in the XML that is returned in the application's response, to make use of the defined external entity.
For example, suppose a shopping application checks for the stock level of a product by submitting the following XML to the server:
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<?xml version="1.0" encoding="UTF-8"?>
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<stockCheck><productId>381</productId></stockCheck>
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The application performs no particular defenses against XXE attacks, so you can exploit the XXE vulnerability to retrieve the /etc/passwd file by submitting the following XXE payload:
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE foo [
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<!ENTITY xxe SYSTEM "file:///etc/passwd">
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]>
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<stockCheck><productId>&xxe;</productId></stockCheck>
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XXE + SSRF

Aside from retrieval of sensitive data, the other main impact of XXE attacks is that they can be used to perform Server-Side Request Forgery (SSRF). This is a potentially serious vulnerability in which the server-side application can be induced to make HTTP requests to any URL that the server can access.
To exploit an XXE vulnerability to perform an SSRF attack, you need to define an external XML entity using the URL that you want to target, and use the defined entity within a data value. If you can use the defined entity within a data value that is returned in the application's response, then you will be able to view the response from the URL within the application's response, and so gain two-way interaction with the back-end system. If not, then you will only be able to perform blind SSRF attacks (which can still have critical consequences).
In the following XXE example, the external entity will cause the server to make a back-end HTTP request to an internal system within the organization's infrastructure:
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<!DOCTYPE foo [
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<!ENTITY xxe SYSTEM "http://internal.vulnerable-website.com/">
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]>
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Blind XXE

Many instances of XXE vulnerabilities are blind. This means that the application does not return the values of any defined external entities in its responses, and so direct retrieval of server-side files is not possible.
Blind XXE vulnerabilities can still be detected and exploited, but more advanced techniques are required. You can sometimes use out-of-band techniques to find vulnerabilities and exploit them to exfiltrate data. And you can sometimes trigger XML parsing errors that lead to disclosure of sensitive data within error messages.

XInclude Attack

Some applications receive client-submitted data, embed it on the server-side into an XML document, and then parse the document. An example of this occurs when client-submitted data is placed into a back-end SOAP request, which is then processed by the backend SOAP service.
In this situation, you cannot carry out a classic XXE attack, because you don't control the entire XML document and so cannot define or modify a DOCTYPE element. However, you might be able to use XInclude instead. XInclude is a part of the XML specification that allows an XML document to be built from sub-documents. You can place an XInclude attack within any data value in an XML document, so the attack can be performed in situations where you only control a single item of data that is placed into a server-side XML document.
To perform an XInclude attack, you need to reference the XInclude namespace and provide the path to the file that you wish to include. For example:
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<foo xmlns:xi="http://www.w3.org/2001/XInclude">
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<xi:include parse="text" href="file:///etc/passwd"/></foo>
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File Upload

Some applications allow users to upload files which are then processed server-side. Some common file formats use XML or contain XML subcomponents. Examples of XML-based formats are office document formats like DOCX and image formats like SVG.
For example, an application might allow users to upload images, and process or validate these on the server after they are uploaded. Even if the application expects to receive a format like PNG or JPEG, the image processing library that is being used might support SVG images. Since the SVG format uses XML, an attacker can submit a malicious SVG image and so reach hidden attack surface for XXE vulnerabilities.

Modified Content Type

Most POST requests use a default content type that is generated by HTML forms, such as application/x-www-form-urlencoded. Some web sites expect to receive requests in this format but will tolerate other content types, including XML.
For example, if a normal request contains the following:
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POST /action HTTP/1.0
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Content-Type: application/x-www-form-urlencoded
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Content-Length: 7
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foo=bar
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Then you might be able submit the following request, with the same result:
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POST /action HTTP/1.0
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Content-Type: text/xml
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Content-Length: 52
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<?xml version="1.0" encoding="UTF-8"?><foo>bar</foo>
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If the application tolerates requests containing XML in the message body, and parses the body content as XML, then you can reach the hidden XXE attack surface simply by reformatting requests to use the XML format.

Prevention

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.

Reference

What is XXE (XML external entity) injection? Tutorial & Examples | Web Security Academy
WebSecAcademy
XML external entity (XXE) injection - Web Security Academy