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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. 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.
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.
If you’re already familiar with the basic concepts behind XXE vulnerabilities and just want to practice exploiting them on some realistic, deliberately vulnerable targets, you can access all of the labs in this topic from the link below. 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. 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.
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:. For example, suppose a shopping application checks for the stock level of a product by submitting the following XML to the server:. This causes the application’s response to include the contents of the file:.
With real-world XXE vulnerabilities, there will often be a large number of data values within the submitted XML, any one of which might be used within the application’s response. To test systematically for XXE vulnerabilities, you will generally need to test each data node in the XML individually, by making use of your defined entity and seeing whether it appears within the response. 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. 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:. 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. In other cases, the attack surface is less visible. Some applications receive client-submitted data, embed it on the server-side into an XML document, and then parse the document.
However, you might be able to use XInclude instead. 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:. Some applications allow users to upload files which are then processed server-side. For example, an application might allow users to upload images, and process or validate these on the server after they are uploaded. Some web sites expect to receive requests in this format but will tolerate other content types, including XML.
The vast majority of XXE vulnerabilities can be found quickly and reliably using Burp Suite’s web vulnerability scanner. 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. Want to track your progress and have a more personalized learning experience?
It’s free! Labs If you’re already familiar with the basic concepts behind XXE vulnerabilities and just want to practice exploiting them on some realistic, deliberately vulnerable targets, you can access all of the labs in this topic from the link below. View all XXE labs. Note With real-world XXE vulnerabilities, there will often be a large number of data values within the submitted XML, any one of which might be used within the application’s response.
Read more Finding and exploiting blind XXE vulnerabilities. Sign up Login. Register for free to track your learning progress. Practise exploiting vulnerabilities on realistic targets. Record your progression from Apprentice to Expert. See where you rank in our Hall of Fame.
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