XML entities
In this section, we’ll explain some key features of XML that are relevant to understanding XXE vulnerabilities.
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.
What are XML entities?
XML entities are a way of representing an item of data within an XML document, instead of using the data itself. Various entities are built in to the specification of the XML language. For example, the entities < and > 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.
What is document type definition?
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.
What are XML 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".
What are XML 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 attacks arise.
XML external entity (XXE) injection
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 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.
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.
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Learn about the XML format, DTDs, and external entities
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.
What are the types of XXE attacks?
There are various 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.
Conclusion:
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