XML (eXtensible Markup Language): This is a markup language used to encode data in a format that is both human-readable and machine-readable. It's similar to HTML, but while HTML is used to display data and focuses on how data looks, XML is designed to store and transport data, focusing on what data is. XML allows developers to create their own tags to describe and structure data, making it highly flexible.
DTD (Document Type Definition): This is a set of markup declarations that define a document type for an XML document. It provides a list of the legal elements, attributes, and entities that can be used within the document. A DTD can be either specified inside the XML document (internal DTD) or in an external file (external DTD).
Entity: In the context of XML, an entity is a way of representing an item of data. Entities can be declared in the DTD and can then be used elsewhere in the XML document. Entities can be internal (where the data is defined within the XML itself) or external (where the data is retrieved from an external source).
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This is a type of attack against an application that parses XML input. The attack occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This attack can lead to the disclosure of confidential data
In an XXE attack, the attacker defines an entity in the DTD that references an external resource. This could be a file on the server's file system, for example. If the XML parser is configured to resolve external entities, it will attempt to access this resource when it encounters a reference to the entity in the XML document. If the response from the server includes the content of the resource, then the attacker has successfully used XXE to extract data from the server.
For example, the following XML document uses an external entity to attempt to access a system file:
xmlCopy code
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///etc/passwd" >]>
<foo>&xxe;</foo>
Here, &xxe; is an entity that is defined to fetch data from /etc/passwd. If the server processes this XML and includes the content of this file in its response, the attacker has successfully exploited an XXE vulnerability to extract sensitive data.