For a better understanding let's quickly look at what XML is.
XML (Extensible Markup Language) is a markup language that is used for structuring and storing data in a structured format that is both human-readable and machine-readable. XML was developed as a successor to HTML (Hypertext Markup Language) and is widely used for data exchange between different systems and platforms, particularly on the web.
XML uses a set of tags to define the structure and content of the data being represented. These tags are used to identify and describe various elements and attributes of the data, such as tags for opening and closing elements, attributes for specifying additional information about the element, and entities for representing special characters and symbols.
One of the key advantages of XML is its flexibility and extensibility. It is possible to define custom tags and schemas for representing data, making it a powerful tool for representing complex data structures and exchanging data between different systems.
While XML was once widely used for a variety of purposes, its usage has declined in recent years as newer data formats like JSON have gained popularity with its simplicity, ease of use, and better support for modern web technologies.
XXE (XML External Entity) vulnerability is a type of security vulnerability that affects applications that parse XML input. In an XXE attack, an attacker injects malicious XML data into an application that uses an XML parser without proper validation, which can result in the application processing external entities that can be controlled by the attacker.
An external entity is a piece of XML that is defined outside of the XML document, but can be referenced and included within the document. An attacker can exploit an XXE vulnerability to include malicious external entities that can read local files, access internal systems, or perform other malicious actions on the server.
XXE vulnerabilities can be exploited in various ways, such as through web forms that accept XML input, SOAP and REST APIs that use XML-based payloads, or other applications that accept and process XML input. These attacks can lead to sensitive data leaks, server-side request forgery (SSRF), denial of service (DoS) attacks, and other serious security issues.
It is important for developers to be aware of XXE vulnerabilities and take steps to prevent them, such as disabling external entities, validating and sanitizing XML input, and using secure XML parsers that are specifically designed to prevent XXE attacks.
To find XML External Entity (XXE) vulnerabilities in a web application, you can start by examining the application's XML processing code to identify any input points that accept XML input. These input points could include:
Once you have identified the input points that accept XML input, you can test them for XXE vulnerabilities by providing input that includes external entity references and observing the application's response.
You can also use automated vulnerability scanners and penetration testing tools that can detect and exploit XXE vulnerabilities. These tools can send various payloads that include external entity references and observe the response to determine if the application is vulnerable.
XXE attacks can depend on the programming language used by the server-side application. The XXE attack vector exploits a vulnerability in the XML parser of the server-side application, and the specific vulnerabilities and defenses can vary depending on the programming language used.