Return to topic cards

Understanding XML and XXE Injections

XMLXXECybersecurityWeb SecurityData Exchange

XML External Entity (XXE) injection is a critical security vulnerability that occurs when an application processes XML input containing external entity references. This can lead to unauthorized access to files, internal systems, or external websites.

Key Points

  • XML: A format used for storing and transporting data that is both human-readable and machine-parseable.
  • Common Use Cases: Data exchange, storage, and configuration in web applications, often used in web services and APIs.
  • XSLT: A language used to transform and format XML documents, which can facilitate XXE attacks.
  • DTDs: Document Type Definitions that define the structure and constraints of an XML document.
  • XML Entities: Placeholders for data or code, including internal, external, parameter, general, and character entities.
  • XML Parsing: The process of reading and manipulating XML files, which can lead to security risks if external entities are processed.

XML and Its Uses

What is XML?

XML (eXtensible Markup Language) is a versatile format used for storing and transporting data. It is both human-readable and machine-parseable, making it a popular choice for various applications.

Common Use Cases in Web Applications

  • Data Exchange: Often used for web services and APIs, such as SOAP and REST, to exchange data between systems.
  • Storage: Used for configuration files, such as web server configurations or application settings.
  • Configuration: Helps in defining settings and parameters for applications.

XSLT and Its Role in XXE Attacks

What is XSLT?

XSLT (Extensible Stylesheet Language Transformations) is a language used to transform and format XML documents. It can be exploited in several ways to facilitate XXE attacks:

  • Data Extraction: Extracting sensitive data from an XML document.
  • Entity Expansion: Allowing attackers to inject malicious entities.
  • Data Manipulation: Injecting malicious data into the XML document.
  • Blind XXE: Injecting without seeing the server's response.

DTDs and XML Entities

Document Type Definitions (DTDs)

DTDs define the structure and constraints of an XML document. They can declare external entities, which can reference external files or URLs.

Types of XML Entities

XML entities are placeholders for data or code. There are five types:

  • Internal
  • External
  • Parameter
  • General
  • Character

XML Parsing and Security Risks

What is XML Parsing?

XML Parsing is the process by which an XML file is read, and its information is accessed and manipulated by code. If a parser is configured to process external entities, it can lead to unauthorized access to files, internal systems, or external websites.

Common XML Parsers

  • DOM Parser
  • SAX Parser
  • StAX Parser
  • XPath Parser

Learn More