XML external entity (XXE) injection

XML External Entity (XXE) injection is a type of security vulnerability that affects applications or systems parsing XML data. It allows an attacker to manipulate the processing of XML input and potentially gain unauthorized access to sensitive data or perform other malicious actions on the system. XXE attacks typically target applications that parse XML documents without proper input validation and sanitization.

To understand XXE, it's essential to know about entities in XML. An entity is a variable that can be referenced within an XML document, using the following syntax:

1. Internal Entity: Defined within the DTD (Document Type Definition):

    <!ENTITY entity_name "entity_value">
   

2. External Entity: Reference an entity from an external source (like a file):

    <!ENTITY entity_name SYSTEM "file_path">
   

The vulnerability arises when an XML parser resolves external entities automatically without proper restrictions. An attacker can craft a malicious XML input that references an external entity controlled by them. When the parser processes the XML, it fetches the external entity, and the attacker can read files from the server, initiate server-side request forgery (SSRF) attacks, or perform other actions depending on the application's context and permissions.

Here's an example of a vulnerable XML document:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE data [
  <!ELEMENT data (#PCDATA)>
  <!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<data>&xxe;</data>

In this example, the XML document includes an external entity xxe that reads the contents of the /etc/passwd file. If this XML is parsed without proper protection, an attacker can obtain sensitive information from the server.

To prevent XXE attacks, developers should take the following precautions:

1. Disable external entity processing: Ensure that the XML parser is configured not to resolve external entities, or use features like "external entity expansion" to block external entity references.

2. Use whitelisting: If external entity processing is necessary, create a whitelist of trusted entities and restrict external references to only those entities.

3. Validate input: Always validate and sanitize XML input to prevent maliciously crafted XML documents from being processed.

4. Use the latest XML parsers: Keep the XML parsing libraries up-to-date to benefit from the latest security fixes.

By following these best practices, developers can mitigate the risk of XXE vulnerabilities in their applications.