Lab: Exploiting XXE using external entities to retrieve files

Lab Scenario: Our mission is to exploit XXE through a web application's "Check stock" feature, specifically using external entities to retrieve files. By intercepting and manipulating a POST request, we intend to use XXE to trigger the retrieval of sensitive information from the server, in this case, the contents of the /etc/passwd file. Let's proceed with the solution:

Intercepting the POST Request:
- Visit a product page and click "Check stock."
- Intercept the resulting POST request using Burp Suite.
Inserting External Entity Definition:
- Insert the following external entity definition between the XML declaration and the stockCheck element:
```
  <!DOCTYPE test [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>
```
Replacing the productId with External Entity Reference:
- Replace the productId number with a reference to the external entity: &xxe;.
- The response should contain "Invalid product ID:" followed by the contents of the /etc/passwd file.
Confirming the File Retrieval:
- Inspect the response to ensure that it contains the expected content of the /etc/passwd file.
Submitting the Solution:
- Use the appropriate method provided by the lab to submit the solution, confirming the successful retrieval of sensitive information.

Conclusion: This lab exercise provides hands-on experience in exploiting XXE vulnerabilities to retrieve files from a web application. By following this step-by-step guide, users can deepen their understanding of XXE attacks and the potential risks associated with improper handling of XML input. Stay informed, keep learning, and continue exploring the dynamic field of cybersecurity to contribute to a more secure online environment.

Reference:

https://portswigger.net/web-security/xxe

https://portswigger.net/web-security/xxe/lab-exploiting-xxe-to-retrieve-files