Exploiting PDF generation vulnerability: a case study from real pentest

Summary

In a recent penetration test conducted by our team, we identified a critical vulnerability within a web application that allowed unauthorized access to sensitive resources. This flaw permits an attacker to access both local server files and data on other servers within the same network. The vulnerability stems from improper handling of user-input data, presenting a severe security risk.

Prerequisites for the Attack

To exploit this vulnerability, an attacker must have a valid user account within the application. This access level is necessary to interact with specific application functionalities and craft the malicious payload.

This vulnerability was identified in the PDF generation component of the application. The inadequate validation of user-input data in this feature is the root cause.

Technical Details and Proof of Concept

Case #1: Unauthorized File Reading

Our test demonstrated the ability to read arbitrary files from the server by injecting HTML payloads. Here are the steps taken:

1. Authentication: Log in to the application using any user account.
2. Navigation: Navigate to the ‘Users’ page.
3. Payload Injection: Edit any part of a user profile, save the changes, capture the outgoing request, and inject the following HTML payload into the request:

4. Execution: Click the ‘Preview PDF’ button.
5. Result: The application generates a PDF file containing the contents of the /etc/passwd file revealing sensitive server information.

Case #2: Interaction with Attacker’s Host

A similar approach can be used to interact with an external attacker-controlled server:

1. Follow Steps 1-3 as in Case #1.
2. Payload: We can use the following payload instead:

3. Result: The application interacts with the attacker’s server.

Recommendations

To mitigate this vulnerability, we recommend implementing the following measures:

1. Input Validation: Enhance the input validation mechanism to restrict access only to pre-approved resources. Implement a whitelist of acceptable locations.

2. HTML Sanitization: Disable HTML parsing or enable sanitization of HTML tags in the library responsible for generating PDF files. This measure will prevent the injection of malicious content.

Conclusion

The discovery of this vulnerability highlights the importance of rigorous input validation and sanitization in web applications. By addressing these issues, organizations can significantly improve their security posture and protect sensitive information from unauthorized access.