Detailed Technical Analysis of "XSS in Google Colab via HTML sanitization bypass"

Overview:
In this detailed bug bounty write-up, the researcher describes the discovery and exploitation of a Cross-Site Scripting (XSS) vulnerability in Google Colab through HTML sanitization bypass. The vulnerability originated from inadequate HTML sanitization and allowed the injection and execution of arbitrary JavaScript code in a user’s browser.

Key Technical Details:

  1. Context and Setup:
  2. Google Colab: An interactive environment for running Jupyter notebooks, often utilized for data science and machine learning tasks.

  3. Vulnerable Input Field: The researcher identified the input field vulnerable to HTML injection. This input field was inadequately sanitized before reflecting user content in the browser.

  4. Initial Discovery:

  5. During an exploration phase, the researcher found that certain HTML entities and tags were not being properly sanitized. This was discovered by experimenting with various inputs and observing their outcomes when rendered in the browser.

  6. Initial payloads such as plain <script> tags failed, indicating some level of sanitization or filtering in place.

  7. Bypassing Sanitization:

  8. Obfuscation Techniques: The research involved using character encoding and entity tricks to bypass the HTML sanitization. For example: html <imG sRc="x" oNerror="alert(document.domain)"> Here, different capitalization (imG and sRc) and event handler (oNerror) were used to trigger JavaScript, capitalizing on the insufficiently robust sanitization process.

  9. Event Handlers: Attributes like onerror can execute JavaScript when certain events occur (e.g., an error in loading an image). This was pivotal in payload delivery, as the image source was deliberately malformed to trigger the onerror event.

  10. Crafting a Working Payload:

  11. Step-by-Step Refinement: The write-up shows a progression of different payload attempts and refinements. By understanding which parts of the HTML were being neutralized and which were not, the researcher refined the payload iteratively:
    • Alternative methods like SVG tags and utilizing custom attributes were also tested.
    • Escalating the attack via nested payloads was essential to uncovering weaknesses in the sanitization routine.

  12. Successful Payload: Eventually, the payload: html <img src="x" onerror="alert(document.domain)"> was successfully executed, demonstrating a clear bypass of the sanitization mechanism.

  13. Exploitation Impact:

  14. With the ability to execute JavaScript in the context of Google Colab, an attacker gains several attack vectors:
    • Stealing sensitive user data (e.g., session cookies, CSRF tokens).
    • Performing unauthorized actions on behalf of the user.
    • Setting up further credential harvesting mechanisms.

  15. The vulnerability has severe implications given the number of users that trust and utilize Google Colab for their sensitive computation tasks.

  16. Reporting and Response:

  17. Responsibility: The researcher followed proper disclosure practices, reporting the issue to Google’s Vulnerability Reward Program (VRP).
  18. Acknowledgment and Fix: Google acknowledged the vulnerability, confirmed it, and took appropriate measures to patch the issue to prevent similar HTML sanitization bypass attempts in the future.

Key Takeaways:

  1. Entity Encoding & Escalation: Malicious actors can use various encoding techniques to bypass sanitization. It's important for developers to be aware of and defend against such obfuscation methods.
  2. Adequate Input Sanitization: Critical fields should employ rigorous sanitization routines, leveraging well-maintained libraries reputed for their security resilience.
  3. Event Handler Triggers: Developers should be cautious about attributes like onerror, onclick, etc., within HTML tags, which can be exploited if not properly sanitized.
  4. Incremental Payload Testing: The success of XSS attacks often relies on iterative testing and refinement of payloads, requiring a methodical approach to identifying weak points in security measures.
  5. Robust Security Testing: Regular and thorough security testing (including penetration testing and code reviews) is essential, particularly for platforms handling sensitive user data and services.

Conclusion:

The write-up demonstrates the complexity and tenacity required for identifying and exploiting XSS vulnerabilities. It underscores the importance of thorough and precise sanitization for user inputs in web applications. By revealing the methods to bypass such security mechanisms, this case serves as a critical learning point for both security researchers and developers.

For more exhaustive details, you can visit the original hunt write-up here.