Technical Analysis of "Cookieless DuoDrop: IIS Authentication Bypass and App Pool Privilege Escalation in ASP.NET Framework (CVE-2023-36899)"

Overview:
Soroush Dalili’s detailed blog post discusses a critical vulnerability (CVE-2023-36899) in the ASP.NET framework that leverages cookieless forms authentication. The vulnerability involves bypassing IIS authentication and potentially escalating privileges in an application pool context.

Key Technical Details:

1. Cookieless Forms Authentication:
2. Context: ASP.NET provides a cookieless option for forms authentication by embedding session identifiers directly into URLs instead of via cookies.

3. Mechanism: The session identifier is included in the URL structure as (S(session_id))/. Example: http://example.com/(S(wszuadx3xdjebw455tmtfo30))/default.aspx.

4. Vulnerability Mechanics:

5. Authentication Bypass: By manipulating the session identifier and its placement in the URL, attackers can bypass IIS authentication layers.
6. IIS URL Processing: IIS processes URLs in a manner where the embedded session ID in the URL results in the Session ID attribute being treated as part of the URL path rather than a separate entity. This leads to an incorrect parsing sequence allowing for an authentication bypass.

7. Privileged Context: If this is exploited within the application pool, it can result in higher privilege execution within that specific context.

8. Step-by-Step Exploitation:

9. Initial Investigation: The researcher discovered the URL structure behavior that ASP.NET’s cookieless authentication uses, specifically how URLs with different placements of the session identifier are processed.

10. Bypassing Normal Checks:

    1. Typical URL: http://example.com/secure/page
    2. Cookieless URL: http://example.com/(S(session_id))/secure/page
    3. Manipulation: Moving the session segment in problematic ways such as:
       • Nested within paths or directories.
       • Replacing or misplacing directory sequences causing misinterpretation.

11. Practical Example:

    • Attempting access: http://example.com/(S(AUTHED_SESSION_ID))/secure/page
    • If parsed incorrectly, this can result in unauthorized access: http://example.com/secure/(S(AUTHED_SESSION_ID)page
    • This results in IIS and ASP.NET interpreting the session as authenticated for certain requested resources, bypassing intended access controls.

12. Further Exploitation - Privilege Escalation:

13. Application Pool Context: The vulnerability could further be utilized to escalate privileges within the application pool, especially if certain aspects of session or identity management are handled insecurely within that context.

14. Analyzing App Pool Characteristics: Specific configurations of application pools can lead to means whereby compromised or misleading session identifiers can elevate permissions unintentionally.

15. Mitigation Strategies:

16. Upgrade & Patch: The primary recommendation is to upgrade ASP.NET to versions where this vulnerability is patched.
17. URL Handling & Validation: Ensuring robust URL validation mechanisms to prevent malformed session manipulations.
18. Application Pool Security: Strict auditing of app pool configurations to mitigate privilege escalation vectors and secure identification management more rigorously.

Key Takeaways:

• Cookieless Authentication: While cookieless forms of session management can simplify scenarios where cookies might be restricted, they introduce complex URL handling that can be exploited.
• URL Manipulation: This vulnerability underscores the dangers of inadequate parsing and validation of URLs and session identifiers.
• App Pool Context: The impact of such vulnerabilities is magnified in contexts where application pools handle elevated or sensitive processes.
• Defense in Depth: Routine updates, adhering to updated security practices during configuration, and validating all forged paths of user data remain critical.

Conclusion:

The detailed breakdown of CVE-2023-36899 in this write-up serves as a critical insight into handling session identifiers within URLs, exposing the potential risks for authentication and subsequent privilege escalation attacks. Effective responses include robust validation, timely updates, and defensive configurations within ASP.NET environments.

For the original technical write-up, refer to Soroush’s blog.