Technical Analysis of "Denial of Service using Cookie Bombing"

Overview:
The write-up by Ronak Desai on Medium delves into a Denial of Service (DoS) attack method known as "Cookie Bombing." This technique exploits various web application weaknesses related to cookie management, leading to resource exhaustion and potential service disruption.

Key Technical Details:

1. Cookie Handling in Web Applications:
2. Cookies are small pieces of data stored by the user's browser, often used for session management, personalization, and tracking.
3. Web servers include cookies in HTTP headers, which browsers then send back with each subsequent request.

4. Key attributes of cookies include name, value, domain, path, expiration date, and security flags (HttpOnly, Secure).

5. Concept of Cookie Bombing:

6. The attacker sets an excessive number of cookies or excessively large cookie values on the target domain.

7. Each HTTP request sent by the user's browser includes these bloated cookies, resulting in:

   • Increased request size: HTTP request headers grow significantly, impacting bandwidth and server processing time.
   • Resource exhaustion: Servers and intermediary devices like load balancers and firewalls might slow down or crash trying to handle oversized headers.

8. Technical Workflow:

9. Initial Setup: The attacker leverages a subdomain or an endpoint on the target site that reflects or stores cookies.
   • Example: Suppose vulnerable.example.com has a subdomain attacker.vulnerable.example.com under attacker control.
10. Bombing Process:
    • The attacker injects numerous cookies or sets cookies with massive values on attacker.vulnerable.example.com.
    • These cookies are sent on every request to vulnerable.example.com and its subdomains.

11. Impact:

    • The web server's processing time increases as it parses each oversized request.
    • Bandwidth consumption rises due to the larger-than-usual request sizes.
    • Potentially, the server's memory or CPU resources are exhausted, leading to a DoS condition.

12. Practical Demonstration:

13. Cookie Setting Script:
    • A simple JavaScript snippet in the attacker's control could be: javascript const largeValue = 'a'.repeat(4096); // 4KB payload for (let i = 0; i < 50; i++) { document.cookie = `bomb_${i}=${largeValue}; path=/; domain=vulnerable.example.com`; }
14. This script sets 50 cookies, each with a 4KB payload, resulting in an additional 200KB per request.

15. Execution: The attacker convinces users to visit a page on attacker.vulnerable.example.com, executing the cookie-setting script.

16. Mitigation Strategies:

17. Limit Cookie Size: Implementing back-end and front-end policies to limit individual cookie sizes (e.g., 4KB max per cookie).
18. Restrict Number of Cookies: Enforce limits on the total number of cookies per domain.
19. Validation and Filtering: Implementing strict validation on cookies to ensure they don't exceed expected bounds and filtering out excessive cookies.
20. Secure Cookie Attributes: Utilizing HttpOnly and Secure flags makes it difficult for JavaScript to manipulate cookies.

21. Separate Subdomains: Using different subdomains for unrelated features or minimizing the sharing of cookies across subdomains prevents cross-domain cookie abuse.

22. Real-World Implications:

23. Websites and applications vulnerable to cookie bombing may face service disruptions, affecting availability and performance.
24. Load balancers and web application firewalls might also crash or become ineffective, exposing further vulnerabilities.

Key Takeaways:

• Vulnerability Awareness: Developers and administrators should be aware of how cookie management practices can lead to resource exhaustion.
• Effective Limits: Enforcing constraints on cookie size and quantity can mitigate the risk of cookie bombing.
• Defense-in-Depth: Combining multiple security measures (cookie validation, proper use of attributes, domain segregation) enhances robustness against such attacks.
• Monitoring and Response: Setting up monitoring for abnormal traffic patterns and large request headers can help in early detection and response to potential cookie bombing attacks.

Conclusion:

Ronak Desai’s article illustrates the critical yet often overlooked threat that improper cookie handling can pose. By understanding and implementing effective cookie management practices, web applications can avoid the pitfalls of resource exhaustion and DoS attacks driven by cookie bombing.

For full details, refer to the original article here.