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CVE-2025-59830: Rack::QueryParser Parameter Count Limit Bypass Vulnerability in Ruby

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Overview

This report covers a significant vulnerability found in Rack, a modular Ruby web server interface. The flaw, identified as CVE-2025-59830, is a parameter count limit bypass vulnerability that may lead to increased CPU and memory consumption, potentially causing a denial of service. This vulnerability is of importance to all businesses and individuals using affected versions of Rack, as it poses a risk to system stability and data security.

Vulnerability Summary

CVE ID: CVE-2025-59830
Severity: High (CVSS: 7.5)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Potential system compromise or data leakage due to increased CPU and memory consumption

Affected Products

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Product | Affected Versions

Rack | Prior to 2.2.18

How the Exploit Works

The vulnerability lies in the Rack::QueryParser module which enforces its params_limit only for parameters separated by &, while it should also split on ;. This allows attackers to bypass the parameter count limit by using ; separators to submit more parameters than intended. The impact is increased CPU and memory usage, which can be leveraged for a denial-of-service attack.

Conceptual Example Code

The following is a conceptual example of how the vulnerability might be exploited. This example uses a hypothetical HTTP request with a payload that abuses the parameter count bypass vulnerability.

POST /vulnerable/endpoint HTTP/1.1
Host: target.example.com
Content-Type: application/x-www-form-urlencoded
param1=value1&param2=value2;param3=value3;param4=value4;param5=value5;...;paramN=valueN

In the above example, the attacker is sending a POST request with more parameters than the server is designed to handle, using the ; character to bypass the parameter count limit.

Mitigation

It is recommended to apply the vendor patch as soon as possible. If immediate patching is not possible, using a Web Application Firewall (WAF) or Intrusion Detection System (IDS) can serve as temporary mitigation. The vulnerability has been patched in Rack version 2.2.18.

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Disclaimer:

The information and code presented in this article are provided for educational and defensive cybersecurity purposes only. Any conceptual or pseudocode examples are simplified representations intended to raise awareness and promote secure development and system configuration practices.

Do not use this information to attempt unauthorized access or exploit vulnerabilities on systems that you do not own or have explicit permission to test.

Ameeba and its authors do not endorse or condone malicious behavior and are not responsible for misuse of the content. Always follow ethical hacking guidelines, responsible disclosure practices, and local laws.

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