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CVE-2025-53890: Critical JavaScript Evaluation Vulnerability in pyLoad’s CAPTCHA Processing Code

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Overview

We are currently investigating a critical vulnerability, CVE-2025-53890, that resides within the CAPTCHA processing code of pyLoad, a popular open-source download manager written in Python. This vulnerability could potentially affect thousands of users who rely on pyLoad for managing their downloads. The severity of this issue is underlined by its CVSS Severity Score of 9.8, which signifies a critical impact. The flaw can allow unauthenticated remote attackers to execute arbitrary code, resulting in severe consequences such as session hijacking, credential theft, and even full system remote code execution.

Vulnerability Summary

CVE ID: CVE-2025-53890
Severity: Critical (9.8/10)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Session hijacking, Credential theft, Full system remote code execution

Affected Products

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Product | Affected Versions

pyLoad | Prior to 0.5.0b3.dev89

How the Exploit Works

The vulnerability lies in pyLoad’s CAPTCHA processing code. It is an unsafe JavaScript evaluation vulnerability, which means it allows the execution of arbitrary code in the client browser without any form of user interaction or authentication. This code execution can extend to the backend server and can be exploited by remote attackers. The vulnerability can lead to a full system compromise, allowing attackers to hijack sessions, steal credentials, and execute code remotely.

Conceptual Example Code

To illustrate how an attacker might exploit this vulnerability, consider the following hypothetical HTTP request:

POST /pyload/captcha/endpoint HTTP/1.1
Host: target.example.com
Content-Type: application/json
{ "captcha_solution": "eval('malicious_code')" }

In this example, the attacker is embedding malicious JavaScript code in the `captcha_solution` field. When this request is processed by the server, it evaluates the malicious JavaScript code leading to the potential compromise of the system.

Mitigation Guidance

Users are strongly urged to update their pyLoad software to version 0.5.0b3.dev89 or later where the patch for this issue has been included. If updating is not an immediate option, using a Web Application Firewall (WAF) or Intrusion Detection System (IDS) can offer temporary mitigation. However, these are temporary measures and updating the software is the most reliable way to ensure protection against this severe vulnerability.

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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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