CVE-2026-25521 is a prototype pollution vulnerability classified under CWE-1321 affecting the locutus JavaScript library, which provides standard libraries from other programming languages for JavaScript usage primarily for educational purposes. The vulnerability exists in versions from 2.0.12 up to but not including 2.0.39. Despite a prior fix that attempted to block prototype pollution by filtering forbidden keys in user input, attackers can still exploit the vulnerability by crafting inputs that manipulate String.prototype, thereby polluting Object.prototype. Prototype pollution occurs when an attacker can inject or modify properties on the base Object prototype, which all JavaScript objects inherit from, potentially altering application logic, bypassing security controls, or causing application crashes. This vulnerability does not require authentication or user interaction, making it easier to exploit in environments where locutus is used to process untrusted input. The CVSS 4.0 base score is 9.4 (critical), reflecting high impact on confidentiality, integrity, and availability, with low attack complexity and no privileges required. Although no known exploits are currently reported in the wild, the severity and ease of exploitation make this a significant threat. The issue was publicly disclosed on February 4, 2026, and patched in locutus version 2.0.39. Organizations using affected versions should upgrade immediately and review any code that utilizes locutus functions to handle external input to prevent exploitation.

For European organizations, the impact of this vulnerability can be substantial, especially for those relying on locutus in development environments, educational platforms, or production systems that process untrusted JavaScript inputs. Exploitation can lead to arbitrary code execution, data tampering, or denial of service by corrupting the prototype chain, which can compromise application logic and security controls. This can result in data breaches, service outages, and loss of trust. Since locutus is used to emulate standard libraries from other languages, the vulnerability could affect a wide range of applications that depend on it for cross-language functionality or educational purposes. The lack of authentication and user interaction requirements increases the risk of automated exploitation attempts. Additionally, the critical severity score indicates that the vulnerability could be leveraged in targeted attacks against European organizations, potentially impacting sectors such as finance, education, and technology where JavaScript usage is prevalent.

The primary mitigation is to upgrade locutus to version 2.0.39 or later, where the vulnerability has been patched. Organizations should audit their codebases to identify any usage of locutus versions between 2.0.12 and 2.0.38, especially where user input is processed or passed to locutus functions. Implement strict input validation and sanitization to prevent malicious payloads from reaching vulnerable code paths. Employ runtime application self-protection (RASP) or Web Application Firewalls (WAFs) with rules designed to detect and block prototype pollution attack patterns. Conduct thorough testing after upgrading to ensure no residual vulnerabilities remain. Additionally, monitor security advisories for any emerging exploit code or related vulnerabilities. For development teams, educate about the risks of prototype pollution and encourage secure coding practices when handling JavaScript objects and third-party libraries.