The vulnerability identified as CVE-2026-25521 affects the locutus JavaScript library, which provides standard libraries from other programming languages for JavaScript, primarily for educational purposes. Versions from 2.0.12 up to but not including 2.0.39 contain a prototype pollution flaw classified under CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes). Prototype pollution occurs when an attacker can modify the prototype of a base object, such as Object.prototype, thereby influencing all objects inheriting from it. Despite a prior fix that attempted to block forbidden keys in user input, attackers can still exploit the vulnerability by crafting input that manipulates String.prototype, effectively bypassing the check. This manipulation can lead to severe security consequences including arbitrary code execution, privilege escalation, denial of service, or data integrity issues. The vulnerability is exploitable without authentication or user interaction but requires local access (AV:L), indicating that the attacker must have some level of access to the environment where locutus is used. The CVSS 4.0 score of 9.4 reflects the critical nature of this vulnerability, with high impact on confidentiality, integrity, and availability, and low attack complexity. The issue was publicly disclosed on February 4, 2026, and patched in locutus version 2.0.39. No known exploits are currently reported in the wild, but the severity and ease of exploitation make it a significant threat. Organizations using affected versions should prioritize upgrading and review their code for unsafe handling of user inputs that could lead to prototype pollution.

For European organizations, the impact of this vulnerability can be substantial. Locutus is used in educational contexts and potentially in development environments that rely on JavaScript standard libraries. Prototype pollution can allow attackers to alter application behavior globally, leading to unauthorized access, data corruption, or denial of service. This can compromise sensitive data confidentiality, disrupt service availability, and undermine data integrity. Organizations involved in software development, education technology, or web application deployment are particularly at risk. The local attack vector means that insider threats or attackers who gain initial foothold can escalate their privileges or cause widespread damage. Given the critical CVSS score, the vulnerability could be leveraged in targeted attacks against European companies, especially those with complex JavaScript-based infrastructures or those using locutus as part of their development toolchain. The lack of known exploits in the wild currently provides a window for proactive mitigation before widespread exploitation occurs.

1. Immediate upgrade to locutus version 2.0.39 or later, which contains the patch for this vulnerability. 2. Conduct a thorough audit of all JavaScript codebases and dependencies to identify and remediate unsafe input handling that could lead to prototype pollution, especially where user input is used to modify objects. 3. Implement strict input validation and sanitization routines to prevent injection of malicious keys or properties. 4. Employ runtime protection mechanisms such as JavaScript security linters and static analysis tools that can detect prototype pollution patterns. 5. Restrict local access to environments running vulnerable locutus versions to trusted personnel only, minimizing the risk of local exploitation. 6. Monitor application logs and behavior for anomalies indicative of prototype pollution attacks, such as unexpected changes in object properties or application crashes. 7. Educate developers about prototype pollution risks and secure coding practices to prevent similar vulnerabilities in future code. 8. Consider using security-focused JavaScript frameworks or libraries that have built-in protections against prototype pollution.