CVE-2025-57351 is a prototype pollution vulnerability identified in the ts-fns package, a JavaScript utility library. The vulnerability arises from insufficient validation of user-supplied keys in the assign function, which allows attackers to manipulate the Object.prototype chain. Prototype pollution occurs when an attacker can inject or modify properties on the global Object prototype, thereby affecting all objects that inherit from it. In this case, the flaw is due to improper handling of deep property assignments within the library's public API, enabling adversaries to inject arbitrary properties into the global prototype. This manipulation can lead to various adverse effects, including application crashes, unexpected or malicious code execution behaviors, and bypasses of security-critical validation logic that relies on the integrity of the prototype chain. The vulnerability affects versions of ts-fns prior to 13.0.7 and, notably, remains unpatched as of the latest available version. The CVSS score assigned is 6.5 (medium severity), with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), limited confidentiality impact (C:L), no integrity impact (I:N), and low availability impact (A:L). No known exploits are reported in the wild yet. The underlying CWE is 1321, which relates to improper handling of prototype pollution vulnerabilities. This vulnerability is critical in environments where ts-fns is used to process untrusted input, as it can be exploited remotely without authentication or user interaction, potentially compromising application stability and security.

For European organizations, the impact of CVE-2025-57351 can be significant, especially for those relying on JavaScript-based applications or services that incorporate the ts-fns package for date/time or utility functions. Prototype pollution can lead to application instability, crashes, or unexpected behavior, which may disrupt business operations. More critically, the ability to inject arbitrary properties into the global prototype can be leveraged to bypass security controls, potentially allowing attackers to evade input validation or security checks, leading to further exploitation such as remote code execution or privilege escalation in complex attack chains. This risk is heightened in web applications exposed to the internet or processing untrusted user input. The medium CVSS score reflects a moderate risk, but the lack of required privileges and user interaction increases the likelihood of exploitation. European organizations in sectors such as finance, healthcare, and critical infrastructure, which often use JavaScript frameworks extensively, could face operational disruptions and data integrity issues. Additionally, regulatory frameworks like GDPR impose strict requirements on data protection and security; exploitation of this vulnerability could lead to compliance violations and reputational damage.

To mitigate CVE-2025-57351, European organizations should take the following specific actions: 1) Identify all applications and services using the ts-fns package, particularly versions prior to 13.0.7. 2) Since no official patch is currently available, consider temporarily removing or replacing the assign function usage with safer alternatives that do not allow prototype pollution, or implement strict input validation and sanitization to reject keys that could manipulate the prototype chain (e.g., keys like '__proto__', 'constructor', or 'prototype'). 3) Employ runtime application self-protection (RASP) or Web Application Firewalls (WAFs) configured to detect and block prototype pollution attack patterns. 4) Conduct thorough code reviews and static analysis to identify unsafe deep property assignments in the codebase. 5) Monitor application logs and behavior for anomalies indicative of prototype pollution exploitation attempts. 6) Engage with the ts-fns maintainers or community to track patch releases and apply updates promptly once available. 7) Educate developers about the risks of prototype pollution and secure coding practices to prevent similar vulnerabilities. These steps go beyond generic advice by focusing on immediate code-level mitigations and proactive monitoring in the absence of an official patch.