CVE-2025-13204 identifies a prototype pollution vulnerability in the npm package expr-eval, maintained by silentmatt. Prototype pollution occurs when an attacker can modify the prototype of a base object, thereby influencing all objects inheriting from it. In this case, the expr-eval package, which parses and evaluates mathematical expressions in JavaScript, improperly controls modifications to object prototype attributes. An attacker with access to the expression evaluation interface can inject malicious payloads that alter the prototype chain, enabling arbitrary code execution within the context of the vulnerable application. This vulnerability is particularly dangerous because it requires no authentication or user interaction, and can be exploited remotely over the network. The CVSS v3.1 base score of 7.3 reflects a high severity, with network attack vector, low attack complexity, no privileges required, and no user interaction needed. The impact includes potential leakage of sensitive data (confidentiality), unauthorized modification of application state or data (integrity), and disruption or denial of service (availability). The vulnerability affects all versions of expr-eval, with no official patch available from silentmatt at the time of publication; however, the expr-eval-fork package addresses the issue. No known exploits have been reported in the wild yet, but the nature of prototype pollution and the widespread use of expr-eval in JavaScript projects make this a critical risk.

For European organizations, the impact of CVE-2025-13204 can be substantial, especially for those relying on JavaScript-based web applications, server-side Node.js services, or any software components that incorporate the expr-eval package. Exploitation could lead to unauthorized code execution, allowing attackers to escalate privileges, exfiltrate sensitive data, manipulate business logic, or disrupt services. This can result in data breaches, financial losses, reputational damage, and regulatory penalties under GDPR. The vulnerability’s network accessibility and lack of authentication requirements increase the attack surface, making it easier for threat actors to target organizations remotely. Industries with critical infrastructure, financial services, healthcare, and government sectors in Europe are particularly vulnerable due to the sensitivity of their data and services. Additionally, supply chain risks arise if expr-eval is embedded in third-party software or libraries used by European companies, potentially propagating the vulnerability widely.

European organizations should immediately audit their software dependencies to identify usage of the expr-eval package. Where found, they should replace expr-eval with the expr-eval-fork package or other secure alternatives that have addressed the prototype pollution issue. If replacement is not immediately feasible, implement runtime protections such as JavaScript sandboxing, prototype pollution detection libraries, or strict input validation to prevent malicious payloads from reaching the expression evaluation interface. Conduct thorough code reviews focusing on areas where user input is parsed or evaluated dynamically. Employ Web Application Firewalls (WAFs) with custom rules to detect and block suspicious prototype pollution attack patterns. Monitor application logs for anomalies indicative of prototype manipulation attempts. Finally, maintain an incident response plan tailored to code injection and prototype pollution scenarios to minimize impact if exploitation occurs.