CVE-2025-13204 identifies a prototype pollution vulnerability in the silentmatt npm package expr-eval, which is widely used for parsing and evaluating mathematical expressions in JavaScript environments. Prototype pollution occurs when an attacker is able to modify the prototype of a base object, thereby influencing all objects inheriting from that prototype. In this case, the vulnerability stems from insufficient validation or control over input that is processed by expr-eval, allowing an attacker to inject or modify prototype attributes. This can lead to unexpected behavior, including arbitrary code execution, by exploiting JavaScript's prototype-based inheritance model. The vulnerability is classified under CWE-1321, which concerns improper control of modifications to object prototype attributes. Exploitation requires the attacker to have access to the expression evaluation interface, which may be exposed in web applications or services that use expr-eval to process user-supplied expressions. No CVSS score has been assigned yet, and no known exploits are reported in the wild. The expr-eval-fork package is noted as a remediation that addresses this issue, suggesting that the original expr-eval package has not yet been patched. This vulnerability is critical because prototype pollution can lead to severe impacts such as remote code execution, privilege escalation, or denial of service, depending on the context in which the vulnerable package is used.

For European organizations, the impact of this vulnerability can be significant, particularly for those relying on expr-eval within web applications, APIs, or backend services that evaluate user-supplied expressions. Successful exploitation could allow attackers to execute arbitrary code on affected systems, leading to data breaches, system compromise, or disruption of services. This is especially critical for sectors with high-value data or critical infrastructure, such as finance, healthcare, and government. Additionally, organizations using expr-eval in cloud-native or containerized environments may face lateral movement risks if attackers gain initial footholds. The lack of authentication requirements for exploitation (assuming the expression evaluation interface is accessible) increases the threat surface. The absence of known exploits currently provides a window for proactive mitigation, but the potential for rapid weaponization exists given the nature of prototype pollution vulnerabilities. The impact on confidentiality, integrity, and availability is high, as arbitrary code execution can compromise all three.

European organizations should immediately audit their software dependencies to identify usage of the expr-eval package. Where possible, migrate to the expr-eval-fork package, which addresses this vulnerability. If migration is not immediately feasible, implement strict input validation and sanitization on all user-supplied expressions processed by expr-eval to prevent injection of malicious prototype modifications. Restrict access to the expression evaluation interface to trusted users or internal networks only, employing network segmentation and access controls. Employ runtime application self-protection (RASP) or Web Application Firewalls (WAFs) with custom rules to detect and block suspicious payloads targeting prototype pollution. Monitor application logs for anomalous behavior indicative of prototype manipulation attempts. Additionally, consider implementing Content Security Policy (CSP) and other browser security features if the vulnerable package is used in client-side code. Finally, maintain up-to-date software inventories and subscribe to vulnerability advisories to apply patches promptly once available.