CVE-2025-55164 is a high-severity prototype pollution vulnerability identified in the content-security-policy-parser component of the helmetjs project, affecting all versions prior to 0.6.0. The vulnerability arises from improper handling of object prototype attributes when parsing Content Security Policy (CSP) directives. Specifically, if an attacker crafts a CSP directive with the policy name '__proto__', they can manipulate the Object prototype in JavaScript. This manipulation allows the attacker to inject or modify properties on the global Object prototype, potentially altering the behavior of the application or its dependencies in unexpected ways. Prototype pollution can lead to severe consequences including denial of service, arbitrary code execution, or bypassing security controls. The vulnerability does not require any authentication or user interaction and can be exploited remotely over the network. The issue has been addressed in version 0.6.0 of content-security-policy-parser. As a temporary mitigation, Node.js offers a runtime flag (--disable-proto=delete or --disable-proto=throw) to disable prototype modification methods, effectively neutralizing prototype pollution attacks. The CVSS 4.0 base score of 8.8 reflects the vulnerability’s high impact on confidentiality and integrity, with no required privileges or user interaction, and network attack vector. No known exploits are currently reported in the wild, but the severity and ease of exploitation make it a critical patching priority for affected systems.

For European organizations, this vulnerability poses a significant risk, especially for those relying on helmetjs for securing web applications via Content Security Policy enforcement. Exploitation could allow attackers to manipulate application logic, bypass security policies, or cause denial of service, potentially leading to data breaches or service disruptions. Given the widespread use of Node.js and helmetjs in web development across Europe, especially in sectors like finance, healthcare, and government where CSP is critical for preventing cross-site scripting and other web attacks, the impact could be substantial. Compromised web applications could lead to exposure of sensitive personal data protected under GDPR, resulting in regulatory penalties and reputational damage. Additionally, prototype pollution can be a stepping stone for further attacks, increasing the overall threat surface.

European organizations should immediately upgrade the content-security-policy-parser package to version 0.6.0 or later to apply the official patch. Until the upgrade can be performed, Node.js applications should be run with the --disable-proto=delete flag (recommended) or --disable-proto=throw to prevent prototype pollution at runtime. Code audits should be conducted to identify any usage of vulnerable versions of helmetjs or its dependencies. Security teams should monitor for unusual application behavior indicative of prototype pollution exploitation. Incorporating runtime application self-protection (RASP) or Web Application Firewalls (WAFs) with rules targeting prototype pollution patterns can provide additional defense layers. Finally, developers should be trained to avoid unsafe handling of untrusted input in JavaScript objects and to follow secure coding practices to minimize prototype pollution risks.