The vulnerability identified as CVE-2025-55164 affects the content-security-policy-parser component of the helmetjs project, specifically versions prior to 0.6.0. This component is responsible for parsing Content Security Policy (CSP) directives, which are critical for defining security policies that restrict resources the browser is allowed to load. The vulnerability is a prototype pollution issue (CWE-1321), where an attacker can manipulate the JavaScript Object prototype by using a specially crafted policy name '__proto__'. This allows the attacker to override or inject properties into the Object prototype, potentially altering the behavior of all objects in the application. Prototype pollution can lead to severe consequences including arbitrary code execution, denial of service, or bypassing security controls. The vulnerability is exploitable remotely without authentication or user interaction, as indicated by the CVSS vector (AV:N/AC:L/AT:N/UI:N/PR:N). The CVSS score of 8.8 (high severity) reflects the critical nature of this flaw, given its potential to impact confidentiality, integrity, and availability. The issue has been patched in version 0.6.0 of content-security-policy-parser. As a workaround, Node.js can be started with the --disable-proto=delete or --disable-proto=throw flags to neutralize prototype pollution attacks by disabling prototype modification methods. No known exploits are currently reported in the wild, but the vulnerability's characteristics suggest it could be weaponized if discovered by attackers.

For European organizations, this vulnerability poses a significant risk, especially for those relying on helmetjs for securing web applications. Since helmetjs is widely used in Node.js environments to enforce security headers including CSP, exploitation could allow attackers to bypass CSP protections, leading to cross-site scripting (XSS) or other injection attacks. This undermines the confidentiality and integrity of user data and can disrupt service availability. Organizations in sectors such as finance, healthcare, government, and critical infrastructure are particularly at risk due to the sensitive nature of their data and regulatory requirements like GDPR. Exploitation could result in data breaches, regulatory fines, reputational damage, and operational disruptions. The remote, unauthenticated nature of the exploit increases the threat surface, making it easier for attackers to target vulnerable systems at scale. Additionally, compromised CSP enforcement could facilitate further attacks against European users and systems, amplifying the impact.

European organizations should immediately upgrade the content-security-policy-parser dependency to version 0.6.0 or later to apply the official patch. For environments where immediate upgrade is not feasible, Node.js should be run with the --disable-proto=delete flag, which is recommended to prevent prototype pollution by disabling prototype modification methods. Alternatively, --disable-proto=throw can be used to throw errors on such attempts. Security teams should audit their dependency trees to identify all instances of helmetjs and its content-security-policy-parser component, including transitive dependencies, to ensure no vulnerable versions remain. Implement runtime application self-protection (RASP) or Web Application Firewall (WAF) rules to detect and block suspicious payloads attempting prototype pollution. Continuous monitoring and logging should be enhanced to detect anomalous behavior indicative of exploitation attempts. Finally, developers should be trained to recognize prototype pollution risks and avoid unsafe object property assignments in custom code.