The vulnerability identified as CVE-2025-64718 affects the js-yaml library, a widely used JavaScript YAML parser and dumper maintained by nodeca. Versions 4.0.0 up to but not including 4.1.1, and all versions below 3.14.2, are vulnerable to prototype pollution via the __proto__ property in YAML documents. Prototype pollution occurs when an attacker can inject or modify properties on an object's prototype, thereby influencing all objects inheriting from that prototype. In this case, parsing a maliciously crafted YAML document allows an attacker to alter the prototype of the resulting JavaScript object, potentially causing unexpected behavior or security issues in applications relying on js-yaml for configuration or data processing. The attack vector is remote and requires no privileges or user interaction, making it feasible for attackers to exploit exposed services that parse untrusted YAML input. The vulnerability impacts integrity (I) but not confidentiality (C) or availability (A), as per the CVSS vector. The issue was addressed in js-yaml versions 4.1.1 and 3.14.2 by sanitizing or restricting prototype modifications during parsing. Additional mitigation includes running Node.js with the --disable-proto=delete flag, which prevents prototype pollution via deletion operations, or using the Deno runtime environment, which has pollution protections enabled by default. No public exploits have been reported yet, but the vulnerability's nature and ease of exploitation warrant prompt remediation.

For European organizations, this vulnerability poses a risk primarily to the integrity of applications that parse YAML input using vulnerable js-yaml versions. Attackers could manipulate application logic, bypass security controls, or cause erratic behavior by injecting prototype pollution payloads. This can lead to compromised application workflows, data corruption, or indirect privilege escalation if the polluted prototype affects security-critical code paths. Organizations in sectors such as finance, healthcare, and critical infrastructure that rely on Node.js-based services or microservices parsing YAML configurations or data are particularly at risk. The medium severity rating reflects that while confidentiality and availability are not directly impacted, the integrity compromise can have cascading effects on trustworthiness and operational correctness. Given the widespread use of js-yaml in the JavaScript ecosystem, the scope of affected systems is broad, increasing the potential attack surface. The lack of required authentication and user interaction further elevates the risk, especially for publicly accessible services.

European organizations should immediately identify all instances of js-yaml usage in their software stacks, including direct dependencies and transitive dependencies in Node.js projects. Upgrading to js-yaml versions 4.1.1 or 3.14.2 is the most effective mitigation. For environments where immediate upgrade is not feasible, running Node.js with the --disable-proto=delete flag can help prevent prototype pollution attacks by disabling prototype deletion operations. Additionally, migrating to the Deno runtime environment can provide built-in protection against such pollution. Organizations should also implement strict input validation and sanitization for YAML inputs, especially if sourced from untrusted or external origins. Employing runtime application self-protection (RASP) or behavior monitoring to detect anomalous prototype modifications can provide additional defense layers. Regular dependency audits and integrating software composition analysis (SCA) tools into CI/CD pipelines will help detect vulnerable versions early. Finally, educating developers about the risks of prototype pollution and secure YAML parsing practices is crucial to prevent future vulnerabilities.