CVE-2025-64718 is a medium-severity prototype pollution vulnerability affecting the js-yaml library, a widely used JavaScript YAML parser and dumper. Versions 4.1.0 and earlier allow an attacker to manipulate the prototype of the object returned after parsing a maliciously crafted YAML document. This occurs because the library does not properly sanitize or restrict the use of the `__proto__` property within YAML input, enabling modification of the object's prototype chain. Prototype pollution can lead to unexpected behavior in applications, such as altering object properties or methods globally, which may be leveraged to bypass security controls or cause logic errors. The vulnerability requires no privileges or user interaction and can be exploited remotely if untrusted YAML input is parsed. The issue is addressed in js-yaml 4.1.1, which includes proper validation to prevent prototype pollution. Additionally, Node.js runtime mitigations like `--disable-proto=delete` can reduce risk by disabling prototype pollution via deletion operations. Deno runtime users benefit from built-in pollution protection by default. No known exploits are currently reported in the wild, but the ease of exploitation and common usage of js-yaml in server-side applications make timely remediation important.

For European organizations, this vulnerability primarily threatens the integrity of applications that parse untrusted YAML data using vulnerable js-yaml versions. Potential impacts include unauthorized modification of application logic, bypassing security checks, or causing application errors, which can lead to further exploitation or data manipulation. While confidentiality and availability are not directly affected, integrity compromises can undermine trust in application behavior and data correctness. Industries relying on Node.js backend services, configuration management, or CI/CD pipelines that utilize YAML parsing are particularly at risk. Attackers exploiting this vulnerability could gain footholds or escalate privileges indirectly by manipulating application state. The medium CVSS score reflects moderate risk, but the widespread use of js-yaml in European tech ecosystems means the vulnerability could have broad reach if unpatched. Organizations processing YAML from external or user-generated sources must consider this a significant risk vector.

1. Upgrade all instances of js-yaml to version 4.1.1 or later immediately to apply the official patch preventing prototype pollution. 2. Audit codebases to identify where untrusted YAML input is parsed and restrict or sanitize such inputs rigorously. 3. Employ Node.js runtime flag `--disable-proto=delete` to disable prototype pollution via deletion operations as an additional safeguard. 4. For environments using Deno, ensure the latest runtime is used to benefit from built-in pollution protections. 5. Implement strict input validation and schema enforcement for YAML data to prevent injection of malicious properties like `__proto__`. 6. Monitor application logs for unusual object behavior or errors that might indicate exploitation attempts. 7. Educate development teams about prototype pollution risks and secure coding practices related to object property handling. 8. Consider employing runtime application self-protection (RASP) or Web Application Firewalls (WAFs) with rules targeting prototype pollution attack patterns. These steps go beyond generic patching by incorporating runtime and input validation defenses tailored to this vulnerability.