CVE-2026-26021 is a prototype pollution vulnerability classified under CWE-1321 affecting the npm package set-in, specifically versions from 2.0.1 up to but not including 2.0.5. The set-in package is designed to set values in nested associative structures based on an array of keys. Prototype pollution occurs when an attacker can manipulate the prototype of a base object, such as Object.prototype, thereby injecting or modifying properties that affect all objects inheriting from it. Despite a prior fix that attempted to block forbidden keys, attackers can still exploit this vulnerability by leveraging Array.prototype keys to bypass these checks and pollute Object.prototype. This can lead to unexpected behavior in applications, including arbitrary code execution, denial of service, or data corruption. The vulnerability does not require authentication or user interaction, making it easier to exploit in vulnerable environments. The issue was publicly disclosed on February 11, 2026, with a CVSS 4.0 score of 9.4, indicating critical severity. The fix was introduced in version 2.0.5 of set-in, which properly mitigates the prototype pollution vector. No known exploits are currently reported in the wild, but the high severity and ease of exploitation make it a significant threat to applications relying on this package.

For European organizations, the impact of CVE-2026-26021 can be substantial, especially those heavily reliant on Node.js and npm packages in their software development and production environments. Prototype pollution can lead to widespread application instability, data integrity issues, and potential remote code execution, which threatens confidentiality, integrity, and availability of systems. This vulnerability can be exploited remotely without authentication or user interaction, increasing the risk of automated attacks or supply chain compromises. Organizations using vulnerable versions of set-in in web applications, backend services, or internal tools may face service disruptions, data breaches, or unauthorized privilege escalation. The risk is amplified in sectors such as finance, healthcare, and critical infrastructure, where data sensitivity and operational continuity are paramount. Additionally, the vulnerability could be leveraged as part of multi-stage attacks targeting European enterprises, potentially impacting compliance with GDPR and other regulatory frameworks.

1. Immediately upgrade all instances of the set-in package to version 2.0.5 or later, which contains the fix for this vulnerability. 2. Conduct a thorough audit of all software dependencies to identify and remediate any transitive dependencies that include vulnerable versions of set-in. 3. Implement strict dependency management policies, including the use of tools like npm audit, Snyk, or Dependabot, to detect and alert on vulnerable packages proactively. 4. Employ runtime application self-protection (RASP) or Web Application Firewalls (WAFs) with rules designed to detect and block prototype pollution attack patterns. 5. Review and harden application code to avoid unsafe usage of user-supplied input in object property assignments, especially when dealing with nested structures. 6. Incorporate static and dynamic code analysis tools in the CI/CD pipeline to catch prototype pollution risks early. 7. Educate development teams about prototype pollution risks and secure coding practices related to object manipulation in JavaScript. 8. Monitor logs and application behavior for anomalies that could indicate exploitation attempts.