CVE-2025-66415 identifies a vulnerability in the fastify-reply-from plugin, a component used within the Fastify Node.js framework to forward HTTP requests to other servers. The vulnerability is classified as CWE-441, an unintended proxy or intermediary ('confused deputy') issue. In versions prior to 12.5.0, the plugin fails to properly restrict which routes can be forwarded. By crafting a malicious URL, an attacker can exploit this flaw to access backend routes that should be off-limits, effectively bypassing route-based access controls. This occurs because the plugin forwards requests based on the URL without sufficient validation, allowing unauthorized access to internal services or sensitive endpoints. The vulnerability has a CVSS 4.0 base score of 6.9 (medium severity), reflecting its network attack vector, low attack complexity, no privileges or user interaction required, and limited impact on confidentiality and integrity. No known exploits have been reported in the wild as of the publication date. The issue is resolved in fastify-reply-from version 12.5.0, which implements stricter route validation and forwarding controls to prevent unauthorized access. Organizations using Fastify with this plugin should audit their versions and upgrade promptly to avoid exploitation.

For European organizations, exploitation of this vulnerability could lead to unauthorized access to internal or sensitive backend routes, potentially exposing confidential data or allowing attackers to interact with internal services not intended for public access. This could undermine the confidentiality and integrity of data processed by web applications using fastify-reply-from. The vulnerability does not require authentication or user interaction, increasing the risk of automated exploitation. While availability impact is minimal, unauthorized access could facilitate further attacks or data leakage. Organizations in sectors with high reliance on web services—such as finance, healthcare, and government—may face increased risk due to the sensitive nature of their data and services. Additionally, the widespread use of Node.js and Fastify in European startups and enterprises means a broad attack surface. Failure to patch could also lead to reputational damage and regulatory penalties under GDPR if personal data is exposed.

The primary mitigation is to upgrade fastify-reply-from to version 12.5.0 or later, where the vulnerability is fixed. Organizations should audit their codebases and dependencies to identify any usage of vulnerable versions. Beyond upgrading, developers should implement strict validation and whitelisting of URLs and routes that can be forwarded by the plugin to prevent unauthorized access. Employing Web Application Firewalls (WAFs) with rules to detect and block suspicious URL patterns targeting proxy endpoints can provide additional defense. Regular security testing, including penetration testing focused on proxy and forwarding mechanisms, can help detect misconfigurations. Monitoring logs for unusual access patterns to internal routes can aid in early detection of exploitation attempts. Finally, adopting a defense-in-depth approach by segmenting internal services and enforcing least privilege access reduces the impact of any potential compromise.