The vulnerability identified as CVE-2025-66415 affects the fastify-reply-from plugin, a component of the Fastify web framework used to forward HTTP requests to other servers. Prior to version 12.5.0, the plugin improperly validates or restricts URLs used in forwarding requests, allowing an attacker to craft malicious URLs that bypass intended route restrictions. This results in a confused deputy scenario (CWE-441), where the plugin inadvertently acts as an intermediary proxy to routes that should be inaccessible. Because the plugin forwards requests without sufficient validation, attackers can access internal or protected endpoints, potentially exposing sensitive data or enabling unauthorized actions. The vulnerability can be exploited remotely without any authentication or user interaction, increasing its risk profile. The CVSS 4.0 vector indicates network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), and limited impact on confidentiality and integrity (VC:L, VI:L), with no impact on availability or other security properties. The issue was publicly disclosed on December 1, 2025, and fixed in fastify-reply-from version 12.5.0. No public exploits have been reported yet, but the vulnerability's nature makes it a candidate for exploitation in environments where the plugin is used without patching. Fastify is popular in modern Node.js web applications, especially in microservices and API gateway scenarios, making this vulnerability relevant for organizations relying on these architectures.

For European organizations, this vulnerability poses a risk of unauthorized access to internal or restricted web routes, potentially exposing sensitive business logic, user data, or internal APIs. This could lead to data breaches, unauthorized data manipulation, or lateral movement within the network. Organizations using fastify-reply-from in microservices or API gateway roles are particularly vulnerable, as attackers could leverage this flaw to pivot into backend systems. The lack of authentication or user interaction requirements means attackers can exploit the vulnerability remotely and at scale. This could impact confidentiality and integrity of data, disrupt trust in web applications, and lead to regulatory compliance issues under GDPR if personal data is exposed. The medium severity score reflects the moderate but tangible risk, especially in environments with sensitive data or critical services exposed via Fastify-based applications.

European organizations should immediately upgrade fastify-reply-from to version 12.5.0 or later, where the vulnerability is fixed. If upgrading is not immediately feasible, implement strict input validation and URL filtering at the application or reverse proxy level to prevent malicious URL patterns from reaching the plugin. Employ network segmentation and access controls to limit exposure of internal routes. Conduct thorough code reviews and penetration testing focused on proxy and forwarding logic in Fastify applications. Monitor web application logs for unusual request patterns indicative of exploitation attempts. Additionally, consider deploying Web Application Firewalls (WAFs) with custom rules to detect and block suspicious forwarding requests. Educate developers about the risks of confused deputy vulnerabilities and secure coding practices related to request forwarding.