CVE-2025-66415 Overview
CVE-2025-66415 is a medium-severity authorization bypass vulnerability in @fastify/reply-from, a popular Fastify plugin used to forward HTTP requests to another server. Prior to version 12.5.0, attackers could craft malicious URLs to access routes that are explicitly restricted, bypassing the route-specific forwarding configurations defined in reply.from. This vulnerability is classified as CWE-441 (Unintended Proxy or Intermediary), which occurs when a product acts as an intermediary that can be used to bypass access controls.
Critical Impact
Attackers can bypass route restrictions in Fastify applications using @fastify/reply-from to access unauthorized endpoints by crafting malicious URLs, potentially exposing sensitive internal routes or backend services.
Affected Products
- @fastify/reply-from versions prior to 12.5.0
- Fastify applications using vulnerable versions of the reply-from plugin
- Backend services proxied through affected Fastify instances
Discovery Timeline
- 2025-12-01 - CVE CVE-2025-66415 published to NVD
- 2025-12-02 - Last updated in NVD database
Technical Details for CVE-2025-66415
Vulnerability Analysis
This vulnerability has a CVSS 4.0 score of 6.9 (Medium severity) with the vector string CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X.
The attack vector is Network-based, requires low complexity, needs no privileges or user interaction, and affects both confidentiality and integrity at a low level. The EPSS (Exploit Prediction Scoring System) probability is 0.054% (16.928th percentile as of 2025-12-16), indicating a relatively low likelihood of exploitation in the wild.
Root Cause
The root cause of this vulnerability lies in improper URL validation within the @fastify/reply-from plugin. When the plugin processes incoming requests to determine which routes should be forwarded, it fails to properly sanitize or validate crafted URLs. This allows attackers to manipulate the URL structure in a way that bypasses the intended route restrictions defined by developers using reply.from for specific endpoints.
The vulnerability is categorized under CWE-441 (Unintended Proxy or Intermediary), which describes situations where a software component acting as a proxy or intermediary can be exploited to bypass access controls that would normally prevent direct access to protected resources.
Attack Vector
The attack exploits the URL parsing logic in @fastify/reply-from. When developers configure the plugin to forward requests only from specific routes, they expect that requests to other routes will not be proxied. However, by crafting a malicious URL with specific characters or encoding, an attacker can trick the plugin into forwarding requests that should be blocked.
This could allow unauthorized access to internal backend services, administrative endpoints, or sensitive API routes that were intended to be protected by the route-specific reply.from configuration. The vulnerability is particularly dangerous in microservice architectures where @fastify/reply-from is used as a reverse proxy to route traffic to internal services.
For technical details on the vulnerability mechanism, refer to the security advisory published by the Fastify maintainers.
Detection Methods for CVE-2025-66415
Indicators of Compromise
- Unusual URL patterns in access logs containing encoded characters or path traversal sequences targeting Fastify proxy endpoints
- Requests to internal routes or backend services that should not be accessible through the public-facing proxy
- Anomalous traffic patterns showing access to administrative or restricted endpoints without proper authentication
Detection Strategies
Organizations should monitor HTTP access logs for requests with malformed or suspiciously crafted URLs targeting Fastify applications using @fastify/reply-from. Web Application Firewalls (WAF) can be configured to detect and block URL manipulation attempts, including encoded path sequences and anomalous URL structures.
SentinelOne Singularity provides comprehensive protection against exploitation attempts through behavioral analysis and real-time monitoring of application-layer attacks. The platform can detect unauthorized access patterns and URL manipulation attempts targeting vulnerable proxy configurations.
Monitoring Recommendations
Implement logging for all proxied requests in Fastify applications, paying particular attention to URL patterns that differ from expected formats. Enable detailed access logging on backend services to correlate with frontend proxy logs and identify potential bypass attempts. Consider implementing rate limiting and anomaly detection for requests to proxy endpoints.
How to Mitigate CVE-2025-66415
Immediate Actions Required
- Upgrade @fastify/reply-from to version 12.5.0 or later immediately
- Audit current route configurations to ensure reply.from is properly restricting access
- Review access logs for any signs of exploitation or unauthorized route access
Patch Information
The vulnerability is fixed in @fastify/reply-from version 12.5.0. The fix is available in commit 4d9795cd5b57a36756d37b7f036eae369f69fa66. Organizations should update their dependencies as soon as possible using their package manager.
Workarounds
If immediate patching is not possible, consider implementing additional access controls at the network layer or through a Web Application Firewall. Restrict access to sensitive backend routes using IP allowlisting or VPN requirements. Implement strict URL validation at the application level before requests reach the @fastify/reply-from plugin.
# Update @fastify/reply-from to the patched version
npm update @fastify/reply-from@12.5.0
# Or with yarn
yarn upgrade @fastify/reply-from@12.5.0
# Verify the installed version
npm list @fastify/reply-from
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
