@fastify/middie is a middleware plugin for the Fastify web framework that enhances middleware support. The vulnerability identified as CVE-2026-22031 arises from improper handling of URL encoding (CWE-177) in path prefix matching within @fastify/middie versions prior to 9.1.0. Specifically, when middleware is registered with a path prefix (e.g., /admin), the middleware engine matches requests against this prefix to determine if middleware should execute. However, if an attacker sends a request with URL-encoded characters representing the prefix (e.g., /%61dmin where %61 is 'a'), the middleware engine fails to decode and match this path, thus skipping middleware execution. Meanwhile, Fastify's internal router correctly decodes the URL-encoded path and matches the route handler, allowing the request to reach protected endpoints without middleware constraints such as authentication or logging. This discrepancy creates an access control bypass, potentially exposing sensitive endpoints. The vulnerability requires network access and low privileges but no user interaction. The CVSS 3.1 score of 8.4 indicates high severity, with critical impacts on confidentiality and integrity, and limited impact on availability. No known exploits have been reported in the wild as of the publication date. The issue is fixed in @fastify/middie version 9.1.0. Organizations using affected versions should upgrade promptly to prevent unauthorized access.

For European organizations, this vulnerability poses a significant risk of unauthorized access to protected web application endpoints that rely on @fastify/middie for middleware enforcement. This can lead to exposure of sensitive data, unauthorized actions, and potential compromise of application integrity. Industries with strict data protection requirements such as finance, healthcare, and government are particularly at risk. The bypass of middleware could circumvent authentication, logging, or input validation middleware, increasing the risk of data breaches and regulatory non-compliance under GDPR. Since Fastify is popular in modern Node.js web applications, organizations using it in critical infrastructure or customer-facing services could face reputational damage and operational disruption. The lack of user interaction and low privilege requirements make exploitation feasible by remote attackers scanning for vulnerable endpoints. Although no exploits are currently known, the high CVSS score and nature of the vulnerability warrant urgent remediation to mitigate potential future attacks.

1. Upgrade @fastify/middie to version 9.1.0 or later immediately to apply the fix for this vulnerability. 2. Audit all Fastify applications to identify usage of @fastify/middie and verify middleware path prefixes for potential bypass. 3. Implement additional server-side validation and access controls at the route handler level to reduce reliance solely on middleware for critical security functions. 4. Employ Web Application Firewalls (WAFs) with rules to detect and block suspicious URL-encoded requests targeting sensitive paths. 5. Monitor application logs for unusual access patterns involving URL-encoded paths that could indicate exploitation attempts. 6. Conduct security testing including fuzzing and penetration testing focusing on URL encoding edge cases to uncover similar issues. 7. Educate development teams on secure middleware usage and the importance of consistent URL decoding in all request processing layers. 8. Maintain an inventory of all web applications using Fastify and middleware plugins to ensure timely patch management.