Vulnerability Description
@fastify/express versions 4.0.6 and earlier only rewrite the plugin prefix for middleware mount paths when the path argument is a string. Non-string mount paths (arrays of paths and regular expressions) are left unprefixed inside prefixed plugin scopes, so middleware registered with those forms does not match the actual prefixed request path. Applications that use path-scoped middleware for authentication, authorization, rate limiting, or auditing on routes inside a prefixed scope can be bypassed by sending a request to the prefixed route, because Fastify still matches the route but the middleware is skipped. Patches: upgrade to @fastify/express 4.0.7. Workarounds: use string mount paths instead of arrays or regular expressions in prefixed plugins, or register one use call per path.
CVSS Score
CRITICAL
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Fastify | Fastify\/Express | < 4.0.7 |
Related Weaknesses (CWE)
References
- https://cna.openjsf.org/security-advisories.htmlVendor Advisory
- https://github.com/fastify/fastify-express/security/advisories/GHSA-3wf5-7852-vcMitigationVendor Advisory
FAQ
What is CVE-2026-6556?
CVE-2026-6556 is a vulnerability with a CVSS score of 9.1 (CRITICAL). @fastify/express versions 4.0.6 and earlier only rewrite the plugin prefix for middleware mount paths when the path argument is a string. Non-string mount paths (arrays of paths and regular expression...
How severe is CVE-2026-6556?
CVE-2026-6556 has been rated CRITICAL with a CVSS base score of 9.1/10. This is considered a critical vulnerability requiring immediate attention.
Is there a patch for CVE-2026-6556?
Check the references section above for vendor advisories and patch information. Affected products include: Fastify Fastify\/Express.