Vulnerability Description
@fastify/middie versions 9.1.0 through 9.3.2 decode the encoded slash %2F inside path parameter values before matching middleware paths, while Fastify's underlying router preserves the encoding during route lookup. The two layers disagree on the canonical request path, so the middleware fails to match a URL that the route handler does match. When middleware is used for authentication, authorization, rate limiting, or auditing on parameterized paths, an attacker can reach the protected handler by sending a single crafted URL with an encoded slash in the parameter position. The bypass is HTTP method agnostic and requires no authentication or special preconditions. Patches: upgrade to @fastify/middie 9.3.3. Workarounds: avoid parameterized middleware paths for security decisions, or enforce authentication at the route handler or via a Fastify hook that runs after the router has resolved the request.
CVSS Score
CRITICAL
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Fastify | Fastify\/Middie | >= 9.1.0, < 9.3.3 |
Related Weaknesses (CWE)
References
- https://cna.openjsf.org/security-advisories.htmlVendor Advisory
- https://github.com/fastify/middie/security/advisories/GHSA-2v46-jxjm-7q3vMitigationVendor Advisory
FAQ
What is CVE-2026-14198?
CVE-2026-14198 is a vulnerability with a CVSS score of 9.1 (CRITICAL). @fastify/middie versions 9.1.0 through 9.3.2 decode the encoded slash %2F inside path parameter values before matching middleware paths, while Fastify's underlying router preserves the encoding during...
How severe is CVE-2026-14198?
CVE-2026-14198 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-2026-14198?
Check the references section above for vendor advisories and patch information. Affected products include: Fastify Fastify\/Middie.