Fastify Middie Middleware Path Bypass

published 2026-01-19 9 mo ago modified 2026-07-08 4 mo ago also GHSA-cxrg-g7r8-w69p , EUVD-2026-3321

cvss: 8.4; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.5%; EPSS 0.5% chance of exploitation in 30 days, percentile 45%
public exploits: 0; none known

@fastify/middie is the plugin that adds middleware support on steroids to Fastify. A security vulnerability exists in @fastify/middie prior to version 9.1.0 where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., `/%61dmin` instead of `/admin`). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints. Version 9.1.0 fixes the issue.

Patch / fix

fixed in 2 links 3

npm @fastify/middie 9.1.0
fastify fastify/middie 9.1.0

github.com/fastify/middie@d44cd56 commit
github.com/fastify/middie#245 pull request
github.com/fastify/middie/releases/tag/v9.1.0 release

CISA SSVC

Exploitation: Proof of concept is public
Automatable: no
Technical impact: partial
Assessed: 9 mo ago (2026-01-20)

Exploit prediction

0.53% chance of exploitation in the next 30 days, higher than 45% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

github.com https://github.com/fastify/middie/security/advisories/GHSA-cxrg-g7r8-w69p referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CNA	3.1	8.4	CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L
NVD	3.1	8.8	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
GitHub	3.1	8.4	CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L

Affected packages

ecosystem	package	affected	fixed in
npm	@fastify/middie	<9.1.0	9.1.0

Affected products

fastify middie

Classification

Weakness

CWE-177 Improper Handling of URL Encoding (Hex Encoding)

References

github.com /fastify/middie/pull/245
web, issue-tracking, patch
github.com /fastify/middie/commit/d44cd56eb724490babf7b452fdbbdd37ea2effba
web, patch
github.com /fastify/middie/security/advisories/GHSA-cxrg-g7r8-w69p
web, exploit, vendor-advisory
github.com /fastify/middie/releases/tag/v9.1.0
web, product, release-notes
nvd.nist.gov /vuln/detail/CVE-2026-22031
advisory
github.com /fastify/middie
package

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary