@fastify/express vulnerable to middleware path doubling causing authentication bypass in child plugin scopes

published 2026-04-15 6 mo ago modified 2026-10-26 13 d ago also GHSA-hrwm-hgmj-7p9c , EUVD-2026-22880

cvss: 9.1; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.4%; EPSS 0.4% chance of exploitation in 30 days, percentile 37%
public exploits: 0; none known

@fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed a second time, causing it to never match incoming requests. This results in complete bypass of Express middleware security controls, including authentication, authorization, and rate limiting, for all routes defined within affected child plugin scopes. No special configuration or request crafting is required.

Upgrade to @fastify/express v4.0.5 or later.

Patch / fix

fixed in 2

npm @fastify/express 4.0.5
fastify @fastify/express 4.0.5

CISA SSVC

Exploitation: Proof of concept is public
Automatable: yes
Technical impact: total
Assessed: 6 mo ago (2026-04-15)

Exploit prediction

0.43% chance of exploitation in the next 30 days, higher than 37% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

github.com https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CNA	3.1	9.1	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
GitHub	3.1	9.1	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Affected packages

ecosystem	package	affected	fixed in
npm	@fastify/express	<4.0.5	4.0.5

Affected products

fastify @fastify/express

Classification

Weakness

CWE-436 Interpretation Conflict

References

github.com /fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c
exploit, web, mitigation, vendor-advisory
cna.openjsf.org /security-advisories.html
web, vendor-advisory
nvd.nist.gov /vuln/detail/CVE-2026-33807
advisory
github.com /fastify/fastify-express
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