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Vulnerability record · CVE-2026-33807 · published 15 April 2026

CVE-2026-33807: Fastify\/express interpretation conflict vulnerability

Fastify · Fastify\/Express

@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.

9.1 CVSS 3.1 Critical EPSS 0.53% · top 57.2% CWE-436 · Interpretation conflict
9.1CVSS 3.1 base score
0.53%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
3References, 2 tagged exploit
17 Jun 2026Last modified by NVD

Description

@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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://cna.openjsf.org/security-advisories.html Vendor Advisory
https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c ExploitMitigationVendor Advisory
https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c ExploitMitigationVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

9.1CVE-2026-6556Fastify\/express improper authorization vulnerability@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 …EPSS 0.52%9.1CVE-2026-33808Fastify\/express interpretation conflict vulnerabilityImpact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to Express middleware when Fastify router normalization options…EPSS 0.55%9.8CVE-2026-63030WordPress REST API route confusion leads to SQL injection and RCEWordPress 6.9.x before 6.9.5 and 7.0.x before 7.0.2 contain a REST API batch endpoint route confusion flaw (CWE-436). Chained with the author__not_in…KEVEPSS 10%analysed8.0CVE-2025-48384Git config CRLF handling allows submodule path link-following code executionGit mishandles trailing carriage returns when writing config values, so a submodule path ending in CR is read back altered during submodule initializ…KEVEPSS 4.1%analysed

Source: NIST National Vulnerability Database (record CVE-2026-33807), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.